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Table of Contents
Assembling your own cart ROM image
Cartridge ROM images are burnt or flashed into ROM/flash chips. The file formats of the ROM images vary, but common formats are either raw binary dumps or the special .crt file format.
Most emulators can handle both of these formats, and hardware such as Retro Replay and 1541U with flash ROM can be used if you want to run your cartridge image on the real hardware without having a EPROM burner or other special hardware.
Suitable assemblers for cartridge ROM development
Most assemblers can be used to write your own cartridge ROM, but some are more suitable than others. For example, if you are planning to write a cartridge ROM that uses more than one ROM bank, you might want to use an assembler that, in one way or another, supports writing several segments that have the same start address. DreamAss deserves a special comment here since it has even more features in this direction. In addition to plain support of segments, it also has:
- a pseudo op that returns the bank number of a symbol. this makes it very easy to build tables with bank/address for a cross-bank-call-api
- the assembler can put functions into banks as they fit automatically, so you don't have to worry about memory layout.
Simple cartridge skeleton sources
Here are two small sources from the net that might help you to get up and running with an autostarting cartridge.
CBM80 Autostart Cartridge
Version 1.0
By Alphaworks (?)
CHRGET = $0073
TXTPTR = $7A
IERROR = $0300
IMAIN = $0302
IGONE = $0308
GONE = $A7E4
CHKCOM = $AEFD
FRMNUM = $AD8A
GETADR = $B7F7
CHROUT = $FFD2
BORDER = $D020
SCREEN = $D021
TEXT = $0286
* = $8000
.BYTE $09, $80 ; Cartridge cold-start vector = $8009
.BYTE $25, $80 ; Cartridge warm-start vector = $8025
.BYTE $C3, $C2, $CD, $38, $30 ; CBM8O - Autostart key
; KERNAL RESET ROUTINE
STX $D016 ; Turn on VIC for PAL / NTSC check
JSR $FDA3 ; IOINIT - Init CIA chips
JSR $FD50 ; RANTAM - Clear/test system RAM
JSR $FD15 ; RESTOR - Init KERNAL RAM vectors
JSR $FF5B ; CINT - Init VIC and screen editor
CLI ; Re-enable IRQ interrupts
; BASIC RESET Routine
JSR $E453 ; Init BASIC RAM vectors
JSR $E3BF ; Main BASIC RAM Init routine
JSR $E422 ; Power-up message / NEW command
LDX #$FB
TXS ; Reduce stack pointer for BASIC
; START YOUR PROGRAM HERE ($8025)
LDA #4 ; CHANGE BORDER COLOUR TO
STA BORDER ; BLACK
LDA #147 ; PRINT CHR$(147) TO CLEAR
JSR CHROUT ; SCREEN
RTS
Copy a program from ROM to RAM and autostart it
By Enthusi
Taken from this CSDb discussion thread.
This code is used to create a .crt ROM image which includes a copy of an executable program. When started, the code in the ROM copies the program to C64 RAM (from $0801) and executes it. The cart-reset-routine copies the “launcher” to $0400, executes it and somewhere near $0430 it says JMP $080b.
These CRTs will run in VICE and MMCR Bios 0.55 - source is in XA syntax
;this is for a 8KB cart!! *=$0000 .asc "C64 CARTRIDGE " .byte $00,$00 ;header length .byte $00,$40 ;header length .word $0001 ;version .word $0000 ;crt type .byte $00 ;exrom line .byte $01 ;game line .byte $00,$00,$00,$00,$00,$00 ;unused .asc "CRT TITLE" name .dsb ($0040-name),0 ;chip packets .asc "CHIP" .byte $00,$00,$20,$10 ;chip length .byte $00,$00 ;chip type .byte $00,$00 ;bank .byte $80,$00 ;adress .byte $20,$00 ;length ;ROM part ;--------------------------------- *=$8000 .word launcher .word launcher .byte $c3 ;c .byte $c2 ;b .byte $cd ;m .byte $38 ;8 .byte $30 ;0 launcher sei stx $d016 jsr $fda3 ;prepare irq jsr $fd50 ;init memory jsr $fd15 ;init i/o jsr $ff5b ;init video cli ;jsr $e453 ;load basic vectors ;jsr $e3bf ;init basic ram ;jsr $a68e ;jmp $a7ae ;ldx #$fb ; txs ;--------------------------------- start_all lda $d011 and #%11101111 sta $d011 ldx #$00 sa1 lda movecode1,x sta $0400,x inx bne sa1 jmp $0400 ;------------------------------ movecode1 *=$0400 movecode2 basic_move ldx #$00 bm1 lda main_file_start,x bm2 sta $0801,x inx bne bm1 inc bm1+2 inc bm2+2 lda bm1+2 cmp #$c0 bne basic_move jmp $080b movecode3 ;--------------------------------- *=movecode1+(movecode3-movecode2) main_file_start .bin 2,0,"game.bin" main_file_end .dsb ($a000-main_file_end),0