org &0040   ;Game Boy's vblank IRQ goes here.
;This is not part of the standard Z80 vector table - interrupts work differently on the Game Boy.
jp &ff80

;more code goes here

;during setup, we'll CALL SetupDMA before enabling the vblank IRQ.


SetupDMA:
	ld bc,DMACopyEnd-DMACopy               ;how many bytes to copy
	ld hl,DMACopy                          ;pointer to source
	ld de,&ff80                            ;pointer to destination
	z_ldir                                 ;macro for LDIR which the game boy doesn't have.
        ret

DMACopy:					;must be run from &ff80
	push af
	ld a,>GBSpriteCache			;high byte of wherever we're storing our object attribute memory
	gb_out <dma				;start the transfer - DMA auto-copies 256 bytes from xx00-xxFF where xx = >GBSpriteCache
	ld a,&28				;delay - this ensures the DMA is done before we exit. Adjust to your liking.

;game boy doesn't have in/out commands, rather all its I/O ports are at &FFxx so there are special commands just for accessing them faster
;gb_out is a macro that inlines the bytecode, since not all assemblers auto-convert LD (&FFxx),a.

DMACopyWait:
	dec a
	jr nz,DMACopyWait
	pop af
        reti
DMACopyEnd: