Description

The VDP can perform [1] 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

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

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

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.

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 :

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.

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 :

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.

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
Memory to VRAM	H40	During active scan : 18 During VBLANK : 205
VRAM Fill	H32	During active scan : 15 During VBLANK : 166
VRAM Fill	H40	During active scan : 17 During VBLANK : 204
VRAM Copy	H32	During active scan : 8 During VBLANK : 83
VRAM Copy	H40	During active scan : 9 During VBLANK : 102

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