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;****************************************************************;
;                                                                ;
;               Tiny BASIC for the Raptor64                              ;
;                                                                ;
; Derived from a 68000 derivative of Palo Alto Tiny BASIC as     ;
; published in the May 1976 issue of Dr. Dobb's Journal.         ;
; Adapted to the 68000 by:                                       ;
;       Gordon brndly                                                                    ;
;       12147 - 51 Street                                                                ;
;       Edmonton AB  T5W 3G8                                                         ;
;       Canada                                                                               ;
;       (updated mailing address for 1996)                                       ;
;                                                                ;
; Adapted to the RTF65002 by:                                    ;
;    Robert Finch                                                ;
;    Ontario, Canada                                             ;
;        robfinch@opencores.org                              ;
;****************************************************************;
;    Copyright (C) 2012 by Robert Finch. This program may be     ;
;    freely distributed for personal use only. All commercial    ;
;                      rights are reserved.                                          ;
;****************************************************************;
;
; Register Usage
; r8 = text pointer (global usage)
; r3,r4 = inputs parameters to subroutines
; r2 = return value
;
;* Vers. 1.0  1984/7/17 - Original version by Gordon brndly
;*      1.1  1984/12/9  - Addition of '0x' print term by Marvin Lipford
;*      1.2  1985/4/9   - Bug fix in multiply routine by Rick Murray
 
;
; Standard jump table. You can change these addresses if you are
; customizing this interpreter for a different environment.
;
CR      EQU     0x0D            ;ASCII equates
LF      EQU     0x0A
TAB     EQU     0x09
CTRLC   EQU     0x03
CTRLH   EQU     0x08
CTRLI   EQU     0x09
CTRLJ   EQU     0x0A
CTRLK   EQU     0x0B
CTRLM   EQU 0x0D
CTRLS   EQU     0x13
CTRLX   EQU     0x18
XON             EQU     0x11
XOFF    EQU     0x13
 
CursorRow       EQU             0x7C2
CursorCol       EQU             0x7C3
CursorFlash     EQU             0x7C4
IRQFlag         EQU             0x7C6
 
OSSP            EQU             0x700
TXTUNF          EQU             0x701
VARBGN          EQU             0x702
LOPVAR          EQU             0x703
STKGOS          EQU             0x704
CURRNT          EQU             0x705
BUFFER          EQU             0x706
BUFLEN          EQU             84
LOPPT           EQU             0x760
LOPLN           EQU             0x761
LOPINC          EQU             0x762
LOPLMT          EQU             0x763
NUMWKA          EQU             0x764
STKINP          EQU             0x774
STKBOT          EQU             0x775
usrJmp          EQU             0x776
IRQROUT         EQU             0x777
 
 
 
                cpu     rtf65002
                code
                org             $FFFFE800
GOSTART:
                jmp     CSTART  ;       Cold Start entry point
GOWARM:
                jmp     WSTART  ;       Warm Start entry point
GOOUT:
                jmp     OUTC    ;       Jump to character-out routine
GOIN:
                jmp     INCH    ;Jump to character-in routine
GOAUXO:
                jmp     AUXOUT  ;       Jump to auxiliary-out routine
GOAUXI:
                jmp     AUXIN   ;       Jump to auxiliary-in routine
GOBYE:
                jmp     BYEBYE  ;       Jump to monitor, DOS, etc.
;
; Modifiable system constants:
;
                align   4
;THRD_AREA      dw      0x04000000      ; threading switch area 0x04000000-0x40FFFFF
;bitmap dw      0x04100000      ; bitmap graphics memory 0x04100000-0x417FFFF
TXTBGN  dw      0x04180000      ;TXT            ;beginning of program memory
ENDMEM  dw      0x057FFFFF      ;       end of available memory
STACKOFFS       dw      0x058FFFFF      ; stack offset - leave a little room for the BIOS stacks
;
; The main interpreter starts here:
;
; Usage
; r1 = temp
; r8 = text buffer pointer
; r12 = end of text in text buffer
;
        align   4
message "CSTART"
public CSTART:
        ; First save off the link register and OS sp value
        tsx
        stx             OSSP
        ldx             STACKOFFS>>2    ; initialize stack pointer
        txs
        jsr             RequestIOFocus
        jsr             HomeCursor
        lda             #0                              ; turn off keyboard echoing
        jsr             SetKeyboardEcho
        stz             CursorFlash
        ldx             #0x10000020     ; black chars, yellow background
;       stx             charToPrint
        jsr             ClearScreen
        lda             #msgInit        ;       tell who we are
        jsr             PRMESG
        lda             TXTBGN>>2       ;       init. end-of-program pointer
        sta             TXTUNF
        lda             ENDMEM>>2       ;       get address of end of memory
        sub             #4096   ;       reserve 4K for the stack
        sta             STKBOT
        sub             #16384 ;   1000 vars
        sta     VARBGN
        jsr     clearVars   ; clear the variable area
        stz             IRQROUT
        lda     VARBGN   ; calculate number of bytes free
        ldy             TXTUNF
        sub     r1,r1,r3
        ldx             #12             ; max 12 digits
        jsr     PRTNUM
        lda             #msgBytesFree
        jsr             PRMESG
WSTART:
        stz             LOPVAR   ; initialize internal variables
        stz             STKGOS
        stz             CURRNT  ;       current line number pointer = 0
        ldx             ENDMEM>>2       ;       init S.P. again, just in case
        txs
        lda             #msgReady       ;       display "Ready"
        jsr             PRMESG
ST3:
        lda             #'>'            ; Prompt with a '>' and
        jsr             GETLN           ; read a line.
        jsr             TOUPBUF         ; convert to upper case
        ld              r12,r8          ; save pointer to end of line
        ld              r8,#BUFFER      ; point to the beginning of line
        jsr             TSTNUM          ; is there a number there?
        jsr             IGNBLK          ; skip trailing blanks
; does line no. exist? (or nonzero?)
        cpx             #0
        beq             DIRECT          ; if not, it's a direct statement
        cmp             #$FFFF          ; see if line no. is <= 16 bits
        bcc             ST2
        beq             ST2
        lda             #msgLineRange   ; if not, we've overflowed
        jmp             ERROR
ST2:
    ; ugliness - store a character at potentially an
    ; odd address (unaligned).
    tax                                 ; r2 = line number
        dec             r8
    stx         (r8)            ;
        jsr             FNDLN           ; find this line in save area
        ld              r13,r9          ; save possible line pointer
        cmp             #0
        beq             ST4                     ; if not found, insert
        ; here we found the line, so we're replacing the line
        ; in the text area
        ; first step - delete the line
        lda             #0
        jsr             FNDNXT          ; find the next line (into r9)
        cmp             #0
        bne             ST7
        cmp             r9,TXTUNF
        beq             ST6                     ; no more lines
        bcs             ST6
        cmp             r9,r0
        beq             ST6
ST7:
        ld              r1,r9           ; r1 = pointer to next line
        ld              r2,r13          ; pointer to line to be deleted
        ldy             TXTUNF          ; points to top of save area
        sub             r1,r3,r9        ; r1 = length to move TXTUNF-pointer to next line
;       dea                                     ; count is one less
        ld              r2,r9           ; r2 = pointer to next line
        ld              r3,r13          ; r3 = pointer to line to delete
        push    r4
ST8:
        ld              r4,(x)
        st              r4,(y)
        inx
        iny
        dea
        bne             ST8
        pop             r4
;       mvn
;       jsr             MVUP            ; move up to delete
        sty             TXTUNF          ; update the end pointer
        ; we moved the lines of text after the line being
        ; deleted down, so the pointer to the next line
        ; needs to be reset
        ld              r9,r13
        bra             ST4
        ; here there were no more lines, so just move the
        ; end of text pointer down
ST6:
        st              r13,TXTUNF
        ld              r9,r13
ST4:
        ; here we're inserting because the line wasn't found
        ; or it was deleted     from the text area
        sub             r1,r12,r8               ; calculate the length of new line
        cmp             #2                              ; is it just a line no. & CR? if so, it was just a delete
        beq             ST3
        bcc             ST3
 
        ; compute new end of text
        ld              r10,TXTUNF              ; r10 = old TXTUNF
        add             r11,r10,r1              ; r11 = new top of TXTUNF (r1=line length)
 
        cmp             r11,VARBGN      ; see if there's enough room
        bcc             ST5
        lda             #msgTooBig      ; if not, say so
        jmp             ERROR
 
        ; open a space in the text area
ST5:
        st              r11,TXTUNF      ; if so, store new end position
        ld              r1,r10          ; points to old end of text
        ld              r2,r11          ; points to new end of text
        ld              r3,r9       ; points to start of line after insert line
        jsr             MVDOWN          ; move things out of the way
 
        ; copy line into text space
        ld              r1,r8           ; set up to do the insertion; move from buffer
        ld              r2,r13          ; to vacated space
        ld              r3,r12          ; until end of buffer
        jsr             MVUP            ; do it
        jmp             ST3                     ; go back and get another line
 
;******************************************************************
;
; *** Tables *** DIRECT *** EXEC ***
;
; This section of the code tests a string against a table. When
; a match is found, control is transferred to the section of
; code according to the table.
;
; At 'EXEC', r8 should point to the string, r9 should point to
; the character table, and r10 should point to the execution
; table. At 'DIRECT', r8 should point to the string, r9 and
; r10 will be set up to point to TAB1 and TAB1_1, which are
; the tables of all direct and statement commands.
;
; A '.' in the string will terminate the test and the partial
; match will be considered as a match, e.g. 'P.', 'PR.','PRI.',
; 'PRIN.', or 'PRINT' will all match 'PRINT'.
;
; There are two tables: the character table and the execution
; table. The character table consists of any number of text items.
; Each item is a string of characters with the last character's
; high bit set to one. The execution table holds a 32-bit
; execution addresses that correspond to each entry in the
; character table.
;
; The end of the character table is a 0 byte which corresponds
; to the default routine in the execution table, which is
; executed if none of the other table items are matched.
;
; Character-matching tables:
message "TAB1"
TAB1:
        db      "LIS",'T'+0x80        ; Direct commands
        db      "LOA",'D'+0x80
        db      "NE",'W'+0x80
        db      "RU",'N'+0x80
        db      "SAV",'E'+0x80
TAB2:
        db      "NEX",'T'+0x80         ; Direct / statement
        db      "LE",'T'+0x80
        db      "I",'F'+0x80
        db      "GOT",'O'+0x80
        db      "GOSU",'B'+0x80
        db      "RETUR",'N'+0x80
        db      "RE",'M'+0x80
        db      "FO",'R'+0x80
        db      "INPU",'T'+0x80
        db      "PRIN",'T'+0x80
        db      "POK",'E'+0x80
        db      "STO",'P'+0x80
        db      "BY",'E'+0x80
        db      "SY",'S'+0x80
        db      "CL",'S'+0x80
    db  "CL",'R'+0x80
    db  "RDC",'F'+0x80
    db  "ONIR",'Q'+0x80
    db  "WAI",'T'+0x80
        db      0
TAB4:
        db      "PEE",'K'+0x80         ;Functions
        db      "RN",'D'+0x80
        db      "AB",'S'+0x80
        db  "SG",'N'+0x80
        db      "TIC",'K'+0x80
        db      "SIZ",'E'+0x80
        db  "US",'R'+0x80
        db      0
TAB5:
        db      "T",'O'+0x80           ;"TO" in "FOR"
        db      0
TAB6:
        db      "STE",'P'+0x80         ;"STEP" in "FOR"
        db      0
TAB8:
        db      '>','='+0x80           ;Relational operators
        db      '<','>'+0x80
        db      '>'+0x80
        db      '='+0x80
        db      '<','='+0x80
        db      '<'+0x80
        db      0
TAB9:
    db  "AN",'D'+0x80
    db  0
TAB10:
    db  "O",'R'+0x80
    db  0
 
;* Execution address tables:
; We save some bytes by specifiying only the low order 16 bits of the address
;
TAB1_1:
        dh      LISTX                   ;Direct commands
        dh      LOAD
        dh      NEW
        dh      RUN
        dh      SAVE
TAB2_1:
        dh      NEXT            ;       Direct / statement
        dh      LET
        dh      IF
        dh      GOTO
        dh      GOSUB
        dh      RETURN
        dh      IF2                     ; REM
        dh      FOR
        dh      INPUT
        dh      PRINT
        dh      POKE
        dh      STOP
        dh      GOBYE
        dh      SYSX
        dh      _cls
        dh  _clr
        dh      _rdcf
        dh  ONIRQ
        dh      WAITIRQ
        dh      DEFLT
TAB4_1:
        dh      PEEK                    ;Functions
        dh      RND
        dh      ABS
        dh  SGN
        dh      TICKX
        dh      SIZEX
        dh  USRX
        dh      XP40
TAB5_1
        dh      FR1                     ;"TO" in "FOR"
        dh      QWHAT
TAB6_1
        dh      FR2                     ;"STEP" in "FOR"
        dh      FR3
TAB8_1
        dh      XP11    ;>=             Relational operators
        dh      XP12    ;<>
        dh      XP13    ;>
        dh      XP15    ;=
        dh      XP14    ;<=
        dh      XP16    ;<
        dh      XP17
TAB9_1
    dh  XP_AND
    dh  XP_ANDX
TAB10_1
    dh  XP_OR
    dh  XP_ORX
 
