VDP command reference

• Set register
• Set write address
• Set read address
• Start DMA

Set register

Writing to a VDP register is done by writing a 16-bit value (a single word) to $C00004 (VdpCtrl) with this format:

$8000 OR (register << 8) OR value

For a full list of registers and their values, check the VDP register reference.

Set write address

To write to video memory you need to write a 32-bit value to $C00004 (VdpCtrl), either as a longword or two separate words. Bits 13-10 of the address are placed in the high word, bits 15-14 are placed in the low word's bits 1-0:

((address AND $3FFF) << 16) OR
((address AND $C000) >> 14) OR
command

The command is one of the following:

After that, you can write the data to $C00000 (VdpData), either words or longwords (the latter are seen as two words). After every word written the address is automatically incremented by the autoincrement amount.

Set read address

Reading from video memory is similar to writing, the main difference is the command:

After that, you can read the data from $C00000 (VdpData), either words or longwords (the latter are seen as two words). After every word read the address is automatically incremented by the autoincrement amount.

Start DMA

Again, setting the DMA destination address is similar to setting up an address for writing, with the only difference being the command used (in fact, they're the same aside from setting bit 7 of the command's second word):

In the case of DMA transfer or DMA copy, the operation will start immediately after writing this value. In the case of DMA fill, you need to write the fill value to $C00000 (VdpData) after this to initiate the DMA fill.