Kjetil's Information Center: A Blog About My Projects

Commodore 64 Game Cheats

I recently tried out the same tricks from my older article about DOS games, but for a Commodore 64 game instead. This time using the VICE emulator for Blagger by Alligata, a game I could never finish as a kid.

I present some of the steps I used to create the "cheat" patch for infinite lives.
1) Start the game (should start with 5 lives) and go into the VICE monitor.
2) Dump the memory and state of the C64 with the "dump" command.
3) Continue the game, and just loose 1 life.
4) Re-enter the monitor and dump the memory and state again with the "dump" command.
5) Convert the dumps to hex and do a diff on them, to see what has changed. It may take a while to find what you are looking for, which may require additional dumps.
Eventually I found this suspicious memory area, where two values were decremented twice:

< 00000740  20 20 20 20 1e 57 57 1e  57 57 1e 7a 3a 3b 42 43  |    .WW.WW.z:;BC|
> 00000740  20 20 20 20 1e 57 57 1e  57 57 1e 7a 3a 3b 44 45  |    .WW.WW.z:;DE|

Although this is at address 0x74e and 0x74f in the dump file, the actual memory addresses are 0x6ca and 0x6cb due to the format of the dump.
6) Now, with an address to look at, set a watch point on this in the VICE monitor.
7) Continue the game again and loose another life.
8) The monitor should now stop execution automatically if done correctly. In my case it displayed:

(C:$1102) w $06ca
WATCH: 1  C:$06ca  (Stop on load store)
(C:$1102) x
#1 (Stop on  load 06ca)  076 050
.C:0de6  AE CA 06    LDX $06CA      - A:01 X:44 Y:00 SP:f6 ..-....C  184810550
.C:0de9  CA          DEX            - A:01 X:44 Y:00 SP:f6 ..-....C  184810550

This assembly instruction (DEX) means the memory area is being decremented, exactly what we are looking for.
9) After investigation of the assembly code in that area I found two decrement instructions. These could be patched with NOP (No Operation) instructions (6502 machine code 0xEA) from the VICE monitor like so:

(C:$0e0c) > $0de9 ea
(C:$0e0c) > $0dea ea

10) Now, when continuing to play the game, any lost life is simply ignored.

To apply a patch like this permanently, do it on the file (in this case a PRG file) instead of directly in memory of course. The code location will be different, so it must be searched for manually.

Topic: Configuration, by Kjetil @ 02/07-2019, Article Link