Sega Mega Drive/DMA

DMA (Direct Memory Access) is a method for transfering data from the 68k RAM or ROM to VRAM, CRAM or VSRAM without needing to write code to send it via the data port. It can also be used to fill VRAM with a specified value, or perform a VRAM to VRAM copy.

DMA is generally faster than the equivalent 68k code, though during transfer the 68k CPU is frozen. For this reason, it is recommended only to use DMA during VBlank. The 68k is not frozen during VRAM fill and VRAM to VRAM copy operations, and the Z80 CPU will run as normal during any DMA operation unless it attempts to access the bus.

68k to VRAM/CRAM/VSRAM copy

1: Set length and source

Registers $13 and $14 set the DMA length:

7

6

5

4

3

2

1

0

L7-L0

7

6

5

4

3

2

1

0

H7-H0

• L7-L0: Low byte of DMA length in bytes, divided by 2.
• H7-H0: High byte of DMA length in bytes, divided by 2.

Registers $15, $16 and $17 set the DMA source (68k address in RAM, ROM or SRAM):

7

6

5

4

3

2

1

0

L7-L0

7

6

5

4

3

2

1

0

M7-M0

7

6

5

4

3

2

1

0

0

H6-H0

• L7-L0: Low byte of DMA source address, divided by 2.
• M7-M0: Middle byte of DMA source address, divided by 2.
• H6-H0: High byte of DMA source address, divided by 2.

The following code will set the appropriate registers:

move.w #$8F02,($c00004).l

move.w #$9300+(($xxxx>>1)&$FF),($c00004).l
move.w #$9400+((($xxxx>>1)&$FF00)>>8),($c00004).l

move.w #$9500+(($yyyyyy>>1)&$FF),($c00004).l
move.w #$9600+((($yyyyyy>>1)&$FF00)>>8),($c00004).l
move.w #$9700+((($yyyyyy>>1)&$7F0000)>>16),($c00004).l

N.B. The source data should all be located within the same 128kB ($20000) section of ROM. For example, attempting to read $400 bytes from the address $1FFE00 will result in half the data being read from the wrong location, because it overflows the first 128kB section. Section boundaries are located at $20000, $40000, $60000, $80000, $A0000 and so on.

2: Set destination

Sending the following longword to the control port will both set the target VRAM/CRAM/VSRAM address and trigger the DMA:

7

6

5

4

3

2

1

0

CD1

1

A13-A8

7

6

5

4

3

2

1

0

A7-A0

7

6

5

4

3

2

1

0

0

0

0

0

0

0

0

0

7

6

5

4

3

2

1

0

1

0

0

CD2

0

0

A15

A14

• A15-A0: VRAM/CRAM/VSRAM address.
• CD2-CD1: 00 = VRAM; 01 = CRAM; 10 = VSRAM.

Memory space	CD2-CD1	Code
VRAM	00	move.l #$40000080+(($zzzz&$3FFF)<<16)+(($zzzz&$C000)>>14),($C00004).l
CRAM	01	move.l #$C0000080+($zz<<16),($C00004).l
VSRAM	10	move.l #$40000090+($zz<<16),($C00004).l

VRAM to VRAM copy

1: Set length and source

Registers $13 and $14 set the DMA length:

7

6

5

4

3

2

1

0

L7-L0

7

6

5

4

3

2

1

0

H7-H0

• L7-L0: Low byte of DMA length in bytes, divided by 2.
• H7-H0: High byte of DMA length in bytes, divided by 2.

Registers $15, $16 and $17 set the DMA source (VRAM address):

7

6

5

4

3

2

1

0

L7-L0

7

6

5

4

3

2

1

0

H7-H0

7

6

5

4

3

2

1

0

1

1

0

0

0

0

0

0

• L7-L0: Low byte of DMA source address.
• H7-H0: High byte of DMA source address.

The following code will set the appropriate registers:

move.w #$8F01,($c00004).l

move.w #$9300+(($xxxx>>1)&$FF),($c00004).l
move.w #$9400+((($xxxx>>1)&$FF00)>>8),($c00004).l

move.w #$9500+($yyyy&$FF),($c00004).l
move.w #$9600+(($yyyy&$FF00)>>8),($c00004).l
move.w #$97C0,($c00004).l

2: Set destination

Sending the following longword to the control port will both set the target VRAM address and trigger the DMA:

7

6

5

4

3

2

1

0

0

0

A13-A8

7

6

5

4

3

2

1

0

A7-A0

7

6

5

4

3

2

1

0

0

0

0

0

0

0

0

0

7

6

5

4

3

2

1

0

1

1

0

0

0

0

A15

A14

• A15-A0: VRAM address.

The following code will set the target VRAM address and trigger the DMA:

move.l #$000000C0+(($zzzz&$3FFF)<<16)+(($zzzz&$C000)>>14),($C00004).l

VRAM fill

1: Set length

Registers $13 and $14 set the DMA length:

7

6

5

4

3

2

1

0

L7-L0

7

6

5

4

3

2

1

0

H7-H0

• L7-L0: Low byte of DMA length in bytes, divided by 2.
• H7-H0: High byte of DMA length in bytes, divided by 2.

