bB to assembly question

[Lillapojkenpåön]

I noticed that this line..

if player2y < player1y && player1y < player2y + 24 then _wait

outputs this assembly

LDA player2y

CMP player1y

BCS .skipL01207

; complex condition detected

LDA player2y

CLC

ADC #24

PHA

TSX

PLA

LDA player1y

CMP 1,x

if ((* - ._wait) < 127) && ((* - ._wait) > -128)

bcc ._wait

else

bcs .20skip_wait

jmp ._wait

.20skip_wait

endif

.skipL01207

what does this do and why?

PHA

TSX

PLA

LDA player1y

CMP 1,x

 

Is that why you should use <= and >= instead?

My wild guess is that there either isn't a smaller then or bigger then in assembly, that one of them also checks if equal to, so that part adds 1 to compensate for that?

[+splendidnut]

Have you tried experimenting with flipping the second condition around?

 

Try:

if player2y < player1y && player2y + 24 > player1y then _wait

 

and look at the code that produces.

 

 

what does this do and why?

PHA

TSX

PLA

LDA player1y

CMP 1,x

 

Is that why you should use <= and >= instead?

My wild guess is that there either isn't a smaller then or bigger then in assembly, that one of them also checks if equal to, so that part adds 1 to compensate for that?

 

The code puts a copy of A, which is the result of player2y+24), onto the stack. Then performs the comparison with player1y.

PHA TSX PLA -- puts A on the stack and puts the stack pointer into X and then "pops" A from the stack... A still exists on the stack since pop only reads the value and moves the stack pointer without clearing that spot.

 

CMP 1,x access that copy of A on the stack using the saved pointer which needs 1 added since the stack pointer always points at the next available spot and not the last value added.

 

If TSX was moved outside of the PHA PLA then it would be CMP 0,x instead.

 

 

To answer the other portion of your last question, < and >= are the simplest cases to handle in assembly (equal and not equal are also simple cases).

 

For good information about doing comparisons in assembly:

http://6502.org/tutorials/compare_instructions.html

 

Hopefully I explained that clearly enough

[Lillapojkenpåön]

Thanks for the quick and detailed info!

 

I will try changing it around next time I encounter it, I changed it to this which seems to work fine

 

asm

LDA player2y

CMP player1y

BCS .skipwait

CLC

ADC #24

CMP player1y

bcs ._wait

.skipwait

end