Subversion Repositories eco32

;

; mbr.s -- the master boot record

;

        .set    tba,0xF0300000          ; terminal base address

        .set    tos,0xC0011000          ; top of stack

start:

        add     $29,$0,tos               ; set stackpointer

        jal     msgout                  ; output message

stop:

        j       stop                    ; halt by looping

msgout:

        sub     $29,$29,8               ; allocate stack frame

        stw     $31,$29,0                ; save return register

        stw     $16,$29,4               ; save local variable

        add     $16,$0,msg               ; pointer to string

loop:

        ldbu    $4,$16,0         ; get char

        beq     $4,$0,exit               ; null - finished

        jal     out                     ; output char

        add     $16,$16,1               ; bump pointer

        j       loop                    ; next char

exit:

        ldw     $31,$29,0                ; restore return register

        ldw     $16,$29,4               ; restore local variable

        add     $29,$29,8               ; release stack frame

        jr      $31                     ; return

out:

        add     $8,$0,tba                ; set I/O base address

out1:

        ldw     $9,$8,8                 ; get xmtr status

        and     $9,$9,1                 ; xmtr ready?

        beq     $9,$0,out1               ; no - wait

        stw     $4,$8,12                ; send char

        jr      $31                     ; return

msg:

        .byte   0x0D, 0x0A

        .byte   "Error: This is the default MBR, "

        .byte   "which cannot load anything."

        .byte   0x0D, 0x0A

        .byte   "Please replace the disk, or "

        .byte   "write an operating system onto it."

        .byte   0x0D, 0x0A

        .byte   "Execution halted."

        .byte   0x0D, 0x0A

        .byte   0x0D, 0x0A, 0

        .locate 512-2

sign:

        .byte   0x55, 0xAA