VDP DMA

Description

The VDP can perform DMA actions for copying data from ROM/RAM to its RAM, filling its RAM with a value or even copying from its RAM to its RAM.

It's quite useful cause DMA transfer is faster than doing direct access (in VBLANK only, during active scan it's the same).

During DMA the VDP stop the 68k as it's using it bus to access the data.

DMA Operations

Memory to VRAM

Step to perform a memory (ROM or RAM) to one of the VDP RAMs :

• Enable DMA in VDP register 0x1

• Set VDP auto-increment register (Usually 2 because source address in is increased by 2)

• Set DMA length (VDP registers 0x13 and 0x14)

• Set DMA source address (VDP registers 0x15, 0x16 and 0x17)

• Set the destination address (See setting RAM address) (Don't forget to set CD5 to 1)

• As soon as you set the destination address, the VDP halt the 68k and DMA operation starts.

• Disable DMA in VDP register 0x1

VRAM Fill

VRAM Fill is a DMA operation that allows you to fill the VRAM with one 16 bits value (a word).

Step to perform a VRAM Fill :

• Enable DMA in VDP register 0x1

• Set VDP auto-increment register (Usually 1)

• Set DMA length (VDP registers 0x13 and 0x14)

• Set VDP register 0x17 to 0x80 (Enable VRAM Fill)

• Set the destination address (See setting RAM address) (Don't forget to set CD5 to 1)

• Set the Fill data by writing the desired value to the data port

• As soon as you set the data, the DMA operation starts.

• Disable DMA in VDP register 0x1

VRAM Copy

VRAM Copy is a DMA operation that allows you to duplicate a region of the VRAM to another one.

Steps to perform a VRAM Copy :

• Enable DMA in VDP register 0x1

• Set DMA length (VDP registers 0x13 and 0x14)

• Set DMA source address (VDP registers 0x15 and 0x16)

• Set VDP register 0x17 to 0xC0

• Set the destination address (See setting RAM address) (Don't forget to set CD5 and CD4 to 1)

• As soon as you set the destination address, the VDP halt the 68k and DMA operation starts.

• Disable DMA in VDP register 0x1

DMA Transfer capacity

Note: This information is from SEGA and hasn't been verified

DMA Mode	Display Mode	Bytes per lines
Memory to VRAM	H32	During active scan : 16 During VBLANK : 167
	H40	During active scan : 18 During VBLANK : 205
VRAM Fill	H32	During active scan : 15 During VBLANK : 166
	H40	During active scan : 17 During VBLANK : 204
VRAM Copy	H32	During active scan : 8 During VBLANK : 83
	H40	During active scan : 9 During VBLANK : 102

When display is disable (See VDP register 0x1), transfer capacity is the same as in VBLANK.