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This is a port of the Commodore C64 Kernal version 3 and BASIC version 2 to the ASM6502 assembler. It can also be built with the not yet released VASM by Fred N. van Kempen. The original source can be found athttps://github.com/mist64/cbmsrc/tree/master/.

• The assembler can be obtained fromhttps://github.com/boeckmann/asm6502.
• A MD5 checksum generator is needed if you want to compare your builds with known to be good ROMs.
• The Commodore ROMs contain a one-byte checksum at somewhat random positions. Thecbmsum utility contained in this repository can generate these checksums and put them into place. Binaries are provided for Windows, DOS, and OS/2 (see releases).

Here are MD5 checksums of the original ROMs:

MD5 (kernal.rom) = 39065497630802346bce17963f13c092MD5 (basic.rom) = 57af4ae21d4b705c2991d98ed5c1f7b8

Build the KERNAL and BASIC binary files:

cd kernalasm6502 kernal.a65 -o kernal.bin -l kernal.lstcd ../basicasm6502 basic.a65 -o basic.bin -l basic.lstcd ..

There are nowkernal/kernal.bin andbasic/basic.bin files and listing fileskernal/kernal.lst andbasic/basic.lst.

The binaries arenot the ROM files, because the BASIC binary is too large to fit into its ROM. The last $4B7 bytes of BASIC reside in the KERNAL ROM.

To generate the KERNAL and BASIC ROM files run the following after creating the binaries above. You may have to adjust the two .a65 files in the rom directory if the directory separator of your operating system is not the forward slash /:

cd romasm6502 kernal.a65 -o kernal.romasm6502 basic.a65 -o basic.rom

This generateskernal.rom andbasic.rom inside therom sub-directory. These are the ROM files.

The checksum bytes are zeroed in the freshly built ROM files. You may put the correct checksums into place by using thecbmsum utility provided in this repository. If you do not want to download it, you may build it by yourself with every ANSI conformant C compiler. The tool can also verify the checksum of a Commodore ROM. Call it without parameters for usage info.

To patch the ROMs to include the checksum run:

cd rom../supp/cbmsum patch old kernal.rom e000 e4ac kernal.rom../supp/cbmsum patch old basic.rom a000 bf52 basic.rom

You may also put the checksums into place with your favourite hex editor. But you may place wrong checksums if the ROMs do not match the original ROMsexactly.

• Basic ROM check sum at offset $1F52 = $EC
• Kernal ROM check sum at offset $4AC = $81

Two flags may be altered in the filekernal/kernal.a65. If you alter one of these you will not produce an exact replica of the Kernal V3 ROM:

• FLAG_RRBY: Most ROMS contain "RRBY" at $FFF6-$FFF9. These are the initials of Commodore engineers. The initials are not included in the source I based my work on, so I patched them in again. SetFLAG_RRBY=0 to disable it.
• FLAG_FIX_FF7E: The source I based my work on containsJMP CLKHI at $FF7D, but it isJMP CLKLO in the Kernal V3 ROM. FLAG_FIX_FF7E may be set to 1 if you want it to beJMP CLKHI.

First build thecbmsum utility by invokingmake in thesupp sub-directory.

AdaptMakefile and call it viamake to build the ROMs. If you have not altered the sources you may compare the checksums of your ROMs viamake -f verify.