subtraction problem

[antron]

i have a ram location (wchman_L) that holds the low byte of a pointer to a graphic

if that pointer is pointing to a certain graphic (#

 

this works fine:

lda wchman_L 

sec

sbc #<MAN4_L 

beq reset

 

 

 

but this does not:

lda #<MAN4_L   

sec

sbc wchman_L

beq reset

 

why?

[antron]

my bad

 

the difference was the state that it left the carry in.

i got it.

[Nukey Shay]

I don't understand...why not just use CMP?

 

lda wchman_L 

cmp #<MAN4_L 

beq reset

 

 

...and you'll save a byte+2 cycles

[CPUWIZ]

I don't understand...why not just use CMP?

 

lda wchman_L 

cmp #<MAN4_L 

beq reset

 

 

...and you'll save a byte+2 cycles

 

Perhaps because the algorithm is something like this ...

 

lda wchman_L

sec

sbc #<MAN4_L

beq reset



asl

tay

lda Location,y

.

.

blah

 

You never know.

[antron]

in my case Nukey is right, thanks.

 

but i don't understand why my original two cases gave different behavior.

 

how could they leave the carry in different states?

Edited May 11, 2005 by antron

[Tom]

basic math : subtraction isn't commutative.

[vdub_bobby]

basic math : subtraction isn't commutative.

Yeah, but if you are testing for equality it doesn't matter.

 

If A==B, A-B==B-A==0

 

If A!=B, A-B!=0 and B-A!=0 even though A-B!=B-A

[vdub_bobby]

in my case he is right' date=' thanks.

 

but i don't understand why my original two cases gave different behavior.

 

how could they leave the carry in different states?[/quote']

if #

 

This is because after a sbc the carry is cleared if the subtraction needed a borrow (i.e., if the result is negative, assuming unsigned numbers) and the carry will be set if the subtraction didn't need a borrow.

 

In other words,

 

If A > B, then A - B will leave the carry set, but B - A will clear the carry.

 

See here: http://www.6502.org/tutorials/6502opcodes.htm#SBC

and here:http://www.geocities.com/oneelkruns/asm1step.html

[antron]

makes sense now.

it is the case when they were not equal that things went wrong.

 

thanks all.