;*
; r3 = match flag (trashed)
; r9 = text table
; r10 = exec table
; r11 = trashed
message "DIRECT"
DIRECT:
        ld              r9,#TAB1
        ld              r10,#TAB1_1
EXEC:
        jsr             IGNBLK          ; ignore leading blanks
        ld              r11,r8          ; save the pointer
        eor             r3,r3,r3        ; clear match flag
EXLP:
        lda             (r8)            ; get the program character
        inc             r8
        lb              r2,$0,r9        ; get the table character
        bne             EXNGO           ; If end of table,
        ld              r8,r11          ;       restore the text pointer and...
        bra             EXGO            ;   execute the default.
EXNGO:
        cmp             r1,r3           ; Else check for period... if so, execute
        beq             EXGO
        and             r2,r2,#0x7f     ; ignore the table's high bit
        cmp             r2,r1           ;               is there a match?
        beq             EXMAT
        inc             r10                     ;if not, try the next entry
        inc             r10
        ld              r8,r11          ; reset the program pointer
        eor             r3,r3,r3        ; sorry, no match
EX1:
        lb              r1,0,r9         ; get to the end of the entry
        inc             r9
        bit             #$80            ; test for bit 7 set
        beq             EX1
        bra             EXLP            ; back for more matching
EXMAT:
        ldy             #'.'            ; we've got a match so far
        lb              r1,0,r9         ; end of table entry?
        inc             r9
        bit             #$80            ; test for bit 7 set
        beq             EXLP            ; if not, go back for more
EXGO:
        ; execute the appropriate routine
        lb              r1,1,r10        ; get the low mid order byte
        asl             r1,r1,#8
        orb             r1,r1,0,r10     ; get the low order byte
        or              r1,r1,#$FFFF0000        ; add in ROM base
        jmp             (r1)
 
 
;******************************************************************
;
; What follows is the code to execute direct and statement
; commands. Control is transferred to these points via the command
; table lookup code of 'DIRECT' and 'EXEC' in the last section.
; After the command is executed, control is transferred to other
; sections as follows:
;
; For 'LISTX', 'NEW', and 'STOP': go back to the warm start point.
; For 'RUN': go execute the first stored line if any; else go
; back to the warm start point.
; For 'GOTO' and 'GOSUB': go execute the target line.
; For 'RETURN' and 'NEXT'; go back to saved return line.
; For all others: if 'CURRNT' is 0, go to warm start; else go
; execute next command. (This is done in 'FINISH'.)
;
;******************************************************************
;
; *** NEW *** STOP *** RUN (& friends) *** GOTO ***
;
; 'NEW' sets TXTUNF to point to TXTBGN
;
 
NEW:
        jsr             ENDCHK
        lda             TXTBGN>>2
        sta             TXTUNF  ;       set the end pointer
        jsr     clearVars
 
; 'STOP' goes back to WSTART
;
STOP:
        jsr             ENDCHK
        jmp             WSTART          ; WSTART will reset the stack
 
; 'RUN' finds the first stored line, stores its address
; in CURRNT, and starts executing it. Note that only those
; commands in TAB2 are legal for a stored program.
;
; There are 3 more entries in 'RUN':
; 'RUNNXL' finds next line, stores it's address and executes it.
; 'RUNTSL' stores the address of this line and executes it.
; 'RUNSML' continues the execution on same line.
;
RUN:
        jsr             ENDCHK
        ld              r8,TXTBGN>>2    ;       set pointer to beginning
        st              r8,CURRNT
        jsr     clearVars
 
RUNNXL                                  ; RUN 
        lda             CURRNT  ; executing a program?
        beq             WSTART  ; if not, we've finished a direct stat.
        lda             IRQROUT         ; are we handling IRQ's ?
        beq             RUN1
        ld              r0,IRQFlag              ; was there an IRQ ?
        beq             RUN1
        stz             IRQFlag
        jsr             PUSHA           ; the same code as a GOSUB
        push    r8
        lda             CURRNT
        pha                                     ; found it, save old 'CURRNT'...
        lda             STKGOS
        pha                                     ; and 'STKGOS'
        stz             LOPVAR          ; load new values
        tsx
        stx             STKGOS
        ld              r9,IRQROUT
        bra             RUNTSL
RUN1
        lda             #0          ; else find the next line number
        ld              r9,r8
        jsr             FNDLNP          ; search for the next line
;       cmp             #0
;       bne             RUNTSL
        cmp             r9,TXTUNF; if we've fallen off the end, stop
        beq             WSTART
        bcs             WSTART
 
RUNTSL                                  ; RUN 
        st              r9,CURRNT       ; set CURRNT to point to the line no.
        add             r8,r9,#1        ; set the text pointer to
 
RUNSML                 ; RUN 
        jsr             CHKIO           ; see if a control-C was pressed
        ld              r9,#TAB2                ; find command in TAB2
        ld              r10,#TAB2_1
        jmp             EXEC            ; and execute it
 
 
; 'GOTO expr' evaluates the expression, finds the target
; line, and jumps to 'RUNTSL' to do it.
;
GOTO
        lda             #'G'
        jsr             DisplayChar
        jsr             OREXPR          ;evaluate the following expression
        jsr             DisplayWord
        ld      r5,r1
        jsr     ENDCHK          ;must find end of line
        ld      r1,r5
        jsr     FNDLN           ; find the target line
        cmp             #0
        bne             RUNTSL          ; go do it
        lda             #msgBadGotoGosub
        jmp             ERROR           ; no such line no.
 
_clr:
    jsr     clearVars
    jmp     FINISH
 
; Clear the variable area of memory
clearVars:
        push    r6
    ld      r6,#2048    ; number of words to clear
    lda     VARBGN
cv1:
    stz     (r1)
    ina
    dec         r6
    bne         cv1
    pop         r6
    rts
 
;******************************************************************
; ONIRQ 
; ONIRQ sets up an interrupt handler which acts like a specialized
; subroutine call. ONIRQ is coded like a GOTO that never executes.
;******************************************************************
;
ONIRQ:
        jsr             OREXPR          ;evaluate the following expression
        ld      r5,r1
        jsr     ENDCHK          ;must find end of line
        ld      r1,r5
        jsr     FNDLN           ; find the target line
        cmp             #0
        bne             ONIRQ1
        stz             IRQROUT
        jmp             FINISH
ONIRQ1:
        st              r9,IRQROUT
        jmp             FINISH
 
 
WAITIRQ:
        jsr             CHKIO           ; see if a control-C was pressed
        ld              r0,IRQFlag
        beq             WAITIRQ
        jmp             FINISH
 
 
;******************************************************************
; LIST
;
; LISTX has two forms:
; 'LIST' lists all saved lines
; 'LIST #' starts listing at the line #
; Control-S pauses the listing, control-C stops it.
;******************************************************************
;
LISTX:
        jsr             TSTNUM          ; see if there's a line no.
        ld      r5,r1
        jsr             ENDCHK          ; if not, we get a zero
        ld      r1,r5
        jsr             FNDLN           ; find this or next line
LS1:
        cmp             #0
        bne             LS4
        cmp             r9,TXTUNF
        beq             WSTART
        bcs             WSTART          ; warm start if we passed the end
LS4:
        ld              r1,r9
        jsr             PRTLN           ; print the line
        ld              r9,r1           ; set pointer for next
        jsr             CHKIO           ; check for listing halt request
        cmp             #0
        beq             LS3
        cmp             #CTRLS          ; pause the listing?
        bne             LS3
LS2:
        jsr     CHKIO           ; if so, wait for another keypress
        cmp             #0
        beq             LS2
LS3:
        lda             #0
        jsr             FNDLNP          ; find the next line
        bra             LS1
 
 
;******************************************************************
; PRINT command is 'PRINT ....:' or 'PRINT ....'
; where '....' is a list of expressions, formats, back-arrows,
; and strings.  These items a separated by commas.
;
; A format is a pound sign followed by a number.  It controls
; the number of spaces the value of an expression is going to
; be printed in.  It stays effective for the rest of the print
; command unless changed by another format.  If no format is
; specified, 11 positions will be used.
;
; A string is quoted in a pair of single- or double-quotes.
;
; An underline (back-arrow) means generate a  without a 
;
; A  is generated after the entire list has been printed
; or if the list is empty.  If the list ends with a semicolon,
; however, no  is generated.
;******************************************************************
;
PRINT:
        ld              r5,#11          ; D4 = number of print spaces
        ldy             #':'
        ld              r4,#PR2
        jsr             TSTC            ; if null list and ":"
        jsr             CRLF            ; give CR-LF and continue
        jmp             RUNSML          ;               execution on the same line
PR2:
        ldy             #CR
        ld              r4,#PR0
        jsr             TSTC            ;if null list and 
        jsr             CRLF            ;also give CR-LF and
        jmp             RUNNXL          ;execute the next line
PR0:
        ldy             #'#'
        ld              r4,#PR1
        jsr             TSTC            ;else is it a format?
        jsr             OREXPR          ; yes, evaluate expression
        ld              r5,r1   ; and save it as print width
        bra             PR3             ; look for more to print
PR1:
        ldy             #'$'
        ld              r4,#PR4
        jsr             TSTC    ;       is character expression? (MRL)
        jsr             OREXPR  ;       yep. Evaluate expression (MRL)
        jsr             GOOUT   ;       print low byte (MRL)
        bra             PR3             ;look for more. (MRL)
PR4:
        jsr             QTSTG   ;       is it a string?
        ; the following branch must occupy only two bytes!
        bra             PR8             ;       if not, must be an expression
PR3:
        ldy             #','
        ld              r4,#PR6
        jsr             TSTC    ;       if ",", go find next
        jsr             FIN             ;in the list.
        bra             PR0
PR6:
        jsr             CRLF            ;list ends here
        jmp             FINISH
PR8:
        jsr             OREXPR          ; evaluate the expression
        ld              r2,r5           ; set the width
        jsr             PRTNUM          ; print its value
        bra             PR3                     ; more to print?
 
FINISH:
        jsr             FIN             ; Check end of command
        jmp             QWHAT   ; print "What?" if wrong
 
 
;*******************************************************************
;
; *** GOSUB *** & RETURN ***
;
; 'GOSUB expr:' or 'GOSUB expr' is like the 'GOTO' command,
; except that the current text pointer, stack pointer, etc. are
; saved so that execution can be continued after the subroutine
; 'RETURN's.  In order that 'GOSUB' can be nested (and even
; recursive), the save area must be stacked.  The stack pointer
; is saved in 'STKGOS'.  The old 'STKGOS' is saved on the stack.
; If we are in the main routine, 'STKGOS' is zero (this was done
; in the initialization section of the interpreter), but we still
; save it as a flag for no further 'RETURN's.
;******************************************************************
;
GOSUB:
        jsr             PUSHA           ; save the current 'FOR' parameters
        jsr             OREXPR          ; get line number
        jsr             FNDLN           ; find the target line
        cmp             #0
        bne             gosub1
        lda             #msgBadGotoGosub
        jmp             ERROR           ; if not there, say "How?"
gosub1:
        push    r8
        lda             CURRNT
        pha                                     ; found it, save old 'CURRNT'...
        lda             STKGOS
        pha                                     ; and 'STKGOS'
        stz             LOPVAR          ; load new values
        tsx
        stx             STKGOS
        jmp             RUNTSL
 
 
;******************************************************************
; 'RETURN' undoes everything that 'GOSUB' did, and thus
; returns the execution to the command after the most recent
; 'GOSUB'.  If 'STKGOS' is zero, it indicates that we never had
; a 'GOSUB' and is thus an error.
;******************************************************************
;
RETURN:
        jsr             ENDCHK          ; there should be just a 
        ldx             STKGOS          ; get old stack pointer
        bne             return1
        lda             #msgRetWoGosub
        jmp             ERROR           ; if zero, it doesn't exist
return1:
        txs                                     ; else restore it
        pla
        sta             STKGOS          ; and the old 'STKGOS'
        pla
        sta             CURRNT          ; and the old 'CURRNT'
        pop             r8                      ; and the old text pointer
        jsr             POPA            ;and the old 'FOR' parameters
        jmp             FINISH          ;and we are back home
 
