SolderGirl wrote: ↑
Fri Aug 10, 2018 10:56 pm
If that is correct, it would be perfect.
Because PCIe works with a packet protocol, with packets of 4 x 32bit words each. So That is exactly 16 bytes packets. That would fit nicely.
"The frame format for PCIe is shown in the graphic below. The frame is made up of a 1-byte Start-of-Frame, 2-byte Sequence Number, 16 or 20-byte Header, 0 to 4096-byte Data field, 0 to 4-byte ECRC field, 4-byte LCRC, and 1-byte End-of Frame. The smaller the number of bits transferred in the data field the greater the over-head becomes. A zero byte data field results in a 100 percent over-head, because no data was transferred."
From this, it looks like a typical Frame would be something like 28 bytes + Data, but i don't know the data formats used.
Also i don't know how flexible the block sizes for the DMA port are
You want to use protocol what is used on min. 1000x faster computers. How looks drawing of 1 letter to screen ?
Here is example in 1 bit plane: write 1 byte, incr. dest address by 80 (640px width) , write next byte ... repeat it 14 time yet (for 16 px height).
With PCI protocol: need to write address in header, I guess, then 1 byte data, then ECRC, LCRC - ah, who will calculate it ? You need extra chips for that, because 8 MHz 68000 will take ages for that. Repeat all it 16 times totally. Even without ECRC it will be at least 10x slower that straight write to video RAM. With more bitplanes it can be little better, still much slower than staright write.
When it can be good ? Indeed in case of transferring larger data block at once. So, let's say drawing some nice photo on screen. Then can use DMA with it's 512 byte blocks well. Even calculating those CRCs with 68000, maybe - depends on speed with 68K and availability of CRC chips.
Desktop and usual VDI, GEM graphic will be just too slow.
I say that this hybrid - of old architecture with latest graphic cards, which btw. have at least 100x more RAM than whole Atari ST is just not good idea.
There are already working solutions with ISA and PCI cards. Basically all solutions with USB on Ataris are slow. This will be case with PCIe too . Just much worse.
There is 2 kind of people: one thinking about moving to Mars after here becomes too bad, the others thinking about how to keep this planet habitable.