Write buffer missing on PiStorm32-lite variant

[michalsc]

In contrast to 68040 or 68060 Emu68 in PiStorm32-lite variant does not provide any sort of write buffer which could merge single byte writes to chip memory into larger operations. This causes large slowdowns on configurations where both DMA and CPU are concurently trying to access the CHIP memory. The high pressure of DMA (e.g. by using 640x512x8bpp mode or larger) extends the wait states for write and read operations. When these are done on byte-basis, as it is the case in e.g. playing RLE-compressed ANIM files on RGB output, the Emu68 will crawl as it will perform single byte writes. Example bus timing during HAM8 anim playback:

Please note this is not a bug of Emu68 or PiStorm32 as write buffers are not mandatory and can be disabled e.g. on 68060 (default mode of operation - whole address space is treated as precise uncached). This is rather an enhancement which can allow to use as much of the memory bandwidth as possible.

Related issue: PiStorm/pistorm32-lite-gateware#1

[michalsc]

Write buffer turned out to be a nightmare with some software. Instead, 2-slot FPGA firmware is used. This solves the issue.