;******************************************************************
; *** FOR *** & NEXT ***
;
; 'FOR' has two forms:
; 'FOR var=exp1 TO exp2 STEP exp1' and 'FOR var=exp1 TO exp2'
; The second form means the same thing as the first form with a
; STEP of positive 1.  The interpreter will find the variable 'var'
; and set its value to the current value of 'exp1'.  It also
; evaluates 'exp2' and 'exp1' and saves all these together with
; the text pointer, etc. in the 'FOR' save area, which consists of
; 'LOPVAR', 'LOPINC', 'LOPLMT', 'LOPLN', and 'LOPPT'.  If there is
; already something in the save area (indicated by a non-zero
; 'LOPVAR'), then the old save area is saved on the stack before
; the new values are stored.  The interpreter will then dig in the
; stack and find out if this same variable was used in another
; currently active 'FOR' loop.  If that is the case, then the old
; 'FOR' loop is deactivated. (i.e. purged from the stack)
;******************************************************************
;
FOR:
        jsr             PUSHA           ; save the old 'FOR' save area
        jsr             SETVAL          ; set the control variable
        sta             LOPVAR          ; save its address
        ld              r9,#TAB5
        ld              r10,#TAB5_1     ; use 'EXEC' to test for 'TO'
        jmp             EXEC
FR1:
        jsr             OREXPR          ; evaluate the limit
        sta             LOPLMT  ; save that
        ld              r9,#TAB6
        ld              r10,#TAB6_1     ; use 'EXEC' to test for the word 'STEP
        jmp             EXEC
FR2:
        jsr             OREXPR          ; found it, get the step value
        bra             FR4
FR3:
        lda             #1              ; not found, step defaults to 1
FR4:
        sta             LOPINC  ; save that too
FR5:
        ldx             CURRNT
        stx             LOPLN   ; save address of current line number
        st              r8,LOPPT        ; and text pointer
        tsx
        txy                                     ; dig into the stack to find 'LOPVAR'
        ld              r6,LOPVAR
        bra             FR7
FR6:
        add             r3,r3,#5        ; look at next stack frame
FR7:
        ldx             (y)                     ; is it zero?
        beq             FR8                     ; if so, we're done
        cmp             r2,r6
        bne             FR6                     ; same as current LOPVAR? nope, look some more
 
    tya                       ; Else remove 5 long words from...
        add             r2,r3,#5   ; inside the stack.
        tsx
        txy
        jsr             MVDOWN
        pla                                     ; set the SP 5 long words up
        pla
        pla
        pla
        pla
FR8:
    jmp     FINISH              ; and continue execution
 
 
;******************************************************************
; 'NEXT var' serves as the logical (not necessarily physical) end
; of the 'FOR' loop.  The control variable 'var' is checked with
; the 'LOPVAR'.  If they are not the same, the interpreter digs in
; the stack to find the right one and purges all those that didn't
; match.  Either way, it then adds the 'STEP' to that variable and
; checks the result with against the limit value.  If it is within
; the limit, control loops back to the command following the
; 'FOR'.  If it's outside the limit, the save area is purged and
; execution continues.
;******************************************************************
;
NEXT:
        lda             #0              ; don't allocate it
        jsr             TSTV            ; get address of variable
        cmp             #0
        bne             NX4
        lda             #msgNextVar
        bra             ERROR           ; if no variable, say "What?"
NX4:
        ld              r9,r1   ; save variable's address
NX0:
        lda             LOPVAR  ; If 'LOPVAR' is zero, we never...
        bne             NX5             ; had a FOR loop
        lda             #msgNextFor
        bra             ERROR
NX5:
        cmp             r1,r9
        beq             NX2             ; else we check them OK, they agree
        jsr             POPA            ; nope, let's see the next frame
        bra             NX0
NX2:
        lda             (r9)            ; get control variable's value
        ldx             LOPINC
        add             r1,r1,r2        ; add in loop increment
;       BVS.L   QHOW            say "How?" for 32-bit overflow
        sta             (r9)            ; save control variable's new value
        ldy             LOPLMT          ; get loop's limit value
        cmp             r2,#1
        beq             NX1
        bpl             NX1                     ; check loop increment, branch if loop increment is positive
        cmp             r1,r3
        beq             NX3
        bmi             NXPurge         ; test against limit
        bra     NX3
NX1:
        cmp             r1,r3
        beq             NX3
        bpl             NXPurge
NX3:
        ld              r8,LOPLN        ; Within limit, go back to the...
        st              r8,CURRNT
        ld              r8,LOPPT        ; saved 'CURRNT' and text pointer.
        jmp             FINISH
NXPurge:
    jsr    POPA        ; purge this loop
    jmp     FINISH
 
 
;******************************************************************
; *** REM *** IF *** INPUT *** LET (& DEFLT) ***
;
; 'REM' can be followed by anything and is ignored by the
; interpreter.
;
;REM
;    br     IF2             ; skip the rest of the line
; 'IF' is followed by an expression, as a condition and one or
; more commands (including other 'IF's) separated by colons.
; Note that the word 'THEN' is not used.  The interpreter evaluates
; the expression.  If it is non-zero, execution continues.  If it
; is zero, the commands that follow are ignored and execution
; continues on the next line.
;******************************************************************
;
IF:
    jsr         OREXPR          ; evaluate the expression
IF1:
        cmp             #0
    bne     RUNSML              ; is it zero? if not, continue
IF2:
    ld          r9,r8   ; set lookup pointer
        lda             #0              ; find line #0 (impossible)
        jsr             FNDSKP          ; if so, skip the rest of the line
        cmp             #0
        bcs             WSTART  ; if no next line, do a warm start
IF3:
        jmp             RUNTSL          ; run the next line
 
 
;******************************************************************
; INPUT is called first and establishes a stack frame
INPERR:
        ldx             STKINP          ; restore the old stack pointer
        txs
        pla
        sta             CURRNT          ; and old 'CURRNT'
        pop             r8                      ; and old text pointer
        tsx
        add             r2,r2,#5        ; fall through will subtract 5
        txs
 
; 'INPUT' is like the 'PRINT' command, and is followed by a list
; of items.  If the item is a string in single or double quotes,
; or is an underline (back arrow), it has the same effect as in
; 'PRINT'.  If an item is a variable, this variable name is
; printed out followed by a colon, then the interpreter waits for
; an expression to be typed in.  The variable is then set to the
; value of this expression.  If the variable is preceeded by a
; string (again in single or double quotes), the string will be
; displayed followed by a colon.  The interpreter the waits for an
; expression to be entered and sets the variable equal to the
; expression's value.  If the input expression is invalid, the
; interpreter will print "What?", "How?", or "Sorry" and reprint
; the prompt and redo the input.  The execution will not terminate
; unless you press control-C.  This is handled in 'INPERR'.
;
INPUT:
        push    r7
        tsr             sp,r7
        sub             r7,r7,#5        ; allocate five words on stack
        trs             r7,sp
        st              r5,4,r7         ; save off r5 into stack var
IP6:
        st              r8,(r7)         ; save in case of error
        jsr             QTSTG           ; is next item a string?
        bra             IP2                     ; nope - this branch must take only two bytes
        lda             #1              ; allocate var
        jsr             TSTV            ; yes, but is it followed by a variable?
        cmp             #0
        beq     IP4   ; if not, brnch
        or              r10,r1,r0               ; put away the variable's address
        bra             IP3                     ; if so, input to variable
IP2:
        st              r8,1,r7         ; save off in stack var for 'PRTSTG'
        lda             #1
        jsr             TSTV            ; must be a variable now
        cmp             #0
        bne             IP7
        lda             #msgInputVar
        add             r7,r7,#5        ; cleanup stack
        trs             r7,sp
        pop             r7                      ; so we can get back r7
        bra             ERROR           ; "What?" it isn't?
IP7:
        or              r10,r1,r0       ; put away the variable's address
        ld              r5,(r8)         ; get ready for 'PRTSTG' by null terminating
        stz             (r8)
        lda             1,r7                    ; get back text pointer
        jsr             PRTSTG          ; print string as prompt
        st              r5,(r8)         ; un-null terminate
IP3
        st              r8,1,r7         ; save in case of error
        lda             CURRNT
        sta             2,r7                    ; also save 'CURRNT'
        lda             #-1
        sta             CURRNT          ; flag that we are in INPUT
        stx             STKINP          ; save the stack pointer too
        st              r10,3,r7        ; save the variable address
        lda             #':'            ; print a colon first
        jsr             GETLN           ; then get an input line
        ld              r8,#BUFFER      ; point to the buffer
        jsr             OREXPR          ; evaluate the input
        ld              r10,3,r7        ; restore the variable address
        sta             (r10)           ; save value in variable
        lda             2,r7            ; restore old 'CURRNT'
        sta             CURRNT
        ld              r8,1,r7         ; and the old text pointer
IP4:
        ldy             #','
        ld              r4,#IP5         ; is the next thing a comma?
        jsr             TSTC
        bra             IP6                     ; yes, more items
IP5:
        ld              r5,4,r7
        add             r7,r7,#5        ; cleanup stack
        trs             r7,sp
        pop             r7
        jmp             FINISH
 
 
DEFLT:
    lda     (r8)
    cmp         #CR
        beq         FINISH          ; empty line is OK else it is 'LET'
 
 
;******************************************************************
; 'LET' is followed by a list of items separated by commas.
; Each item consists of a variable, an equals sign, and an
; expression.  The interpreter evaluates the expression and sets
; the variable to that value.  The interpreter will also handle
; 'LET' commands without the word 'LET'.  This is done by 'DEFLT'.
;******************************************************************
;
LET:
    jsr         SETVAL          ; do the assignment
    ldy         #','
    ld          r4,#FINISH
        jsr             TSTC            ; check for more 'LET' items
        bra         LET
LT1:
    jmp     FINISH              ; until we are finished.
 
 
;******************************************************************
; *** LOAD *** & SAVE ***
;
; These two commands transfer a program to/from an auxiliary
; device such as a cassette, another computer, etc.  The program
; is converted to an easily-stored format: each line starts with
; a colon, the line no. as 4 hex digits, and the rest of the line.
; At the end, a line starting with an '@' sign is sent.  This
; format can be read back with a minimum of processing time by
; the RTF65002
;******************************************************************
;
LOAD
        ld              r8,TXTBGN>>2    ; set pointer to start of prog. area
        lda             #CR                     ; For a CP/M host, tell it we're ready...
        jsr             GOAUXO          ; by sending a CR to finish PIP command.
LOD1:
        jsr             GOAUXI          ; look for start of line
        cmp             #0
        beq             LOD1
        bcc             LOD1
        cmp             #'@'            ; end of program?
        beq             LODEND
        cmp             #$1A
        beq     LODEND  ; or EOF marker
        cmp             #':'
        bne             LOD1    ; if not, is it start of line? if not, wait for it
        jsr             GCHAR           ; get line number
        sta             (r8)            ; store it
        inc             r8
LOD2:
        jsr             GOAUXI          ; get another text char.
        cmp             #0
        beq             LOD2
        bcc             LOD2
        sta             (r8)
        inc             r8                      ; store it
        cmp             #CR
        bne             LOD2            ; is it the end of the line? if not, go back for more
        bra             LOD1            ; if so, start a new line
LODEND:
        st              r8,TXTUNF       ; set end-of program pointer
        jmp             WSTART          ; back to direct mode
 
 
; get character from input (16 bit value)
GCHAR:
        push    r5
        push    r6
        ld              r6,#3       ; repeat four times
        ld              r5,#0
GCHAR1:
        jsr             GOAUXI          ; get a char
        cmp             #0
        beq             GCHAR1
        bcc             GCHAR1
        jsr             asciiToHex
        asl             r5,r5
        asl             r5,r5
        asl             r5,r5
        asl             r5,r5
        or              r5,r5,r1
        dec             r6
        bne             GCHAR1
        ld              r1,r5
        pop             r6
        pop             r5
        rts
 
 
; convert an ascii char to hex code
; input
;       r1 = char to convert
 
asciiToHex:
        cmp             #'9'            ; less than '9'
        beq             a2h1
        bcc             a2h1
        sub             #7                      ; shift 'A' to '9'+1
a2h1:
        sub             #'0'
        and             #15                     ; make sure a nybble
        rts
 
 
 
SAVE:
        ld              r8,TXTBGN>>2    ;set pointer to start of prog. area
        ld              r9,TXTUNF       ;set pointer to end of prog. area
SAVE1:
        jsr             AUXOCRLF    ; send out a CR & LF (CP/M likes this)
        cmp             r8,r9
        bcs             SAVEND          ; are we finished?
        lda             #':'            ; if not, start a line
        jsr             GOAUXO
        lda             (r8)            ; get line number
        inc             r8
        jsr             PWORD       ; output line number as 4-digit hex
SAVE2:
        lda             (r8)            ; get a text char.
        inc             r8
        cmp             #CR
        beq             SAVE1           ; is it the end of the line? if so, send CR & LF and start new line
        jsr             GOAUXO          ; send it out
        bra             SAVE2           ; go back for more text
SAVEND:
        lda             #'@'            ; send end-of-program indicator
        jsr             GOAUXO
        jsr             AUXOCRLF    ; followed by a CR & LF
        lda             #$1A            ; and a control-Z to end the CP/M file
        jsr             GOAUXO
        bra             WSTART          ; then go do a warm start
 
