Difference between revisions of "Sega Mega Drive/DMA"
From Sega Retro
(Created page with "'''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 t...") |
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{{bitfield|L7-L0;8}}{{bitfield|H7-H0;8}} | {{bitfield|L7-L0;8}}{{bitfield|H7-H0;8}} | ||
| − | * '''L7-L0:''' Low byte of DMA length in bytes, divided by | + | * '''L7-L0:''' Low byte of DMA length in bytes, divided by 2. |
| − | * '''H7-H0:''' High byte of DMA length in bytes, divided by | + | * '''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): | Registers $15, $16 and $17 set the DMA source (68k address in RAM, ROM or SRAM): | ||
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: <tt>move.w #$8F02,($c00004).l</tt> | : <tt>move.w #$8F02,($c00004).l</tt> | ||
| − | : <tt>move.w #$9300+(($xxxx | + | : <tt>move.w #$9300+(($xxxx>>1)&$FF),($c00004).l</tt><br><tt>move.w #$9400+((($xxxx>>1)&$FF00)>>8),($c00004).l</tt> |
: <tt>move.w #$9500+(($yyyyyy>>1)&$FF),($c00004).l</tt><br><tt>move.w #$9600+((($yyyyyy>>1)&$FF00)>>8),($c00004).l</tt><br><tt>move.w #$9700+((($yyyyyy>>1)&$7F0000)>>16),($c00004).l</tt> | : <tt>move.w #$9500+(($yyyyyy>>1)&$FF),($c00004).l</tt><br><tt>move.w #$9600+((($yyyyyy>>1)&$FF00)>>8),($c00004).l</tt><br><tt>move.w #$9700+((($yyyyyy>>1)&$7F0000)>>16),($c00004).l</tt> | ||
Revision as of 16:43, 9 June 2018
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:
- 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):
- 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
2: Set destination
Sending the following longword to the control port will both set the target VRAM/CRAM/VSRAM address and trigger the DMA:
- 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 |