GameBoy Color DMA-Transfers v0.0.1                           13.04.2000/GPZ-HIT
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"old" DMA Transfer (aka 'OAM-DMA')
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source:      - anywhere from $0000 to $dfff
             - must be page-aligned
destination: - always OAM-Ram at $fe00 - $fe8f
length:      - always 4*40 (=160 / $a0) bytes

- when cpu speed is 2x, transfer speed is also 2x
- transfer takes 671 cycles (160 microseconds)

rDMA   $FF46 - source LSB

CAUTION:

- dma should be executed during v-blank period
- no memory except for HRAM is available for the cpu while DMA is in progress,
  the common method is to place this routine in HRAM:

dma:
    ld a,$c0        ; oam-buffer is at $c000
    ld [rDMA],a

    ld a,$28        ; wait until DMA completes (160 microseconds)
.lp
    dec a
    jr nz,.lp
    ret


"new" DMA Transfer
------------------

source:      - from $0000 to $7fff and $a000-$dfff
             - must be page-aligned
destination: - always in VRAM at $8000-$9fff
             - must be page-aligned
length:      - can be set in 16-bytes increments (ie, its ALWAYS n*16 long),
               (1*16)=$0010 to (128*16)=$0800 bytes

- DMA speed is constant regardless of CPU-Speed
- cpu is halted during dma transfer
- transfer takes 220+n*7,68 microseconds (1x speed)
                 110+n*7,68 microseconds (2x speed)

- has 2 different modes:
  1)    Horizontal blanking DMA (aka 'HDMA')
        - transfers 16 bytes at each horizontal blank
        - can be stopped/restarted at each h-blank period
  2)    General Purpose DMA (aka 'GDMA')
        - can not be stopped or suspended once started

rHDMA1 $ff51    bit 7-0     bit 7-0 of Source MSB
rHDMA2 $ff52    bit 7-4     bit 7-4 of Source LSB
                bit 3-0     unused (set to 0)
rHDMA3 $ff53    bit 7-5     unused (set to 0)
                bit 4-0     bit 4-0 of Destination MSB
rHDMA4 $ff54    bit 7-4     bit 7-4 of Destination LSB
                bit 3-0     unused (set to 0)
rHDMA5 $ff55    bit 7       DMA Mode / Control

                            read:
                            0 : DMA in progress
                            1 : DMA completed

                            write:
                            - after set to 0, GDMA starts
                            - after set to 1, HDMA starts at next h-blank
                              - once started, HDMA can be stopped at next
                                h-blank by setting this bit to 0

                bit 6-0     length (number of 16-byte blocks - 1)
                            - amount of bytes transferred is (n+1)*$10

CAUTION:

- don't switch ROM banks if DMA source is in the range of $4000-$7fff
- don't switch ROM banks if DMA source is in the range of $d000-$dfff
- don't switch VRAM banks until DMA has completed
- halt can not be used while a DMA is in progress
- dma must have completed before another dma is started or registers are
  altered
- display must be enabled for a HDMA transfer taking place
- GDMA is only reliable during v-blank when display is enabled