I'm working on an assembly-language project using cc65. Although the project is mostly assembly, I write all the unit tests in C. It's just easier to build unit tests that way. I run the tests under sim65, and the project itself also works under sim65.
Unfortunately sim65 doesn't really have any debugging features. After some experimentation I decided the fastest way to get some visibility into what the program is doing is to just dump the registers out at various points in the program. To do that I created the module below.
The install_debug_handler function points the BRK vector to debug_handler, which dumps out the registers and flags via cc65's standard library function fprintf. You probably know that BRK is actually a two-byte instruction where the second byte is ignored. The debug handler takes advantage of this by printing the byte following the BRK along with the registers and flags; I use the second byte to identify where the program is. I defined a debug macro that just accepts an argument and adds it after the BRK:
Code:
.macro debug value
brk
.byte value
.endmacro
This has been pretty helpful; if one of my unit tests fail then I just sprinkle the debug macro around to see where the program wound up going and what the registers were. I've rarely had to spend more than an hour or so tracking down some problem, so I don't miss having a source-level debugger that much.
Here's the module. Let me know if you find any issues or have suggestions for improvement.
Code:
; MIT License
;
; Copyright (c) 2022 Willis Blackburn
;
; Permission is hereby granted, free of charge, to any person obtaining a copy
; of this software and associated documentation files (the "Software"), to deal
; in the Software without restriction, including without limitation the rights
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; copies of the Software, and to permit persons to whom the Software is
; furnished to do so, subject to the following conditions:
;
; The above copyright notice and this permission notice shall be included in all
; copies or substantial portions of the Software.
;
; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
; IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
; FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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; OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
; SOFTWARE
; cc65 runtime
.import pushax, pusha0
; C standard library
.import _fprintf, _stderr
; Point the BRK handler to the debug_handler function.
install_debug_handler:
lda #<debug_handler
sta $FFFE ; BRK vector low byte
lda #>debug_handler
sta $FFFF ; BRK vector high byte
rts
.export install_debug_handler
.zeropage
save_pc: .res 2
.bss
save_a: .res 1
save_x: .res 1
save_y: .res 1
save_sp: .res 1
save_flags: .res 1
flag_indicators: .res 8
.code
format: .byte "$%02X: A=%02X X=%02X Y=%02X SP=%02X %.8s", $0A, $00
flag_names: .byte "NV-BDIZC"
; Prints the register values to stderr.
; Although calling into the C library from an interrupt handler is normally asking for trouble, since sim65
; doesn't generate interrupts, this will only be called by a BRK statement.
debug_handler:
cld ; Clear decimal flag (just in case)
sta save_a ; Save 6502 registers
stx save_x
sty save_y
tsx ; Get stack pointer into X
stx save_sp ; Save it so we can print it
ldy $102,x ; PC low byte
sty save_pc
ldy $103,x ; PC high byte
dey ; Subtract 256 from PC; we will index with Y = 255 to get PC-1
sty save_pc+1
lda $101,x ; Flags
sta save_flags
ldy #0
@next_flag:
lda flag_names,y ; Store name in indicator string if flag on
rol save_flags
bcs @on
lda #'-' ; Store '-' indicator string if flag off
@on:
sta flag_indicators,y
iny
cpy #8
bne @next_flag
lda _stderr ; fprintf(stderr, ...
ldx _stderr+1
jsr pushax
lda #<format ; format, ...
ldx #>format
jsr pushax
ldy #$FF
lda (save_pc),y ; id, ...
jsr pusha0
lda save_a ; A, ...
jsr pusha0
lda save_x ; X, ...
jsr pusha0
lda save_y ; Y, ...
jsr pusha0
lda save_sp ; SP, ...
jsr pusha0
lda #<flag_indicators ; flag_indicators)
ldx #>flag_indicators
jsr pushax
ldy #16 ; 16 bytes on the C stack
jsr _fprintf
lda save_a ; Restore 6502 registers
ldx save_x
ldy save_y
rti