Question for Calamari...

[icbrkr]

I'm working some more in 5200BAS (after a few month vacation from it) and I've got a question on how to program something...

 

I'm trying to figure out how to do a simple IF X>Y routine, but I haven't been able to. In other words, when a level ends, I need to check Player 1's score and Player 2's score and whomever's is higher, they win. But it appears I can't do:

 

A=SCORE1

Y=SCORE2

 

IF A>Y THEN

GOSUB STUFF

 

 

and by the examples on your website, it's not possible either. The only thing I can do is

 

IF A>1 THEN

GOSUB STUFF..

 

Any ideas how I can work around this?

 

Brian

[calamari]

I'm trying to figure out how to do a simple IF X>Y routine, but I haven't been able to. In other words, when a level ends, I need to check Player 1's score and Player 2's score and whomever's is higher, they win. But it appears I can't do:

 

A=SCORE1

Y=SCORE2

 

IF A>Y THEN

GOSUB STUFF

 

Any ideas how I can work around this?

 

This is because the 6502 doesn't allow register,register compares (or adds, subtracts, etc, for that matter).. the only thing I can think of that allows reg-reg are transfers (copy/move).. like tya, tay, txa, etc, even then you cant do txy or tyx directly. You also can't do memory-memory compares. You can compare a register vs memory (of different kinds.. zeropage, x/y indexed, indirect, etc), or a register vs an immediate.. so as you found you can do:

 

CMP #$01  ;does A=1? (reg vs immed)

 

Which is isn't any good for this situation, I agree.. but you can also do:

CMP $EE    ;does A=the value at location $EE? (reg vs memory)

 

The second way works well for what you are trying to do. Let's see it in 5200BAS code:

 

A=SCORE1

IF A>SCORE2 THEN:GOSUB STUFF:END IF

 

As a sidenote, you'll want to avoid > if possible (comparing with > and <= takes two instructions vs. one instruction for < and >=, see the IF entry in the Command Reference).. so I'd suggest this instead:

 

A=SCORE2

IF A<SCORE1 THEN:GOSUB STUFF:END IF

 

I'm assuming that you will have it call different subroutines depending on who wins, so the code might look like this:

 

A=SCORE2

IF A<SCORE1 THEN

 GOSUB STUFF

ELSE

 GOSUB STUFF2

END IF

 

You could also use Y instead of A in the above code.. or X.. whatever you prefer.

 

HTH,

calamari

[icbrkr]

A=SCORE2

IF A<SCORE1 THEN:GOSUB STUFF:END IF

 

I'm assuming that you will have it call different subroutines depending on who wins, so the code might look like this:

 

A=SCORE2

IF A<SCORE1 THEN

 GOSUB STUFF

ELSE

 GOSUB STUFF2

END IF

 

You could also use Y instead of A in the above code.. or X.. whatever you prefer.

 

HTH,

calamari

 

I didn't even realize it was possible to use another variable besides X, Y, and A in a comparison (IF A

 

Brian