 
; output a CR LF sequence to auxillary output
; Registers Affected
;   r3 = LF
AUXOCRLF:
    lda         #CR
    jsr         GOAUXO
    lda         #LF
    jsr         GOAUXO
    rts
 
 
; output a word in hex format
; tricky because of the need to reverse the order of the chars
PWORD:
        push    r5
        ld              r5,#NUMWKA+15
        or              r4,r1,r0        ; r4 = value
pword1:
    or      r1,r4,r0    ; r1 = value
    lsr         r4,r4           ; shift over to next nybble
    lsr         r4,r4
    lsr         r4,r4
    lsr         r4,r4
    jsr         toAsciiHex  ; convert LS nybble to ascii hex
    sta     (r5)                ; save in work area
    sub         r5,r5,#1
    cmp         r5,#NUMWKA
    beq         pword1
    bcs         pword1
pword2:
    add         r5,r5,#1
    lda    (r5)     ; get char to output
        jsr             GOAUXO          ; send it
        cmp             r5,#NUMWKA+15
        bcc             pword2
        pop             r5
        rts
 
 
; convert nybble in r2 to ascii hex char2
; r2 = character to convert
 
toAsciiHex:
        and             #15     ; make sure it's a nybble
        cmp             #10     ; > 10 ?
        bcc             tah1
        add             #7      ; bump it up to the letter 'A'
tah1:
        add             #'0'    ; bump up to ascii '0'
        rts
 
 
 
;******************************************************************
; *** POKE ***
;
; 'POKE expr1,expr2' stores the word from 'expr2' into the memory
; address specified by 'expr1'.
;******************************************************************
;
POKE:
        jsr             OREXPR          ; get the memory address
        ldy             #','
        ld              r4,#PKER        ; it must be followed by a comma
        jsr             TSTC            ; it must be followed by a comma
        pha                                     ; save the address
        jsr             OREXPR          ; get the byte to be POKE'd
        plx                                 ; get the address back
        sta             (x)                     ; store the byte in memory
        jmp             FINISH
PKER:
        lda             #msgComma
        jmp             ERROR           ; if no comma, say "What?"
 
 
;******************************************************************
; 'SYSX expr' jumps to the machine language subroutine whose
; starting address is specified by 'expr'.  The subroutine can use
; all registers but must leave the stack the way it found it.
; The subroutine returns to the interpreter by executing an RTS.
;******************************************************************
 
SYSX:
        jsr             OREXPR          ; get the subroutine's address
        cmp             #0
        bne             sysx1           ; make sure we got a valid address
        lda             #msgSYSBad
        jmp             ERROR
sysx1:
        push    r8                      ; save the text pointer
        jsr             (r1)            ; jump to the subroutine
        pop             r8                  ; restore the text pointer
        jmp             FINISH
 
;******************************************************************
; *** EXPR ***
;
; 'EXPR' evaluates arithmetical or logical expressions.
; ::=  OR  ...
; ::= AND  ...
; ::=
;          
; where  is one of the operators in TAB8 and the result
; of these operations is 1 if true and 0 if false.
; ::=(+ or -)(+ or -)(...
; where () are optional and (... are optional repeats.
; ::=( <* or /> )(...
; ::=
;           
;           ()
;  is recursive so that the variable '@' can have an 
; as an index, functions can have an  as arguments, and
;  can be an  in parenthesis.
;
 
; ::= OR  ...
;
OREXPR:
        jsr             ANDEXPR         ; get first 
XP_OR1:
        pha                                     ; save  value
        ld              r9,#TAB10       ; look up a logical operator
        ld              r10,#TAB10_1
        jmp             EXEC            ; go do it
XP_OR:
    jsr         ANDEXPR
    plx
    or      r1,r1,r2
    bra     XP_OR1
XP_ORX:
        pla
    rts
 
 
; ::= AND  ...
;
ANDEXPR:
        jsr             EXPR            ; get first 
XP_AND1:
        pha                                     ; save  value
        ld              r9,#TAB9                ; look up a logical operator
        ld              r10,#TAB9_1
        jmp             EXEC            ; go do it
XP_AND:
    jsr         EXPR
    plx
    and     r1,r1,r2
    bra     XP_AND1
XP_ANDX:
        pla
    rts
 
 
; Determine if the character is a digit
;   Parameters
;       r1 = char to test
;   Returns
;       r1 = 1 if digit, otherwise 0
;
isDigit:
        cmp             #'0'
        bcc             isDigitFalse
        cmp             #'9'+1
        bcs             isDigitFalse
        lda             #1
    rts
isDigitFalse:
    lda         #0
    rts
 
 
; Determine if the character is a alphabetic
;   Parameters
;       r1 = char to test
;   Returns
;       r1 = 1 if alpha, otherwise 0
;
isAlpha:
        cmp             #'A'
        bcc             isAlphaFalse
        cmp             #'Z'
        beq             isAlphaTrue
        bcc             isAlphaTrue
        cmp             #'a'
        bcc             isAlphaFalse
        cmp             #'z'+1
        bcs             isAlphaFalse
isAlphaTrue:
    lda         #1
    rts
isAlphaFalse:
    lda         #0
    rts
 
 
; Determine if the character is a alphanumeric
;   Parameters
;       r1 = char to test
;   Returns
;       r1 = 1 if alpha, otherwise 0
;
isAlnum:
    tax                                         ; save test char
    jsr         isDigit
    cmp         #0
    bne         isDigitx                ; if it is a digit
    txa                                         ; get back test char
    jsr    isAlpha
isDigitx:
    rts
 
 
EXPR:
        jsr             EXPR2
        pha                                     ; save  value
        ld              r9,#TAB8                ; look up a relational operator
        ld              r10,#TAB8_1
        jmp             EXEC            ; go do it
XP11:
        pla
        jsr             XP18    ; is it ">="?
        cmp             r2,r1
        bpl             XPRT1   ; no, return r2=1
        bra             XPRT0   ; else return r2=0
XP12:
        pla
        jsr             XP18    ; is it "<>"?
        cmp             r2,r1
        bne             XPRT1   ; no, return r2=1
        bra             XPRT0   ; else return r2=0
XP13:
        pla
        jsr             XP18    ; is it ">"?
        cmp             r2,r1
        beq             XPRT0
        bpl             XPRT1   ; no, return r2=1
        bra             XPRT0   ; else return r2=0
XP14:
        pla
        jsr             XP18    ; is it "<="?
        cmp             r2,r1
        beq             XPRT1   ; no, return r2=1
        bmi             XPRT1
        bra             XPRT0   ; else return r2=0
XP15:
        pla
        jsr             XP18    ; is it "="?
        cmp             r2,r1
        beq             XPRT1   ; if not, return r2=1
        bra             XPRT0   ; else return r2=0
XP16:
        pla
        jsr             XP18    ; is it "<"?
        cmp             r2,r1
        bmi             XPRT1   ; if not, return r2=1
        bra             XPRT0   ; else return r2=0
XPRT0:
        lda             #0   ; return r1=0 (false)
        rts
XPRT1:
        lda             #1      ; return r1=1 (true)
        rts
 
XP17:                           ; it's not a rel. operator
        pla                             ; return r2=
        rts
 
XP18:
        pha
        jsr             EXPR2           ; do a second 
        plx
        rts
 
; ::=(+ or -)(+ or -)(...
message "EXPR2"
EXPR2:
        ldy             #'-'
        ld              r4,#XP21
        jsr             TSTC            ; negative sign?
        lda             #0              ; yes, fake '0-'
        pha
        bra             XP26
XP21:
        ldy             #'+'
        ld              r4,#XP22
        jsr             TSTC            ; positive sign? ignore it
XP22:
        jsr             EXPR3           ; first 
XP23:
        pha                                     ; yes, save the value
        ldy             #'+'
        ld              r4,#XP25
        jsr             TSTC            ; add?
        jsr             EXPR3           ; get the second 
XP24:
        plx
        add             r1,r1,r2        ; add it to the first 
;       BVS.L   QHOW            brnch if there's an overflow
        bra             XP23            ; else go back for more operations
XP25:
        ldy             #'-'
        ld              r4,#XP45
        jsr             TSTC            ; subtract?
XP26:
        jsr             EXPR3           ; get second 
        sub             r1,r0,r1        ; change its sign
        bra             XP24            ; and do an addition
XP45:
        pla
        rts
 
 
; ::=( <* or /> )(...
 
EXPR3:
        jsr     EXPR4           ; get first 
XP31:
        pha                             ; yes, save that first result
        ldy             #'*'
        ld              r4,#XP34
        jsr             TSTC            ; multiply?
        jsr             EXPR4           ; get second 
        plx
        muls    r1,r1,r2        ; multiply the two
        bra             XP31        ; then look for more terms
XP34:
        ldy             #'/'
        ld              r4,#XP47
        jsr             TSTC            ; divide?
        jsr             EXPR4           ; get second 
        tax
        pla
        divs    r1,r1,r2        ; do the division
        bra             XP31            ; go back for any more terms
XP47:
        pla
        rts
 
 
; Functions are jsred through EXPR4
; ::=
;           
;           ()
 
EXPR4:
    ld          r9,#TAB4                ; find possible function
    ld          r10,#TAB4_1
        jmp             EXEC        ; branch to function which does subsequent rts for EXPR4
XP40:                   ; we get here if it wasn't a function
        lda             #0
        jsr             TSTV
        cmp             #0
        beq     XP41            ; nor a variable
        lda             (r1)            ; if a variable, return its value in r1
        rts
XP41:
        jsr             TSTNUM          ; or is it a number?
        cmp             r2,#0
        bne             XP46            ; (if not, # of digits will be zero) if so, return it in r1
        jsr             PARN        ; check for (EXPR)
XP46:
        rts
 
 
; Check for a parenthesized expression
PARN:
        ldy             #'('
        ld              r4,#XP43
        jsr             TSTC            ; else look for ( OREXPR )
        jsr             OREXPR
        ldy             #')'
        ld              r4,#XP43
        jsr             TSTC
XP42:
        rts
XP43:
        pla                             ; get rid of return address
        lda             #msgWhat
        jmp             ERROR
 
 
; ===== Test for a valid variable name.  Returns Z=1 if not
;       found, else returns Z=0 and the address of the
;       variable in r1.
; Parameters
;       r1 = 1 = allocate if not found
; Returns
;       r1 = address of variable, zero if not found
 
TSTV:
        push    r5
        ld              r5,r1           ; r5=allocate flag
        jsr             IGNBLK
        lda             (r8)            ; look at the program text
        cmp             #'@'
        bcc             tstv_notfound   ; C=1: not a variable
        bne             TV1                             ; brnch if not "@" array
        inc             r8                      ; If it is, it should be
        jsr             PARN            ; followed by (EXPR) as its index.
;       BCS.L   QHOW            say "How?" if index is too big
    pha                             ; save the index
        jsr             SIZEX           ; get amount of free memory
        plx                                 ; get back the index
        cmp             r2,r1
        bcc             TV2                     ; see if there's enough memory
        jmp     QSORRY          ; if not, say "Sorry"
TV2:
        lda             VARBGN          ; put address of array element...
        sub     r1,r1,r2       ; into r1 (neg. offset is used)
        bra     TSTVRT
TV1:
    jsr         getVarName      ; get variable name
    cmp         #0
    beq     TSTVRT    ; if not, return r1=0
    ld          r2,r5
    jsr         findVar     ; find or allocate
TSTVRT:
        pop             r5
        rts                                     ; r1<>0 (found)
tstv_notfound:
        pop             r5
        lda             #0                      ; r1=0 if not found
    rts
 
 
; Returns
;   r1 = 2 character variable name + type
;
getVarName:
    push        r5
 
    lda     (r8)                ; get first character
    pha                                 ; save off current name
    jsr         isAlpha
    cmp         #0
    beq     gvn1
    ld      r5,#2       ; loop two more times
 
        ; check for second/third character
gvn4:
        inc             r8
        lda     (r8)            ; do we have another char ?
        jsr             isAlnum
        cmp             #0
        beq     gvn2            ; nope
        pla                                     ; get varname
        asl
        asl
        asl
        asl
        asl
        asl
        asl
        asl
        ldx     (r8)
        or      r1,r1,r2   ; add in new char
    pha                            ; save off name again
    dec         r5
    bne         gvn4
 
    ; now ignore extra variable name characters
gvn6:
    inc         r8
    lda    (r8)
    jsr    isAlnum
    cmp         #0
    bne     gvn6        ; keep looping as long as we have identifier chars
 
    ; check for a variable type
gvn2:
        lda             (r8)
        cmp             #'%'
        beq             gvn3
        cmp             #'$'
        beq             gvn3
        lda             #0
    dec         r8
 
    ; insert variable type indicator and return
gvn3:
    inc         r8
    plx
    asl         r2,r2
    asl         r2,r2
    asl         r2,r2
    asl         r2,r2
    asl         r2,r2
    asl         r2,r2
    asl         r2,r2
    asl         r2,r2
    or      r1,r1,r2    ; add in variable type
    pop         r5
    rts                                 ; return Z = 0, r1 = varname
 
    ; not a variable name
gvn1:
        pla
        pop             r5
    lda         #0       ; return Z = 1 if not a varname
    rts
 
 
; Find variable
;   r1 = varname
;       r2 = allocate flag
; Returns
;   r1 = variable address, Z =0 if found / allocated, Z=1 if not found
 
findVar:
        push    r7
    ldy     VARBGN
fv4:
    ld      r7,(y)      ; get varname / type
    beq     fv3                 ; no more vars ?
    cmp         r1,r7
    beq     fv1                 ; match ?
    iny                                 ; move to next var
    iny
    ld      r7,STKBOT
    cmp         r3,r7
    bcc     fv4                 ; loop back to look at next var
 
    ; variable not found
    ; no more memory
    lda         #msgVarSpace
    jmp     ERROR
;    lw      lr,[sp]
;    lw      r7,4[sp]
;    add     sp,sp,#8
;    lw      r1,#0
;    rts
 
    ; variable not found
    ; allocate new ?
fv3:
        cpx             #0
        beq             fv2
    sta     (r3)     ; save varname / type
    ; found variable
    ; return address
fv1:
    add         r1,r3,#1
        pop             r7
    rts                     ; Z = 0, r1 = address
 
    ; didn't find var and not allocating
fv2:
    pop         r7
        lda             #0              ; Z = 1, r1 = 0
    rts
 
 
; ===== The PEEK function returns the byte stored at the address
;       contained in the following expression.
;
PEEK:
        jsr             PARN            ; get the memory address
        lda             (r1)            ; get the addressed byte
        rts
 
 
; user function jsr
; call the user function with argument in r1
USRX:
        jsr             PARN            ; get expression value
        push    r8                      ; save the text pointer
        ldx             #0
        jsr             (usrJmp,x)      ; get usr vector, jump to the subroutine
        pop             r8                      ; restore the text pointer
        rts
 
 
; ===== The RND function returns a random number from 1 to
;       the value of the following expression in D0.
;
RND:
        jsr             PARN            ; get the upper limit
        cmp             #0
        beq             rnd2            ; it must be positive and non-zero
        bcc             rnd1
        tax
        ;gran                           ; generate a random number
        ;mfspr  r1,rand         ; get the number
        tsr             LFSR,r1
;       jsr             modu4           ; RND(n)=MOD(number,n)+1
        mod             r1,r1,r2
        ina
        rts
rnd1:
        lda             #msgRNDBad
        jmp             ERROR
rnd2:
        tsr             LFSR,r1
;       gran
;       mfspr   r1,rand
        rts
 
 
; r = a mod b
; a = r1
; b = r2
; r = r6
;modu4:
;       push    r3
;       push    r5
;       push    r6
;       push    r7
;       ld      r7,#31          ; n = 32
;       eor             r5,r5,r5        ; w = 0
;;      eor             r6,r6,r6        ; r = 0
;mod2:
;       rol                                     ; a <<= 1
;       and             r3,r1,#1
;       asl             r6                      ; r <<= 1
;       or              r6,r6,r3
;       and             #-2
;       cmp             r2,r6
;       bmi             mod1            ; b < r ?
;       sub             r6,r6,r2        ; r -= b
;mod1:
;       dec             r7                      ; n--
;       bne             mod2
;       ld              r1,r6
;       pop             r7
;       pop             r6
;       pop             r5
;       pop             r3
;       rts
;
 
; ===== The ABS function returns an absolute value in r2.
;
ABS:
        jsr             PARN            ; get the following expr.'s value
        cmp             #0
        bmi             ABS1
        rts
ABS1:
        sub             r1,r0,r1
        rts
 
;==== The TICK function returns the cpu tick value in r1.
;
TICKX:
        tsr             TICK,r1
        rts
 
; ===== The SGN function returns the sign in r1. +1,0, or -1
;
SGN:
        jsr             PARN            ; get the following expr.'s value
        cmp             #0
        beq             SGN1
        bmi             SGN2
        lda             #1
        rts
SGN2:
        lda             #-1
        rts
SGN1:
        rts
 
; ===== The SIZE function returns the size of free memory in r1.
;
SIZEX:
        lda             VARBGN          ; get the number of free bytes...
        ldx             TXTUNF          ; between 'TXTUNF' and 'VARBGN'
        sub             r1,r1,r2
        rts                                     ; return the number in r1
 
 
;******************************************************************
;
; *** SETVAL *** FIN *** ENDCHK *** ERROR (& friends) ***
;
; 'SETVAL' expects a variable, followed by an equal sign and then
; an expression.  It evaluates the expression and sets the variable
; to that value.
;
; returns
; r2 = variable's address
;
SETVAL:
    lda         #1              ; allocate var
    jsr         TSTV            ; variable name?
    cmp         #0
    bne         sv2
        lda             #msgVar
        jmp             ERROR
sv2:
        pha                         ; save the variable's address
        ldy             #'='
        ld              r4,#SV1
        jsr             TSTC            ; get past the "=" sign
        jsr             OREXPR          ; evaluate the expression
        plx                                 ; get back the variable's address
        sta     (x)                 ; and save value in the variable
        txa                                     ; return r1 = variable address
        rts
SV1:
    jmp     QWHAT               ; if no "=" sign
 
 
; 'FIN' checks the end of a command.  If it ended with ":",
; execution continues.  If it ended with a CR, it finds the
; the next line and continues from there.
;
FIN:
        ldy             #':'
        ld              r4,#FI1
        jsr             TSTC            ; *** FIN ***
        pla                                     ; if ":", discard return address
        jmp             RUNSML          ; continue on the same line
FI1:
        ldy             #CR
        ld              r4,#FI2
        jsr             TSTC            ; not ":", is it a CR?
                                                ; else return to the caller
        pla                                     ; yes, purge return address
        jmp             RUNNXL          ; execute the next line
FI2:
        rts                                     ; else return to the caller
 
 
; 'ENDCHK' checks if a command is ended with a CR. This is
; required in certain commands, such as GOTO, RETURN, STOP, etc.
;
; Check that there is nothing else on the line
; Registers Affected
;   r1
;
ENDCHK:
        lda             #'E'
        jsr             DisplayChar
        jsr             IGNBLK
        lda             (r8)
        cmp             #CR
        beq             ec1     ; does it end with a CR?
        lda             #msgExtraChars
        jmp             ERROR
ec1:
        rts
 
; 'ERROR' prints the string pointed to by r1. It then prints the
; line pointed to by CURRNT with a "?" inserted at where the
; old text pointer (should be on top of the stack) points to.
; Execution of Tiny BASIC is stopped and a warm start is done.
; If CURRNT is zero (indicating a direct command), the direct
; command is not printed. If CURRNT is -1 (indicating
; 'INPUT' command in progress), the input line is not printed
; and execution is not terminated but continues at 'INPERR'.
;
; Related to 'ERROR' are the following:
; 'QWHAT' saves text pointer on stack and gets "What?" message.
; 'AWHAT' just gets the "What?" message and jumps to 'ERROR'.
; 'QSORRY' and 'ASORRY' do the same kind of thing.
; 'QHOW' and 'AHOW' also do this for "How?".
;
TOOBIG:
        lda             #msgTooBig
        bra             ERROR
QSORRY:
    lda         #SRYMSG
        bra         ERROR
QWHAT:
        lda             #msgWhat
ERROR:
        jsr             PRMESG          ; display the error message
        lda             CURRNT          ; get the current line pointer
        beq             ERROR1          ; if zero, do a warm start
        cmp             #-1
        beq             INPERR          ; is the line no. pointer = -1? if so, redo input
        ld              r5,(r8)         ; save the char. pointed to
        stz             (r8)            ; put a zero where the error is
        lda             CURRNT          ; point to start of current line
        jsr             PRTLN           ; display the line in error up to the 0
        ld      r6,r1       ; save off end pointer
        st              r5,(r8)         ; restore the character
        lda             #'?'            ; display a "?"
        jsr             GOOUT
        ldx             #0                      ; stop char = 0
        sub             r1,r6,#1        ; point back to the error char.
        jsr             PRTSTG          ; display the rest of the line
ERROR1:
        jmp         WSTART              ; and do a warm start
 
;******************************************************************
;
; *** GETLN *** FNDLN (& friends) ***
;
; 'GETLN' reads in input line into 'BUFFER'. It first prompts with
; the character in r3 (given by the caller), then it fills the
; buffer and echos. It ignores LF's but still echos
; them back. Control-H is used to delete the last character
; entered (if there is one), and control-X is used to delete the
; whole line and start over again. CR signals the end of a line,
; and causes 'GETLN' to return.
;
;
GETLN:
        push    r5
        jsr             GOOUT           ; display the prompt
        lda             #1
        sta             CursorFlash     ; turn on cursor flash
        lda             #' '            ; and a space
        jsr             GOOUT
        ld              r8,#BUFFER      ; r8 is the buffer pointer
GL1:
        jsr             CHKIO           ; check keyboard
        cmp             #0
        beq             GL1                     ; wait for a char. to come in
        cmp             #CTRLH
        beq             GL3                     ; delete last character? if so
        cmp             #CTRLX
        beq             GL4                     ; delete the whole line?
        cmp             #CR
        beq             GL2                     ; accept a CR
        cmp             #' '
        bcc             GL1                     ; if other control char., discard it
GL2:
        sta             (r8)            ; save the char.
        inc             r8
        pha
        jsr             GOOUT           ; echo the char back out
        pla                                     ; get char back (GOOUT destroys r1)
        cmp             #CR
        beq             GL7                     ; if it's a CR, end the line
        cmp             r8,#BUFFER+BUFLEN-1     ; any more room?
        bcc             GL1                     ; yes: get some more, else delete last char.
GL3:
        lda             #CTRLH  ; delete a char. if possible
        jsr             GOOUT
        lda             #' '
        jsr             GOOUT
        cmp             r8,#BUFFER      ; any char.'s left?
        bcc             GL1                     ; if not
        beq             GL1
        lda             #CTRLH          ; if so, finish the BS-space-BS sequence
        jsr             GOOUT
        dec             r8                      ; decrement the text pointer
        bra             GL1                     ; back for more
GL4:
        ld              r1,r8           ; delete the whole line
        sub             r5,r1,#BUFFER   ; figure out how many backspaces we need
        beq             GL6                             ; if none needed, brnch
        dec             r5                      ; loop count is one less
GL5:
        lda             #CTRLH          ; and display BS-space-BS sequences
        jsr             GOOUT
        lda             #' '
        jsr             GOOUT
        lda             #CTRLH
        jsr             GOOUT
        dec             r5
        bne             GL5
GL6:
        ld              r8,#BUFFER      ; reinitialize the text pointer
        bra             GL1                     ; and go back for more
GL7:
        lda             #0              ; turn off cursor flash
        stz             (r8)            ; null terminate line
        stz             CursorFlash
        lda             #LF             ; echo a LF for the CR
        jsr             GOOUT
        pop             r5
        rts
 
 
; 'FNDLN' finds a line with a given line no. (in r1) in the
; text save area.  r9 is used as the text pointer. If the line
; is found, r9 will point to the beginning of that line
; (i.e. the high byte of the line no.), and r1 = 1.
; If that line is not there and a line with a higher line no.
; is found, r9 points there and r1 = 0. If we reached
; the end of the text save area and cannot find the line, flags
; r9 = 0, r1 = 0.
; r1=1 if line found
; r0 = 1        <= line is found
;       r9 = pointer to line
; r0 = 0    <= line is not found
;       r9 = zero, if end of text area
;       r9 = otherwise higher line number
;
; 'FNDLN' will initialize r9 to the beginning of the text save
; area to start the search. Some other entries of this routine
; will not initialize r9 and do the search.
; 'FNDLNP' will start with r9 and search for the line no.
; 'FNDNXT' will bump r9 by 2, find a CR and then start search.
; 'FNDSKP' uses r9 to find a CR, and then starts the search.
; return Z=1 if line is found, r9 = pointer to line
;
; Parameters
;       r1 = line number to find
;
FNDLN:
        cmp             #$FFFF
        bcc             fl1     ; line no. must be < 65535
        lda             #msgLineRange
        jmp             ERROR
fl1:
        ld              r9,TXTBGN>>2    ; init. the text save pointer
 
FNDLNP:
        cmp             r9,TXTUNF       ; check if we passed the end
        beq             FNDRET1
        bcs             FNDRET1         ; if so, return with r9=0,r1=0
        ldx             (r9)            ; get line number
        cmp             r1,r2
        beq             FNDRET2
        bcs             FNDNXT  ; is this the line we want? no, not there yet
FNDRET:
        lda             #0      ; line not found, but r9=next line pointer
        rts                     ; return the cond. codes
FNDRET1:
;       eor             r9,r9,r9        ; no higher line
        lda             #0      ; line not found
        rts
FNDRET2:
        lda             #1              ; line found
        rts
 
FNDNXT:
        inc             r9      ; find the next line
 
FNDSKP:
        ldx             (r9)
        inc             r9
        cpx             #CR
        bne             FNDSKP          ; try to find a CR, keep looking
        bra             FNDLNP          ; check if end of text
 
 
;******************************************************************
; 'MVUP' moves a block up from where r1 points to where r2 points
; until r1=r3
;
MVUP1:
        ld              r4,(r1)
        st              r4,(r2)
        ina
        inx
MVUP:
        cmp             r1,r3
        bne             MVUP1
MVRET:
        rts
 
 
; 'MVDOWN' moves a block down from where r1 points to where r2
; points until r1=r3
;
MVDOWN1:
        dea
        dex
        ld              r4,(r1)
        st              r4,(r2)
MVDOWN:
        cmp             r1,r3
        bne             MVDOWN1
        rts
 
 
; 'POPA' restores the 'FOR' loop variable save area from the stack
;
; 'PUSHA' stacks for 'FOR' loop variable save area onto the stack
;
; Note: a single zero word is stored on the stack in the
; case that no FOR loops need to be saved. This needs to be
; done because PUSHA / POPA is called all the time.
message "POPA"
POPA:
        ply
        pla
        sta             LOPVAR  ; restore LOPVAR, but zero means no more
        beq             PP1
        pla
        sta             LOPINC
        pla
        sta             LOPLMT
        pla
        sta             LOPLN
        pla
        sta             LOPPT
PP1:
        jmp             (y)
 
 
PUSHA:
        ply
        lda             STKBOT          ; Are we running out of stack room?
        add             r1,r1,#5        ; we might need this many words
        tsx
        cmp             r2,r1
        bcc             QSORRY          ; out of stack space
        ldx             LOPVAR          ; save loop variables
        beq             PU1                     ; if LOPVAR is zero, that's all
        lda             LOPPT
        pha
        lda             LOPLN
        pha
        lda             LOPLMT
        pha
        lda             LOPINC
        pha
PU1:
        phx
        jmp             (y)
 
 
 
;******************************************************************
;
; 'PRTSTG' prints a string pointed to by r1. It stops printing
; and returns to the caller when either a CR is printed or when
; the next byte is the same as what was passed in r2 by the
; caller.
;
; 'PRTLN' prints the saved text line pointed to by r3
; with line no. and all.
;
 
; r1 = pointer to string
; r2 = stop character
; return r1 = pointer to end of line + 1
 
PRTSTG:
        push    r5
        push    r6
        push    r7
    ld      r5,r1           ; r5 = pointer
    ld      r6,r2           ; r6 = stop char
PS1:
    ld      r7,(r5)     ; get a text character
    inc         r5
    cmp         r7,r6
        beq         PRTRET              ; same as stop character? if so, return
        ld      r1,r7
        jsr             GOOUT           ; display the char.
        cmp             r7,#CR
        bne     PS1                     ; is it a C.R.? no, go back for more
        lda             #LF      ; yes, add a L.F.
        jsr             GOOUT
PRTRET:
    ld      r2,r7           ; return r2 = stop char
        ld              r1,r5           ; return r1 = line pointer
        pop             r7
        pop             r6
        pop             r5
    rts                                 ; then return
 
 
; 'QTSTG' looks for an underline (back-arrow on some systems),
; single-quote, or double-quote.  If none of these are found, returns
; to the caller.  If underline, outputs a CR without a LF.  If single
; or double quote, prints the quoted string and demands a matching
; end quote.  After the printing, the next i-word of the caller is
; skipped over (usually a branch instruction).
;
QTSTG:
        ldy             #'"'
        ld              r4,#QT3
        jsr             TSTC            ; *** QTSTG ***
        ldx             #'"'            ; it is a "
QT1:
        ld              r1,r8
        jsr             PRTSTG          ; print until another
        ld              r8,r1
        cpx             #CR
        bne             QT2                     ; was last one a CR?
        jmp             RUNNXL          ; if so run next line
QT3:
        ldy             #''''
        ld              r4,#QT4
        jsr             TSTC            ; is it a single quote?
        ldx             #''''   ; if so, do same as above
        bra             QT1
QT4:
        ldy             #'_'
        ld              r4,#QT5
        jsr             TSTC            ; is it an underline?
        lda             #CR             ; if so, output a CR without LF
        jsr             GOOUT
QT2:
        pla                                     ; get return address
        ina                                     ; add 2 to it in order to skip following branch
        ina
        jmp             (r1)            ; skip over next i-word when returning
QT5:                                            ; not " ' or _
        rts
 
; Output a CR LF sequence
;
prCRLF:
        lda             #CR
        jsr             GOOUT
        lda             #LF
        jsr             GOOUT
        rts
 
; 'PRTNUM' prints the 32 bit number in r1, leading blanks are added if
; needed to pad the number of spaces to the number in r2.
; However, if the number of digits is larger than the no. in
; r2, all digits are printed anyway. Negative sign is also
; printed and counted in, positive sign is not.
;
; r1 = number to print
; r2 = number of digits
; Register Usage
;       r5 = number of padding spaces
public PRTNUM:
        push    r3
        push    r5
        push    r6
        push    r7
        ld              r7,#NUMWKA      ; r7 = pointer to numeric work area
        ld              r6,r1           ; save number for later
        ld              r5,r2           ; r5 = min number of chars
        cmp             #0
        bpl             PN2                     ; is it negative? if not
        sub             r1,r0,r1        ; else make it positive
        dec             r5                      ; one less for width count
PN2:
        ld              r3,#10
PN1:
        mod             r2,r1,r3        ; r2 = r1 mod 10
        div             r1,r1,r3        ; r1 /= 10 divide by 10
        add             r2,r2,#'0'      ; convert remainder to ascii
        stx             (r7)            ; and store in buffer
        inc             r7
        dec             r5                      ; decrement width
        cmp             #0
        bne             PN1
PN6:
        cmp             r5,r0
        bmi             PN4             ; test pad count, skip padding if not needed
        beq             PN4
PN3:
        lda             #' '            ; display the required leading spaces
        jsr             GOOUT
        dec             r5
        bne             PN3
PN4:
        cmp             r6,r0
        bpl             PN5                     ; is number negative?
        lda             #'-'            ; if so, display the sign
        jsr             GOOUT
PN5:
        dec             r7
        lda             (r7)            ; now unstack the digits and display
        jsr             GOOUT
        cmp             r7,#NUMWKA
        beq             PNRET
        bcs             PN5
PNRET:
        pop             r7
        pop             r6
        pop             r5
        pop             r3
        rts
 
; r1 = number to print
; r2 = number of digits
public PRTHEXNUM:
        push    r4
        push    r5
        push    r6
        push    r7
        push    r8
        ld              r7,#NUMWKA      ; r7 = pointer to numeric work area
        ld              r6,r1           ; save number for later
;       setlo   r5,#20          ; r5 = min number of chars
        ld              r5,r2
        ld              r4,r1
        cmp             r4,r0
        bpl             PHN2            ; is it negative? if not
        sub             r4,r0,r4        ; else make it positive
        dec             r5                      ; one less for width count
PHN2
        ld              r8,#10          ; maximum of 10 digits
PHN1:
        ld              r1,r4
        and             #15
        cmp             #10
        bcc             PHN7
        add             #'A'-10
        bra             PHN8
PHN7:
        add             #'0'            ; convert remainder to ascii
PHN8:
        sta             (r7)            ; and store in buffer
        inc             r7
        dec             r5                      ; decrement width
        lsr             r4,r4
        lsr             r4,r4
        lsr             r4,r4
        lsr             r4,r4
        beq             PHN6                    ; is it zero yet ?
        dec             r8
        bne             PHN1
PHN6:   ; test pad count
        cmp             r5,r0
        beq             PHN4
        bcc             PHN4    ; skip padding if not needed
PHN3:
        lda             #' '            ; display the required leading spaces
        jsr             GOOUT
        dec             r5
        bne             PHN3
PHN4:
        cmp             r6,r0
        bcs             PHN5    ; is number negative?
        lda             #'-'            ; if so, display the sign
        jsr             GOOUT
PHN5:
        dec             r7
        lda             (r7)            ; now unstack the digits and display
        jsr             GOOUT
        cmp             r7,#NUMWKA
        beq             PHNRET
        bcs             PHN5
PHNRET:
        pop             r8
        pop             r7
        pop             r6
        pop             r5
        pop             r4
        rts
 
 
; r1 = pointer to line
; returns r1 = pointer to end of line + 1
PRTLN:
        push    r5
    ld          r5,r1           ; r5 = pointer
    lda         (r5)            ; get the binary line number
    inc         r5
    ldx         #12       ; display a 0 or more digit line no.
        jsr             PRTNUM
        lda             #' '     ; followed by a blank
        jsr             GOOUT
        ldx             #0       ; stop char. is a zero
        ld              r1,r5
        jsr     PRTSTG          ; display the rest of the line
        pop             r5
        rts
 
 
; ===== Test text byte following the call to this subroutine. If it
;       equals the byte pointed to by r8, return to the code following
;       the call. If they are not equal, brnch to the point
;       indicated in r4.
;
; Registers Affected
;   r3,r8
; Returns
;       r8 = updated text pointer
;
TSTC
        pha
        jsr             IGNBLK          ; ignore leading blanks
        lda             (r8)
        cmp             r3,r1
        beq             TC1                     ; is it = to what r8 points to? if so
        pla
        ply                                     ; increment stack pointer (get rid of return address)
        jmp             (r4)            ; jump to the routine
TC1:
        inc             r8                      ; if equal, bump text pointer
        pla
        rts
 
; ===== See if the text pointed to by r8 is a number. If so,
;       return the number in r2 and the number of digits in r3,
;       else return zero in r2 and r3.
; Registers Affected
;   r1,r2,r3,r4
; Returns
;       r1 = number
;       r2 = number of digits in number
;       r8 = updated text pointer
;
TSTNUM:
        phy
        jsr             IGNBLK          ; skip over blanks
        lda             #0              ; initialize return parameters
        ldx             #0
        ld              r15,#10
TN1:
        ldy             (r8)
        cpy             #'0'            ; is it less than zero?
        bcc             TSNMRET
        cpy             #'9'+1          ; is it greater than nine?
        bcs             TSNMRET
        cmp             r1,#$7FFFFFF    ; see if there's room for new digit
        bcc             TN2
        beq             TN2
        lda             #msgNumTooBig
        jmp             ERROR           ; if not, we've overflowd
TN2:
        inc             r8                      ; adjust text pointer
        mul             r1,r1,r15       ; quickly multiply result by 10
        and             r3,r3,#$0F      ; add in the new digit
        add             r1,r1,r3
        inx                                     ; increment the no. of digits
        bra             TN1
TSNMRET:
        ply
        rts
 
 
;===== Skip over blanks in the text pointed to by r8.
;
; Registers Affected:
;       r8
; Returns
;       r8 = pointer updateded past any spaces or tabs
;
IGNBLK:
        pha
IGB2:
        lda             (r8)                    ; get char
        cmp             #' '
        beq             IGB1    ; see if it's a space
        cmp             #'\t'
        bne             IGBRET  ; or a tab
IGB1:
        inc             r8              ; increment the text pointer
        bra             IGB2
IGBRET:
        pla
        rts
 
; ===== Convert the line of text in the input buffer to upper
;       case (except for stuff between quotes).
;
; Registers Affected
;   r1,r3
; Returns
;       r8 = pointing to end of text in buffer
;
TOUPBUF:
        ld              r8,#BUFFER      ; set up text pointer
        eor             r3,r3,r3        ; clear quote flag
TOUPB1:
        lda             (r8)            ; get the next text char.
        inc             r8
        cmp             #CR
        beq             TOUPBRT         ; is it end of line?
        cmp             #'"'
        beq             DOQUO   ; a double quote?
        cmp             #''''
        beq             DOQUO   ; or a single quote?
        cpy             #0
        bne             TOUPB1  ; inside quotes?
        jsr             toUpper         ; convert to upper case
        sta             -1,r8   ; store it
        bra             TOUPB1          ; and go back for more
DOQUO:
        cpy             #0
        bne             DOQUO1; are we inside quotes?
        tay                             ; if not, toggle inside-quotes flag
        bra             TOUPB1
DOQUO1:
        cmp             r3,r1
        bne             TOUPB1          ; make sure we're ending proper quote
        eor             r3,r3,r3        ; else clear quote flag
        bra             TOUPB1
TOUPBRT:
        rts
 
