Writing to this register launches a DMA transfer from ROM or RAM to OAM (Object Attribute Memory). The written value specifies the transfer source address divided by $100, that is, source and destination are:
Source: $XX00-$XX9F ;XX = $00 to $DF Destination: $FE00-$FE9F
The transfer takes 160 machine cycles: 152 microseconds in normal speed or 76 microseconds in CGB Double Speed Mode. On DMG, during this time, the CPU can access only HRAM (memory at $FF80-$FFFE); on CGB, the bus used by the source area cannot be used (this isn’t understood well at the moment; it’s recommended to assume same behavior as DMG). For this reason, the programmer must copy a short procedure into HRAM, and use this procedure to start the transfer from inside HRAM, and wait until the transfer has finished:
run_dma: ld a, HIGH(start address) ldh [$FF46], a ; start DMA transfer (starts right after instruction) ld a, 40 ; delay for a total of 4×40 = 160 cycles .wait dec a ; 1 cycle jr nz, .wait ; 3 cycles ret
Because sprites are not displayed while an OAM DMA transfer is in progress, most programs execute this procedure from inside their VBlank handler. But it is also possible to execute it during display redraw (Modes 2 and 3), allowing to display more than 40 sprites on the screen (that is, for example 40 sprites in the top half, and other 40 sprites in the bottom half of the screen), at the cost of a couple lines that lack sprites due to the fact that during those couple lines the PPU reads OAM as $FF. Besides, graphic glitches may happen if an OAM DMA transfer is started during Mode 3.
A more compact procedure is
run_dma: ; This part is in ROM ld a, HIGH(start address) ld bc, $2846 ; B: wait time; C: LOW($FF46) jp run_dma_hrampart run_dma_hrampart: ldh [c], a .wait dec b jr nz, .wait ret db _NARG, WRAM0
This saves 5 bytes of HRAM, but is slightly slower in most cases due to the jump into the HRAM part.