Optimizing Game Genie Codes              XII-12-2000

Created By Jeff Connelly
from ~/mnt/ad4s2d/jeff/xmax/games/emu/nes/gg/optimize.txt


** MUTATING CLOSE OPCODES

Suppose you have these program instructions, and you want to null out the 1st:

    ; From Tetris, in case you're curious
    00148B A5 41                    LDA $41
    00148D 0A                       ASL A


You could, of course, insert NOP's (EA) over the entire first instruction:

    00148B EA                       NOP
    00148C EA                       NOP
    00148D 0A                       ASL A

    Game Genie codes to do this:

        148B:EA (XVAOLK)
        148C:EA (XVAOGK)

But there's a slight problem with using these codes -- there's two of them.
One NES Game Genie device can only apply up to 3 codes. So users of your
two codes can only use one other code. Certainly we can do better. Best case
is only one code; one byte modification.

Look carefully at the sequence again:

    00148B A5 41                    LDA $41
    00148D 0A                       ASL A

Aha! 41 is the opcode for EOR with indirect addressing via X, and it takes
one byte after it. So if we use the Game Genie code 148B:EA, the instructions
will become:

    00148B EA                       NOP
    00148C 41 0A                    EOR ($0A,X)

That's good -- we effectively nulled out the LDA $41 instruction, although
it sort of transformed into another one thanks to it's neighbors. The EOR
operation may or may not affect the game in a noticable way. The lack of a
small ASL instruction might not be important either. The only way to find
out is to try (either that, or careful analyst of the instruction flow).

In fact, the Game Genie code 148B:EA (XVAOLK) does work with Tetris, it
makes pieces overlap.

Codes can be mutated into other codes in other ways, too. For example,
an opcode that takes a two-byte operand could be placed at 148B. This
opcode would suck up bytes at 148C and 148D, nulling them out. The SKB and
SKW opcodes are suitable for this purpose.

** USING THE UNDOCUMENTED SKB (DOP) AND SKW (TOP) OPCODES

From http://nesdev.parodius.com/undocumented_opcodes.txt:

 DOP (NOP) [SKB]
 =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
 No operation (double NOP). The argument has no signifi-cance. Status
 flags: -
 
 Addressing  |Mnemonics  |Opc|Sz | n
 ------------|-----------|---|---|---
 Zero Page   |DOP arg    |$04| 2 | 3
 Zero Page,X |DOP arg,X  |$14| 2 | 4
 Zero Page,X |DOP arg,X  |$34| 2 | 4
 Zero Page   |DOP arg    |$44| 2 | 3
 Zero Page,X |DOP arg,X  |$54| 2 | 4
 Zero Page   |DOP arg    |$64| 2 | 3
 Zero Page,X |DOP arg,X  |$74| 2 | 4
 Immediate   |DOP #arg   |$80| 2 | 2
 Immediate   |DOP #arg   |$82| 2 | 2
 Immediate   |DOP #arg   |$89| 2 | 2
 Immediate   |DOP #arg   |$C2| 2 | 2
 Zero Page,X |DOP arg,X  |$D4| 2 | 4
 Immediate   |DOP #arg   |$E2| 2 | 2
 Zero Page,X |DOP arg,X  |$F4| 2 | 4

 TOP (NOP) [SKW]
 =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
 No operation (tripple NOP). The argument has no signifi-cance. Status
 flags: -
 
 Addressing  |Mnemonics  |Opc|Sz | n
 ------------|-----------|---|---|---
 Absolute    |TOP arg    |$0C| 3 | 4
 Absolute,X  |TOP arg,X  |$1C| 3 | 4 *
 Absolute,X  |TOP arg,X  |$3C| 3 | 4 *
 Absolute,X  |TOP arg,X  |$5C| 3 | 4 *
 Absolute,X  |TOP arg,X  |$7C| 3 | 4 *
 Absolute,X  |TOP arg,X  |$DC| 3 | 4 *
 Absolute,X  |TOP arg,X  |$FC| 3 | 4 *

The important field here is the instruction size (Sz), which is the length of
the opcode (1) and the length of it's operands. SKB will slurp up one byte
after it, SKW will take two. The major benefit from using these opcodes is
there is no ill side effects -- they do nothing, unlike EOR from the last
section.

If you want to null out two bytes, simply place a SKW opcode right before
them. Note that some NES emulators do not support unofficial opcodes, so
you might have to try your codes on a real NES. (FCE Ultra supports undoc
opcodes)