 
; ===== Convert the character in r1 to upper case
;
toUpper
        cmp             #'a'            ; is it < 'a'?
        bcc             TOUPRET
        cmp             #'z'+1          ; or > 'z'?
        bcs             TOUPRET
        sub             #32     ; if not, make it upper case
TOUPRET
        rts
 
 
; 'CHKIO' checks the input. If there's no input, it will return
; to the caller with the r1=0. If there is input, the input byte is in r1.
; However, if a control-C is read, 'CHKIO' will warm-start BASIC and will
; not return to the caller.
;
message "CHKIO"
CHKIO:
        jsr             GOIN            ; get input if possible
        cmp             #0
        beq             CHKRET2         ; if Zero, no input
        cmp             #CTRLC
        bne             CHKRET  ; is it control-C?
        pla                                     ; dump return address
        jmp             WSTART          ; if so, do a warm start
CHKRET2:
        lda             #0
CHKRET:
        rts
 
; ===== Display a CR-LF sequence
;
CRLF:
        lda             #CLMSG
 
 
; ===== Display a zero-ended string pointed to by register r1
; Registers Affected
;   r1,r2,r4
;
PRMESG:
        push    r5
        or      r5,r1,r0    ; r5 = pointer to message
PRMESG1:
        inc             r5
        lb              r1,-1,r5                ;       get the char.
        beq             PRMRET
        jsr             GOOUT           ;else display it trashes r4
        bra             PRMESG1
PRMRET:
        or              r1,r5,r0
        pop             r5
        rts
 
 
; ===== Display a zero-ended string pointed to by register r1
; Registers Affected
;   r1,r2,r3
;
PRMESGAUX:
        phy
        tay                                     ; y = pointer
PRMESGA1:
        iny
        lb              r1,-1,y         ;       get the char.
        beq             PRMRETA
        jsr             GOAUXO          ;else display it
        bra             PRMESGA1
PRMRETA:
        tya
        ply
        rts
 
;*****************************************************
; The following routines are the only ones that need *
; to be changed for a different I/O environment.     *
;*****************************************************
 
 
; ===== Output character to the console (Port 1) from register r1
;       (Preserves all registers.)
;
OUTC:
        jmp             DisplayChar
 
 
; ===== Input a character from the console into register R1 (or
;       return Zero status if there's no character available).
;
INCH:
;       jsr             KeybdCheckForKeyDirect
;       cmp             #0
;       beq             INCH1
        jsr             KeybdGetChar
        cmp             #-1
        beq             INCH1
        rts
INCH1:
        ina             ; return a zero for no-char
        rts
 
;*
;* ===== Input a character from the host into register r1 (or
;*      return Zero status if there's no character available).
;*
AUXIN:
        jsr             SerialGetChar
        cmp             #-1
        beq             AXIRET_ZERO
        and             #$7F                            ;zero out the high bit
AXIRET:
        rts
AXIRET_ZERO:
        lda             #0
        rts
 
; ===== Output character to the host (Port 2) from register r1
;       (Preserves all registers.)
;
AUXOUT
        jmp             SerialPutChar   ; call boot rom routine
 
 
_cls
        jsr             ClearScreen
        jsr             HomeCursor
        jmp             FINISH
 
_wait10
        rts
_getATAStatus
        rts
_waitCFNotBusy
        rts
_rdcf
        jmp             FINISH
rdcf6
        bra             ERROR
 
 
; ===== Return to the resident monitor, operating system, etc.
;
BYEBYE:
        jsr             ReleaseIOFocus
        ldx             OSSP
        txs
        rts
 
;       MOVE.B  #228,D7         return to Tutor
;       TRAP    #14
 
msgInit db      CR,LF,"RTF65002 Tiny BASIC v1.0",CR,LF,"(C) 2013  Robert Finch",CR,LF,LF,0
OKMSG   db      CR,LF,"OK",CR,LF,0
msgWhat db      "What?",CR,LF,0
SRYMSG  db      "Sorry."
CLMSG   db      CR,LF,0
msgReadError    db      "Compact FLASH read error",CR,LF,0
msgNumTooBig    db      "Number is too big",CR,LF,0
msgDivZero              db      "Division by zero",CR,LF,0
msgVarSpace     db  "Out of variable space",CR,LF,0
msgBytesFree    db      " words free",CR,LF,0
msgReady                db      CR,LF,"Ready",CR,LF,0
msgComma                db      "Expecting a comma",CR,LF,0
msgLineRange    db      "Line number too big",CR,LF,0
msgVar                  db      "Expecting a variable",CR,LF,0
msgRNDBad               db      "RND bad parameter",CR,LF,0
msgSYSBad               db      "SYS bad address",CR,LF,0
msgInputVar             db      "INPUT expecting a variable",CR,LF,0
msgNextFor              db      "NEXT without FOR",CR,LF,0
msgNextVar              db      "NEXT expecting a defined variable",CR,LF,0
msgBadGotoGosub db      "GOTO/GOSUB bad line number",CR,LF,0
msgRetWoGosub   db      "RETURN without GOSUB",CR,LF,0
msgTooBig               db      "Program is too big",CR,LF,0
msgExtraChars   db      "Extra characters on line ignored",CR,LF,0
 
        align   4
LSTROM  equ     *               ; end of possible ROM area
;       END
 
;*
;* ===== Return to the resident monitor, operating system, etc.
;*
;BYEBYE:
;       jmp             Monitor
;    MOVE.B     #228,D7         ;return to Tutor
;       TRAP    #14
 

