6 replies to this topic

[karnov]

I have a question about this code:

	processor 6502

                include vcs.h

	include macro.h

	org $F000



YPosFromBot = $80

VisibleMissileLine = $81



Start

	CLEAN_START



	lda #$00

	sta COLUBK

	lda #66

	sta COLUP0



	lda #80

	sta YPosFromBot

	lda #$20

	sta NUSIZ0



MainLoop

	lda #2

	sta VSYNC

	sta WSYNC

	sta WSYNC

	sta WSYNC

	lda #43

	sta TIM64T

	lda #0

	sta VSYNC



	lda #%00010000

	sta HMM0



WaitForVblankEnd

	lda INTIM

	bne WaitForVblankEnd

	ldy #191

	sta WSYNC

	sta VBLANK



	sta WSYNC

	sta HMOVE



ScanLoop

	sta WSYNC



CheckActivateMissile

	cpy YPosFromBot           ;  question regarding

	bne SkipActivateMissile   ;  this area!!!!

	lda #8                            

	sta VisibleMissileLine        



SkipActivateMissile

	lda #0

	sta ENAM0



	lda VisibleMissileLine

	beq FinishMissile



IsMissileOn

	lda #2

	sta ENAM0

	dec VisibleMissileLine



FinishMissile

	dey

	bne ScanLoop



	lda #2

	sta WSYNC

	sta VBLANK

	ldx #30



OverScanWait

	sta WSYNC

	dex

	bne OverScanWait

	jmp MainLoop

	org $FFFC

	.word Start

	.word Start

In the area commented as "question regarding this area", does this:

bne SkipActivateMissile

mean:

branch to the location labelled as "SkipActivateMissile" if Y is not
equal to YPosFromBot (referencing the compare statement of the
previous line)

because I thought bne meant branch to (label) if the result of the previous line is not equal to 0.

Thanks! Any help is appreciated!

[Bad Panda Bear]

karnov said:

In the area commented as "question regarding this area", does this:

bne SkipActivateMissile

mean:

branch to the location labelled as "SkipActivateMissile" if Y is not
equal to YPosFromBot (referencing the compare statement of the
previous line)

because I thought bne meant branch to (label) if the result of the previous line is not equal to 0.

Thanks! Any help is appreciated!

In a way both those statements are right.

What BNE does is to branch when the Z flag is set to 0. Usually this happens when the result of an instruction is not zero. For CPY, the cpu flags are set the same as if a subtraction occurred. So if YPosFromBot does not equal Y, the result of subtracting the two will not be 0, so CPY sets the Z flag to 0. This triggers the branch.

Alternatively code like this would also branch when YPosFromBot is not equal to Y:

TYA                   ; Transfer Y to A

SEC                   ; Set carry so it does not interfere with subtraction

SBC  YPosFromBot      ; Subtract YPosFromBot from A

BNE SkipActiveMissile

In this case, if Y does not equal YPosFromBot the result of the SBC is non-zero. The Z flag will be set to 0 so the branch will be taken.

Hope this helps.

[Andrew Davie]

karnov said:

I have a question about this code:

CheckActivateMissile

	cpy YPosFromBot           ;  question regarding

	bne SkipActivateMissile   ;  this area!!!!

	lda #8                            

	sta VisibleMissileLine        



SkipActivateMissile

In the area commented as "question regarding this area", does this:

bne SkipActivateMissile

mean:

branch to the location labelled as "SkipActivateMissile" if Y is not
equal to YPosFromBot (referencing the compare statement of the
previous line)

because I thought bne meant branch to (label) if the result of the previous line is not equal to 0.

Thanks! Any help is appreciated!

The branch is taken based on the status of the processor's flags. In this case, the branch is branching based on the state of the Z flag. When you do a comparison, such as in the line before the branch, you cause the processor to set/clear some flags (specifically, the carry flag, the negative flag, and the zero flag). So the CPY op-code essentially compares the contents of YPosFromBot with the value in the Y register, and the branch will be taken if these values are NOT the same (branch NOT equal).

Some flags are set automatically on the loading of a value to a register (eg: the Z and the N flags). Some are only set as a result of comparisons or specific instructions to modify those flags (eg: the C flag). Branches always happen as a result of the current status of the relevant flag at the point the branch instruction is taken. The current status, of course, depends on what has happened just before the branch instruction itself.

Good questions for the "programming for Newbies" forum

Cheers
A

[Bruce Tomlin]

What you are missing here is that the CMP instruction does a subtraction and throws away the result. But it sets the flags. If you subtract one number from another and the result is zero, that means that the two numbers are equal. The C and N flags are also used for various combinations of less than and greater than.

[karnov]

So, the comparison is done by subtracting the two values (the value in Y, and the value in YPosFromBot). And if the numbers are the same the result will be zero, which will set the Z flag to zero, which will trigger the branch. Is that correct? Thanks everyone, I will post the rest of my questions in the newbie section. THANKS!

[Andrew Davie]

karnov said:

So, the comparison is done by subtracting the two values (the value in Y, and the value in YPosFromBot). And if the numbers are the same the result will be zero, which will set the Z flag to zero, which will trigger the branch. Is that correct? Thanks everyone, I will post the rest of my questions in the newbie section. THANKS!

When any result is zero, the Z flag is SET. It gets a value of 1. When the result is NON-zero, the Z flag is CLEAR (it gets a value of 0). It's a bit backward, but that's the way it works. Thus, "BEQ" will branch if the Z flag is 1 (which is set to 1 when the result of a comparison -- which is effectively a virtual subtraction -- is 0). And, "BNE" will branch if the Z flag is 0 (which is set to 0 when the result of a comparison is NON-zero).

So,

lda #0 ; 0 into accumulator, Z-flag becomes 1
beq iszero ; branch IS taken (beq branches if Z=1)

lda #1 ; 1 into accumulator, Z-flag becomes 0
bne isntzero ; branch IS taken



pseudocode for Z-flag
if result after a 'calculation' is 0, set the Z flag to 1
else set the Z flag to 0

a 'calculation' is defined as modifying any value in any register (eg: through a load, an arithmetic or logical operation) OR by performing a comparison operation which will not modify any registers but will modify the flags as if a subtraction had been done with the carry flag assumed set during the pseudo-subtraction.

[karnov]

Thanks guys, I totally understand how the Z flag, BEQ and BNE work. Mr. Davie, thats the kind of detail I appreciate, It's the closest thing to a teacher standing behind me. Thanks again! I will be posting more questions soon. THANKS.