Day 12 - VBlank And Interrupts

VBlank?

	
	If you don't know what VBlank is, pretend I'm hitting you with a trout. If
you do know what VBlank is, pretend I'm hitting you with a trout. If you have
no idea what a trout has to do with the NES, pretend I'm hitting you with a trout.

	Now that we have an imaginary fish stench in the room, let's learn a better
way to sync with VBlank.

How do we use it?

	First, we have a label, say 'VBlank_Routine', so we put it in the addresses
in Bank 1 at $FFFA. Like so:

	.bank 1
	.org $FFFA
	.dw VBlank_Routine ; address to execute on VBlank
	.dw Start ; address to execute at RESET.
	.dw 0  ; address to execute during a BRK instruction (Another Day, Another time).

	.bank 0
	.org $0000  ;variables
VBlankOrNo  .db 0
	.org $8000  ;code
VBlank_Routine:
	;start of function to execute on VBlank
	inc VBlankOrNo
	; add one (1) to VBlankOrNo, will be 1 if VBlank, 0 if not.
	rti  ; RTI is (Interrupt RETurn or ReTurn from Interrupt)

Start:
	;start of main code
	;blah some code
;waitforvblank:  old code
;	lda $2002  old code
;	bpl waitforvblank old code
;  the new code is :

WaitForVBlank:
	lda VBlankOrNo ; A = VBlankOrNO
	cmp #1         ; if A == 1 then is VBlank
	bne WaitForVBlank ; if not VBlank, then loop and do again
	dec VBlankOrNo ; 1-- or VBlankOrNO - 1 . VBlankOrNo will be 0 again.

	; blah, code as usual.


I hope you get the general idea. I may have gotten something backwards or something,
so if you know this is wrong, please tell me (I'm tired and may have not typed right!).

UPDATE: Don't forget to set the "Generate VBlank bit" in one of the PPU Control ports!
	(it's bit 7 of one of them, I think).

This Day In Review

	This method will, appearantely, be much more stable than our old loop.
	
	Greetings!,
		-Mike H a.k.a GbaGuy

Intro - Day 13