Line No.	Rev	Author	Line
1	16	robfinch	`;****************************************************************;`
2			`; ;`
3			`; Tiny BASIC for the Raptor64 ;`
4			`; ;`
5			`; Derived from a 68000 derivative of Palo Alto Tiny BASIC as ;`
6			`; published in the May 1976 issue of Dr. Dobb's Journal. ;`
7			`; Adapted to the 68000 by: ;`
8			`; Gordon brndly ;`
9			`; 12147 - 51 Street ;`
10			`; Edmonton AB T5W 3G8 ;`
11			`; Canada ;`
12			`; (updated mailing address for 1996) ;`
13			`; ;`
14			`; Adapted to the RTF65002 by: ;`
15			`; Robert Finch ;`
16			`; Ontario, Canada ;`
17			`; robfinch@opencores.org ;`
18			`;****************************************************************;`
19			`; Copyright (C) 2012 by Robert Finch. This program may be ;`
20			`; freely distributed for personal use only. All commercial ;`
21			`; rights are reserved. ;`
22			`;****************************************************************;`
23			`;`
24			`; Register Usage`
25			`; r8 = text pointer (global usage)`
26			`; r3,r4 = inputs parameters to subroutines`
27			`; r2 = return value`
28			`;`
29			`;* Vers. 1.0 1984/7/17 - Original version by Gordon brndly`
30			`;* 1.1 1984/12/9 - Addition of '0x' print term by Marvin Lipford`
31			`;* 1.2 1985/4/9 - Bug fix in multiply routine by Rick Murray`
32
33			`;`
34			`; Standard jump table. You can change these addresses if you are`
35			`; customizing this interpreter for a different environment.`
36			`;`
37			`CR EQU 0x0D ;ASCII equates`
38			`LF EQU 0x0A`
39			`TAB EQU 0x09`
40			`CTRLC EQU 0x03`
41			`CTRLH EQU 0x08`
42			`CTRLI EQU 0x09`
43			`CTRLJ EQU 0x0A`
44			`CTRLK EQU 0x0B`
45			`CTRLM EQU 0x0D`
46			`CTRLS EQU 0x13`
47			`CTRLX EQU 0x18`
48			`XON EQU 0x11`
49			`XOFF EQU 0x13`
50
51	18	robfinch	`CursorRow EQU 0x7C2`
52			`CursorCol EQU 0x7C3`
53			`CursorFlash EQU 0x7C4`
54			`IRQFlag EQU 0x7C6`
55	16	robfinch
56	18	robfinch	`OSSP EQU 0x700`
57			`TXTUNF EQU 0x701`
58			`VARBGN EQU 0x702`
59			`LOPVAR EQU 0x703`
60			`STKGOS EQU 0x704`
61			`CURRNT EQU 0x705`
62	26	robfinch	`BUFFER EQU 0x706`
63	16	robfinch	`BUFLEN EQU 84`
64	18	robfinch	`LOPPT EQU 0x760`
65			`LOPLN EQU 0x761`
66			`LOPINC EQU 0x762`
67			`LOPLMT EQU 0x763`
68			`NUMWKA EQU 0x764`
69			`STKINP EQU 0x774`
70			`STKBOT EQU 0x775`
71			`usrJmp EQU 0x776`
72			`IRQROUT EQU 0x777`
73	16	robfinch
74
75
76			`cpu rtf65002`
77			`code`
78			`org $FFFFE800`
79			`GOSTART:`
80			`jmp CSTART ; Cold Start entry point`
81			`GOWARM:`
82			`jmp WSTART ; Warm Start entry point`
83			`GOOUT:`
84			`jmp OUTC ; Jump to character-out routine`
85			`GOIN:`
86			`jmp INCH ;Jump to character-in routine`
87			`GOAUXO:`
88			`jmp AUXOUT ; Jump to auxiliary-out routine`
89			`GOAUXI:`
90			`jmp AUXIN ; Jump to auxiliary-in routine`
91			`GOBYE:`
92			`jmp BYEBYE ; Jump to monitor, DOS, etc.`
93			`;`
94			`; Modifiable system constants:`
95			`;`
96			`align 4`
97	26	robfinch	`;THRD_AREA dw 0x04000000 ; threading switch area 0x04000000-0x40FFFFF`
98			`;bitmap dw 0x04100000 ; bitmap graphics memory 0x04100000-0x417FFFF`
99			`TXTBGN dw 0x04180000 ;TXT ;beginning of program memory`
100			`ENDMEM dw 0x057FFFFF ; end of available memory`
101			`STACKOFFS dw 0x058FFFFF ; stack offset - leave a little room for the BIOS stacks`
102	16	robfinch	`;`
103			`; The main interpreter starts here:`
104			`;`
105			`; Usage`
106			`; r1 = temp`
107			`; r8 = text buffer pointer`
108			`; r12 = end of text in text buffer`
109			`;`
110			`align 4`
111	26	robfinch	`message "CSTART"`
112	16	robfinch	`public CSTART:`
113			`; First save off the link register and OS sp value`
114			`tsx`
115			`stx OSSP`
116			`ldx STACKOFFS>>2 ; initialize stack pointer`
117			`txs`
118	26	robfinch	`jsr RequestIOFocus`
119			`jsr HomeCursor`
120			`lda #0 ; turn off keyboard echoing`
121			`jsr SetKeyboardEcho`
122	16	robfinch	`stz CursorFlash`
123			`ldx #0x10000020 ; black chars, yellow background`
124			`; stx charToPrint`
125			`jsr ClearScreen`
126			`lda #msgInit ; tell who we are`
127			`jsr PRMESG`
128			`lda TXTBGN>>2 ; init. end-of-program pointer`
129			`sta TXTUNF`
130			`lda ENDMEM>>2 ; get address of end of memory`
131			`sub #4096 ; reserve 4K for the stack`
132			`sta STKBOT`
133			`sub #16384 ; 1000 vars`
134			`sta VARBGN`
135			`jsr clearVars ; clear the variable area`
136	18	robfinch	`stz IRQROUT`
137	16	robfinch	`lda VARBGN ; calculate number of bytes free`
138			`ldy TXTUNF`
139			`sub r1,r1,r3`
140			`ldx #12 ; max 12 digits`
141			`jsr PRTNUM`
142			`lda #msgBytesFree`
143			`jsr PRMESG`
144			`WSTART:`
145			`stz LOPVAR ; initialize internal variables`
146			`stz STKGOS`
147			`stz CURRNT ; current line number pointer = 0`
148			`ldx ENDMEM>>2 ; init S.P. again, just in case`
149			`txs`
150			`lda #msgReady ; display "Ready"`
151			`jsr PRMESG`
152			`ST3:`
153			`lda #'>' ; Prompt with a '>' and`
154			`jsr GETLN ; read a line.`
155			`jsr TOUPBUF ; convert to upper case`
156			`ld r12,r8 ; save pointer to end of line`
157			`ld r8,#BUFFER ; point to the beginning of line`
158			`jsr TSTNUM ; is there a number there?`
159			`jsr IGNBLK ; skip trailing blanks`
160			`; does line no. exist? (or nonzero?)`
161			`cpx #0`
162			`beq DIRECT ; if not, it's a direct statement`
163			`cmp #$FFFF ; see if line no. is <= 16 bits`
164			`bcc ST2`
165			`beq ST2`
166			`lda #msgLineRange ; if not, we've overflowed`
167			`jmp ERROR`
168			`ST2:`
169			`; ugliness - store a character at potentially an`
170			`; odd address (unaligned).`
171			`tax ; r2 = line number`
172			`dec r8`
173			`stx (r8) ;`
174			`jsr FNDLN ; find this line in save area`
175			`ld r13,r9 ; save possible line pointer`
176			`cmp #0`
177			`beq ST4 ; if not found, insert`
178			`; here we found the line, so we're replacing the line`
179			`; in the text area`
180			`; first step - delete the line`
181			`lda #0`
182			`jsr FNDNXT ; find the next line (into r9)`
183			`cmp #0`
184			`bne ST7`
185			`cmp r9,TXTUNF`
186			`beq ST6 ; no more lines`
187			`bcs ST6`
188			`cmp r9,r0`
189			`beq ST6`
190			`ST7:`
191			`ld r1,r9 ; r1 = pointer to next line`
192			`ld r2,r13 ; pointer to line to be deleted`
193			`ldy TXTUNF ; points to top of save area`
194	26	robfinch	`sub r1,r3,r9 ; r1 = length to move TXTUNF-pointer to next line`
195			`; dea ; count is one less`
196			`ld r2,r9 ; r2 = pointer to next line`
197			`ld r3,r13 ; r3 = pointer to line to delete`
198			`push r4`
199			`ST8:`
200			`ld r4,(x)`
201			`st r4,(y)`
202			`inx`
203			`iny`
204			`dea`
205			`bne ST8`
206			`pop r4`
207			`; mvn`
208			`; jsr MVUP ; move up to delete`
209			`sty TXTUNF ; update the end pointer`
210	16	robfinch	`; we moved the lines of text after the line being`
211			`; deleted down, so the pointer to the next line`
212			`; needs to be reset`
213			`ld r9,r13`
214			`bra ST4`
215			`; here there were no more lines, so just move the`
216			`; end of text pointer down`
217			`ST6:`
218			`st r13,TXTUNF`
219			`ld r9,r13`
220			`ST4:`
221			`; here we're inserting because the line wasn't found`
222			`; or it was deleted from the text area`
223	18	robfinch	`sub r1,r12,r8 ; calculate the length of new line`
224	16	robfinch	`cmp #2 ; is it just a line no. & CR? if so, it was just a delete`
225			`beq ST3`
226			`bcc ST3`
227
228	18	robfinch	`; compute new end of text`
229			`ld r10,TXTUNF ; r10 = old TXTUNF`
230			`add r11,r10,r1 ; r11 = new top of TXTUNF (r1=line length)`
231	16	robfinch
232			`cmp r11,VARBGN ; see if there's enough room`
233			`bcc ST5`
234			`lda #msgTooBig ; if not, say so`
235			`jmp ERROR`
236
237			`; open a space in the text area`
238			`ST5:`
239			`st r11,TXTUNF ; if so, store new end position`
240			`ld r1,r10 ; points to old end of text`
241			`ld r2,r11 ; points to new end of text`
242			`ld r3,r9 ; points to start of line after insert line`
243			`jsr MVDOWN ; move things out of the way`
244
245			`; copy line into text space`
246			`ld r1,r8 ; set up to do the insertion; move from buffer`
247			`ld r2,r13 ; to vacated space`
248			`ld r3,r12 ; until end of buffer`
249			`jsr MVUP ; do it`
250			`jmp ST3 ; go back and get another line`
251
252			`;******************************************************************`
253			`;`
254			`; * Tables * DIRECT * EXEC *`
255			`;`
256			`; This section of the code tests a string against a table. When`
257			`; a match is found, control is transferred to the section of`
258			`; code according to the table.`
259			`;`
260			`; At 'EXEC', r8 should point to the string, r9 should point to`
261			`; the character table, and r10 should point to the execution`
262			`; table. At 'DIRECT', r8 should point to the string, r9 and`
263			`; r10 will be set up to point to TAB1 and TAB1_1, which are`
264			`; the tables of all direct and statement commands.`
265			`;`
266			`; A '.' in the string will terminate the test and the partial`
267			`; match will be considered as a match, e.g. 'P.', 'PR.','PRI.',`
268			`; 'PRIN.', or 'PRINT' will all match 'PRINT'.`
269			`;`
270			`; There are two tables: the character table and the execution`
271			`; table. The character table consists of any number of text items.`
272			`; Each item is a string of characters with the last character's`
273			`; high bit set to one. The execution table holds a 32-bit`
274			`; execution addresses that correspond to each entry in the`
275			`; character table.`
276			`;`
277			`; The end of the character table is a 0 byte which corresponds`
278			`; to the default routine in the execution table, which is`
279			`; executed if none of the other table items are matched.`
280			`;`
281			`; Character-matching tables:`
282	26	robfinch	`message "TAB1"`
283	16	robfinch	`TAB1:`
284			`db "LIS",'T'+0x80 ; Direct commands`
285			`db "LOA",'D'+0x80`
286			`db "NE",'W'+0x80`
287			`db "RU",'N'+0x80`
288			`db "SAV",'E'+0x80`
289			`TAB2:`
290			`db "NEX",'T'+0x80 ; Direct / statement`
291			`db "LE",'T'+0x80`
292			`db "I",'F'+0x80`
293			`db "GOT",'O'+0x80`
294			`db "GOSU",'B'+0x80`
295			`db "RETUR",'N'+0x80`
296			`db "RE",'M'+0x80`
297			`db "FO",'R'+0x80`
298			`db "INPU",'T'+0x80`
299			`db "PRIN",'T'+0x80`
300			`db "POK",'E'+0x80`
301			`db "STO",'P'+0x80`
302			`db "BY",'E'+0x80`
303			`db "SY",'S'+0x80`
304			`db "CL",'S'+0x80`
305			`db "CL",'R'+0x80`
306			`db "RDC",'F'+0x80`
307	18	robfinch	`db "ONIR",'Q'+0x80`
308			`db "WAI",'T'+0x80`
309	16	robfinch	`db 0`
310			`TAB4:`
311			`db "PEE",'K'+0x80 ;Functions`
312			`db "RN",'D'+0x80`
313			`db "AB",'S'+0x80`
314	18	robfinch	`db "SG",'N'+0x80`
315	16	robfinch	`db "TIC",'K'+0x80`
316			`db "SIZ",'E'+0x80`
317			`db "US",'R'+0x80`
318			`db 0`
319			`TAB5:`
320			`db "T",'O'+0x80 ;"TO" in "FOR"`
321			`db 0`
322			`TAB6:`
323			`db "STE",'P'+0x80 ;"STEP" in "FOR"`
324			`db 0`
325			`TAB8:`
326			`db '>','='+0x80 ;Relational operators`
327			`db '<','>'+0x80`
328			`db '>'+0x80`
329			`db '='+0x80`
330			`db '<','='+0x80`
331			`db '<'+0x80`
332			`db 0`
333			`TAB9:`
334			`db "AN",'D'+0x80`
335			`db 0`
336			`TAB10:`
337			`db "O",'R'+0x80`
338			`db 0`
339
340			`;* Execution address tables:`
341			`; We save some bytes by specifiying only the low order 16 bits of the address`
342			`;`
343			`TAB1_1:`
344			`dh LISTX ;Direct commands`
345			`dh LOAD`
346			`dh NEW`
347			`dh RUN`
348			`dh SAVE`
349			`TAB2_1:`
350			`dh NEXT ; Direct / statement`
351			`dh LET`
352			`dh IF`
353			`dh GOTO`
354			`dh GOSUB`
355			`dh RETURN`
356			`dh IF2 ; REM`
357			`dh FOR`
358			`dh INPUT`
359			`dh PRINT`
360			`dh POKE`
361			`dh STOP`
362			`dh GOBYE`
363			`dh SYSX`
364			`dh _cls`
365			`dh _clr`
366			`dh _rdcf`
367	18	robfinch	`dh ONIRQ`
368			`dh WAITIRQ`
369	16	robfinch	`dh DEFLT`
370			`TAB4_1:`
371			`dh PEEK ;Functions`
372			`dh RND`
373			`dh ABS`
374	18	robfinch	`dh SGN`
375	16	robfinch	`dh TICKX`
376			`dh SIZEX`
377			`dh USRX`
378			`dh XP40`
379			`TAB5_1`
380			`dh FR1 ;"TO" in "FOR"`
381			`dh QWHAT`
382			`TAB6_1`
383			`dh FR2 ;"STEP" in "FOR"`
384			`dh FR3`
385			`TAB8_1`
386			`dh XP11 ;>= Relational operators`
387			`dh XP12 ;<>`
388			`dh XP13 ;>`
389			`dh XP15 ;=`
390			`dh XP14 ;<=`
391			`dh XP16 ;<`
392			`dh XP17`
393			`TAB9_1`
394			`dh XP_AND`
395			`dh XP_ANDX`
396			`TAB10_1`
397			`dh XP_OR`
398			`dh XP_ORX`
399
400			`;*`
401			`; r3 = match flag (trashed)`
402			`; r9 = text table`
403			`; r10 = exec table`
404			`; r11 = trashed`
405	26	robfinch	`message "DIRECT"`
406	16	robfinch	`DIRECT:`
407			`ld r9,#TAB1`
408			`ld r10,#TAB1_1`
409			`EXEC:`
410			`jsr IGNBLK ; ignore leading blanks`
411	18	robfinch	`ld r11,r8 ; save the pointer`
412	16	robfinch	`eor r3,r3,r3 ; clear match flag`
413			`EXLP:`
414			`lda (r8) ; get the program character`
415			`inc r8`
416			`lb r2,$0,r9 ; get the table character`
417			`bne EXNGO ; If end of table,`
418	18	robfinch	`ld r8,r11 ; restore the text pointer and...`
419	16	robfinch	`bra EXGO ; execute the default.`
420			`EXNGO:`
421			`cmp r1,r3 ; Else check for period... if so, execute`
422			`beq EXGO`
423			`and r2,r2,#0x7f ; ignore the table's high bit`
424			`cmp r2,r1 ; is there a match?`
425			`beq EXMAT`
426			`inc r10 ;if not, try the next entry`
427			`inc r10`
428	18	robfinch	`ld r8,r11 ; reset the program pointer`
429	16	robfinch	`eor r3,r3,r3 ; sorry, no match`
430			`EX1:`
431			`lb r1,0,r9 ; get to the end of the entry`
432			`inc r9`
433			`bit #$80 ; test for bit 7 set`
434			`beq EX1`
435			`bra EXLP ; back for more matching`
436			`EXMAT:`
437			`ldy #'.' ; we've got a match so far`
438			`lb r1,0,r9 ; end of table entry?`
439			`inc r9`
440			`bit #$80 ; test for bit 7 set`
441			`beq EXLP ; if not, go back for more`
442			`EXGO:`
443			`; execute the appropriate routine`
444	18	robfinch	`lb r1,1,r10 ; get the low mid order byte`
445	26	robfinch	`asl r1,r1,#8`
446	18	robfinch	`orb r1,r1,0,r10 ; get the low order byte`
447			`or r1,r1,#$FFFF0000 ; add in ROM base`
448			`jmp (r1)`
449	16	robfinch
450
451			`;******************************************************************`
452			`;`
453			`; What follows is the code to execute direct and statement`
454			`; commands. Control is transferred to these points via the command`
455			`; table lookup code of 'DIRECT' and 'EXEC' in the last section.`
456			`; After the command is executed, control is transferred to other`
457			`; sections as follows:`
458			`;`
459			`; For 'LISTX', 'NEW', and 'STOP': go back to the warm start point.`
460			`; For 'RUN': go execute the first stored line if any; else go`
461			`; back to the warm start point.`
462			`; For 'GOTO' and 'GOSUB': go execute the target line.`
463			`; For 'RETURN' and 'NEXT'; go back to saved return line.`
464			`; For all others: if 'CURRNT' is 0, go to warm start; else go`
465			`; execute next command. (This is done in 'FINISH'.)`
466			`;`
467			`;******************************************************************`
468			`;`
469			`; * NEW * STOP * RUN (& friends) * GOTO ***`
470			`;`
471			`; 'NEW' sets TXTUNF to point to TXTBGN`
472			`;`
473
474			`NEW:`
475			`jsr ENDCHK`
476			`lda TXTBGN>>2`
477			`sta TXTUNF ; set the end pointer`
478			`jsr clearVars`
479
480			`; 'STOP' goes back to WSTART`
481			`;`
482			`STOP:`
483			`jsr ENDCHK`
484			`jmp WSTART ; WSTART will reset the stack`
485
486			`; 'RUN' finds the first stored line, stores its address`
487			`; in CURRNT, and starts executing it. Note that only those`
488			`; commands in TAB2 are legal for a stored program.`
489			`;`
490			`; There are 3 more entries in 'RUN':`
491			`; 'RUNNXL' finds next line, stores it's address and executes it.`
492			`; 'RUNTSL' stores the address of this line and executes it.`
493			`; 'RUNSML' continues the execution on same line.`
494			`;`
495			`RUN:`
496			`jsr ENDCHK`
497			`ld r8,TXTBGN>>2 ; set pointer to beginning`
498			`st r8,CURRNT`
499			`jsr clearVars`
500

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