The following code will set the appropriate registers:

move.w #$8F01,($c00004).l

move.w #$9300+(($xxxx>>1)&$FF),($c00004).l
move.w #$9400+((($xxxx>>1)&$FF00)>>8),($c00004).l

move.w #$9780,($c00004).l

2: Set destination

Sending the following longword to the control port will set the target VRAM address:

7

6

5

4

3

2

1

0

0

0

A13-A8

7

6

5

4

3

2

1

0

A7-A0

7

6

5

4

3

2

1

0

0

0

0

0

0

0

0

0

7

6

5

4

3

2

1

0

1

1

0

0

0

0

A15

A14

• A15-A0: VRAM address.

The following code will set the target VRAM address:

move.l #$40000080+(($yyyy&$3FFF)<<16)+(($yyyy&$C000)>>14),($C00004).l

3: Set fill value

Sending the following word to the data port will set the fill value:

7

6

5

4

3

2

1

0

F7-F0

7

6

5

4

3

2

1

0

0

0

0

0

0

0

0

0

• F7-F0: Fill value.

The following code will set the fill value and trigger the DMA:

move.w #$zz00,($c00000).l

Bugs and peculiarities

• If 68k to VRAM length is set to 0, it will interpret this as a length of 65,536 words instead (exceeding the capacity of the VRAM and causing an overflow). This is because the VDP decrements the length counter before checking if it's reached zero. 68k to CRAM or VSRAM copies won't overflow in the same way because they are automatically stopped after copying to the last available byte.
• As mentioned above, source data must all sit within the same 128kB section of ROM. Since the high byte is not incremented, DMA will read from the start of the 128kB section when it overflows.

Bandwidth

The following values are the maximum number of bytes that can be written by DMA in a single frame, either during the frame (not recommended) or between frames (in VBlank). The values most likely to be useful are in bold.

Type		Resolution	During VBlank	During active display	Total
68k to VRAM/CRAM/VSRAM copy	60 Hz	256 x 224	6118	3584	9702
		320 x 224	7524	4032	11556
	50 Hz	256 x 224	14329	3584	17913
		320 x 224	17622	4032	21654
		256 x 240	11753	3840	15593
		320 x 240	14454	4320	18774
VRAM to VRAM copy	60 Hz	256 x 224	6308	3360	9668
		320 x 224	7752	3808	11560
	50 Hz	256 x 224	14774	3360	18134
		320 x 224	18156	3360	18134
		256 x 240	12118	3600	15718
		320 x 240	14892	4080	18972
VRAM fill	60 Hz	256 x 224	3154	1792	4946
		320 x 224	3876	2016	5892
	50 Hz	256 x 224	7387	1792	9179
		320 x 224	9078	2016	11094
		256 x 240	6059	1920	7979
		320 x 240	7446	2160	9606

Summary

Type	Variables	Code
68k to VRAM/CRAM/VSRAM copy	$xxxx = length in bytes $yyyyyy = source address $zzzz = target address	move.w #$8F02,($c00004).l move.w #$9300+(($xxxx>>1)&$FF),($c00004).l move.w #$9400+((($xxxx>>1)&$FF00)>>8),($c00004).l move.w #$9500+(($yyyyyy>>1)&$FF),($c00004).l move.w #$9600+((($yyyyyy>>1)&$FF00)>>8),($c00004).l move.w #$9700+((($yyyyyy>>1)&$7F0000)>>16),($c00004).l to VRAM: move.l #$40000080+(($zzzz&$3FFF)<<16)+(($zzzz&$C000)>>14),($C00004).l to CRAM: move.l #$C0000080+($zz<<16),($C00004).l to VSRAM: move.l #$40000090+($zz<<16),($C00004).l
VRAM to VRAM copy	$xxxx = length in bytes $yyyy = source address $zzzz = target address	move.w #$8F01,($c00004).l move.w #$9300+(($xxxx>>1)&$FF),($c00004).l move.w #$9400+((($xxxx>>1)&$FF00)>>8),($c00004).l move.w #$9500+($yyyy&$FF),($c00004).l move.w #$9600+(($yyyy&$FF00)>>8),($c00004).l move.w #$97C0,($c00004).l move.l #$000000C0+(($zzzz&$3FFF)<<16)+(($zzzz&$C000)>>14),($C00004).l
VRAM fill	$xxxx = length in bytes $yyyy = target address	move.w #$8F01,($c00004).l move.w #$9300+(($xxxx>>1)&$FF),($c00004).l move.w #$9400+((($xxxx>>1)&$FF00)>>8),($c00004).l move.w #$9780,($c00004).l move.l #$40000080+(($yyyy&$3FFF)<<16)+(($yyyy&$C000)>>14),($C00004).l