Monitor program

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For further development it might be good to have a minimal monitor program running on our computer system. In the first version i will only implement three commands. From here on one could write a full featured monitor with an assembler, disassembler, register and stack display, break points and single step, loading and saving programs...

m <address>

Display 16 bytes starting from the specified address as hexadecimal values and characters.

w <address> <byte>

Write the specified byte into the given address.

j <address>

Jump to the specified address.  A RTS returns to monitor command loop.

Sample session

Welcome to the 6502 monitor program!
> m 4000
4000  FF C0 F7 82 FF 10 FF 02  00 BF 21 BF 00 AF 00 FF   |..........!.....|
> w 4000 a5
> m 4000
4000  A5 C0 F7 82 FF 10 FF 02  00 BF 21 BF 00 AF 00 FF   |..........!.....|


Even a small program like this is too big to show it on a web site. Therefore here only the main command loop of the monitor is shown. The main take away for all my 6502 assembler code is: You won't find any fancy optimization for space or performance in it. My main concern was maintainability.

                      .setcpu "6502"
                      .include "zeropage.inc65"
                      .include "acia.inc65"
                      .include "string.inc65"
                      .include "tools.inc65"
                      .include "macros.inc65"

                      .segment "VECTORS"

                      .word   nmi
                      .word   reset
                      .word   irq


                      BUFFER_LENGTH = 80
buffer:               .res BUFFER_LENGTH + 1, 0


reset:                jmp main

nmi:                  rti

irq:                  rti

main:                 cld
                      ldx #$ff

                      jsr acia_init

                      ld16 R0, msg_welcome
                      jsr acia_puts

loop:                 ld16 R0, prompt
                      jsr acia_puts
                      ld16 R0, buffer
                      lda #BUFFER_LENGTH
                      jsr acia_gets
                      lda buffer
@check_m:             cmp #'m'
                      bne @check_w
                      jsr cmd_memory
                      jmp loop
@check_w:             cmp #'w'
                      bne @check_j
                      jsr cmd_write
                      jmp loop
@check_j:             cmp #'j'
                      bne @unknwon
                      jsr cmd_jump
                      jmp loop
@unknown:             ld16 R0, msg_unknown_command
                      jsr acia_puts
                      jmp loop

prompt:               .byte "> ", $00
assign_arrow:         .byte " <- ", $00
msg_welcome:          .byte $0a, $0a, "Welcome to the 6502 monitor program!", $0a, $00
msg_unknown_command:  .byte "Unknown command", $0a, $00

cmd_memory:           ...
cmd_write:            ...
cmd_jump:             ...
This page was last updated on 07. February 2016