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2 |
robfinch |
XON equ $11
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| 2 |
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XOFF equ $13
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| 3 |
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LEDS equ $FFDC0600
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| 4 |
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UART equ $FFDC0A00
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| 5 |
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| 6 |
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code 18 bits
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| 7 |
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org $FFFC0000
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| 8 |
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start:
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| 9 |
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ldi $sp,#$80000-4 ; setup system mode stack pointer
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| 10 |
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; Switch to user mode
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| 11 |
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.0004:
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| 12 |
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eret
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| 13 |
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; When an ECALL instruction is executed it'll start here.
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| 14 |
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bra .0004
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| 15 |
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| 16 |
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;------------------------------------------------------------------------------
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| 17 |
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; User mode code starts here
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| 18 |
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;------------------------------------------------------------------------------
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| 19 |
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org $FFFC0100
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| 20 |
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| 21 |
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ldi $sp,#$80000-1028 ; setup user mode stack pointer
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| 22 |
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add $t0,$x0,#$AB ; turn on the LED
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| 23 |
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stt $t0,LEDS
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| 24 |
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call SerialInit
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| 25 |
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ldi $t2,#16 ; send an XON just in case
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| 26 |
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ldi $a0,#XON
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| 27 |
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.0004:
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| 28 |
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call SerialPutChar
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| 29 |
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sub $t2,$t2,#1
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| 30 |
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bne $t2,$x0,.0004
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| 31 |
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ldi $a0,#'A' ; Try sending the letter 'A'
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| 32 |
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call SerialPutChar
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| 33 |
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.0002:
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| 34 |
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ldi $a0,#msgStart ; spit out a startup message
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| 35 |
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call SerialPutString
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| 36 |
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| 37 |
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; Now a loop to recieve and echo back characters
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| 38 |
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.0003:
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| 39 |
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call SerialPeekChar
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| 40 |
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blt $v0,$x0,.0003
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| 41 |
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mov $a0,$v0
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| 42 |
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call SerialPutChar
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| 43 |
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bra .0003
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| 44 |
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| 45 |
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;------------------------------------------------------------------------------
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| 46 |
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; SerialPeekChar
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| 47 |
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;
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| 48 |
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; Check the serial port status to see if there's a char available. If there's
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| 49 |
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; a char available then return it.
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| 50 |
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;
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| 51 |
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; Modifies:
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| 52 |
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; $t0
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| 53 |
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; Returns:
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| 54 |
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; $v0 = character or -1
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| 55 |
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;------------------------------------------------------------------------------
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| 56 |
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| 57 |
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SerialPeekChar:
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| 58 |
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ldb $t0,UART+4
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| 59 |
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and $t0,$t0,#8 ; look for Rx not empty
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| 60 |
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beq $t0,$x0,.0001
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| 61 |
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ldb $v0,UART
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| 62 |
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ret
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| 63 |
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.0001:
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| 64 |
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ldi $v0,#-1
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| 65 |
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ret
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| 66 |
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| 67 |
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;------------------------------------------------------------------------------
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| 68 |
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; SerialPutChar
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| 69 |
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; Put a character to the serial transmitter. This routine blocks until the
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| 70 |
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; transmitter is empty.
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| 71 |
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;
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| 72 |
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; Parameters:
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| 73 |
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; $a0 = character to put
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| 74 |
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; Modifies:
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| 75 |
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; $t0
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| 76 |
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;------------------------------------------------------------------------------
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| 77 |
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| 78 |
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SerialPutChar:
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.0001:
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| 80 |
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ldb $t0,UART+4 ; wait until the uart indicates tx empty
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| 81 |
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and $t0,$t0,#16 ; bit #4 of the status reg
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| 82 |
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beq $t0,$x0,.0001 ; branch if transmitter is not empty
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| 83 |
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stb $a0,UART ; send the byte
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| 84 |
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ret
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| 85 |
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| 86 |
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;------------------------------------------------------------------------------
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| 87 |
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; SerialPutString
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| 88 |
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; Put a string of characters to the serial transmitter. Calls the
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| 89 |
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; SerialPutChar routine, so this routine also blocks if the transmitter is not
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| 90 |
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; empty.
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| 91 |
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;
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| 92 |
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; Parameters:
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| 93 |
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; $a0 = pointer to null terminated string to put
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| 94 |
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; Modifies:
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| 95 |
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; $t0 and $t1
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| 96 |
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; Stack Space:
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| 97 |
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; 2 words
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| 98 |
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;------------------------------------------------------------------------------
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| 99 |
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| 100 |
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SerialPutString:
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sub $sp,$sp,#8 ; save link register
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| 102 |
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stt $ra,[$sp]
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| 103 |
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stt $a0,4[$sp] ; and argument
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| 104 |
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mov $t1,$a0 ; t1 = pointer to string
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| 105 |
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.0001:
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| 106 |
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ldb $a0,[$t1]
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| 107 |
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add $t1,$t1,#1 ; advance pointer to next byte
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| 108 |
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beq $a0,$x0,.done ; branch if done
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| 109 |
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call SerialPutChar ; output character
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| 110 |
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bra .0001
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| 111 |
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.done:
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| 112 |
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ldt $ra,[$sp] ; restore return address
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| 113 |
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ldt $a0,4[$sp] ; and argument
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| 114 |
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add $sp,$sp,#8
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| 115 |
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ret
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| 116 |
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| 117 |
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;------------------------------------------------------------------------------
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| 118 |
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; Initialize serial port.
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| 119 |
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;
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| 120 |
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; Modifies:
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| 121 |
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; $t0
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| 122 |
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;------------------------------------------------------------------------------
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| 123 |
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| 124 |
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SerialInit:
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| 125 |
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ldi $t0,#$0B ; dtr,rts active, rxint disabled, no parity
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| 126 |
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stt $t0,UART+8
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| 127 |
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ldi $t0,#$08070012 ; reset the fifo's
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| 128 |
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stt $t0,UART+12
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| 129 |
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ldi $t0,#$08010012 ; baud 115200, 1 stop bit, 8 bit, internal baud gen
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| 130 |
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stt $t0,UART+12
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| 131 |
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ret
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| 132 |
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| 133 |
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;------------------------------------------------------------------------------
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| 134 |
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; Message strings
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| 135 |
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;------------------------------------------------------------------------------
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| 136 |
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| 137 |
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msgStart:
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| 138 |
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db "uart6551 Demo Starting.",13,10,0
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