BLTxDAT wassat ?

[LeCaravage]

Hi,

I wonder what are these registers $dff070, $dff072, $dff074 ?

In simple words please ^^

Are they usefull ? Could they be used in certain circumstances ?
Lot of interrogations in fact.

[paraj]

If you haven't already, I suggest reading up on how the blitter works in general.

In simple terms the blitter allows you to combine bits (read as 16-bit words) from 3 sources: A, B and C and write them somewhere (D). How they're combined depends on the MINTERM (part of BLTCON0).

The three sources will often be read from memory (BLTxPT), but if you want to use the same value for either A, B or C for all of the operations, you can write a constant value to BLTxDAT before starting the operation, and it will be used throughout. This is purely an optimization though, you could always achieve the same thing by having a (possibly large) prefilled area with the value you need, and point BLTxPT there.

[LeCaravage]

Originally Posted by paraj

but if you want to use the same value for either A, B or C for all of the operations, you can write a constant value to BLTxDAT before starting the operation, and it will be used throughout. This is purely an optimization though

Interesting, so we could possibly use this feature to fill an area with a pattern.

[DanScott]

Yes, you could do a D=A blit, but disable the A DMA channel, and put a 16bit value into BLTADAT, and it would fill with that value