While it was well designed, I noticed a few inefficiencies. I offer them
not as criticism of Combat, but as little hints on building better 6502
code:
lda TheScore,X
and #$F0
lsr
lsr
sta Temp1 ; * 4
lsr
lsr
clc
adc Temp1 ; * 5
The clc isn't needed because the least significant bit is already 0 from
the and #$F0 instruction.
There's an RTS instruction at $F507 that didn't need to be there - there
were other RTS's that could've been branched to.
Near $F37B, there's code that reads:
lda GameModeInformation
bmi @not_invisible ; planes/jets are never invisible
and #1
beq @not_invisible ; invisibility bit set?
The and #1/beq is less efficient space-wise than an lsr followed by a bcc
in this case, because the code doesn't need to preserve the accumulator
or the carry flag (if you read the rest of the code, you can see that.)
Last, there's definitely code in there to flash an LED wired to bit 4 of
port B when the game is over. Presumably Atari ditched the LED to save
money.
Overall, Combat *IS* an example of really good programming. There's very
little wasted code, and what's there does a lot for being only 2K.
There should be plenty of time left during vblank to do AI for a one
player mode. Does the computer get to cheat during invisible tank mode
and know where the player is, though? ;)
- Jim
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