Details of the AGA double CAS fetch mode?

[Leffmann]

I've always used both 32 bit and double CAS mode, thinking that bitplane DMA will actually fetch 64 bits in a single clock cycle, but isn't the data bus 32 bits only? Does anyone know the electronics so to speak of the double CAS mode, and if double CAS, when used on its own, is faster than plain 16-bit mode on real hardware?

The reason I'm asking is that I've got no A1200 to test with but I've seen that in WinUAE double CAS by itself is faster than plain 16-bit fetch, but not as fast as 32-bit fetch, and so I can't make much sense of it.

Anyone who knows the details?

Thanks

[Toni Wilen]

Originally Posted by Leffmann

I've always used both 32 bit and double CAS mode, thinking that bitplane DMA will actually fetch 64 bits in a single clock cycle, but isn't the data bus 32 bits only? Does anyone know the electronics so to speak of the double CAS mode, and if double CAS, when used on its own, is faster than plain 16-bit mode on real hardware?

I wouldn't call it faster, maybe mode with more bandwidth is better description

It has 2x more bandwidth. 256 color superhires wouldn't be possible without double cas mode. (ecs shres uses all bandwidth = 2planes, 32-bit mode doubles it = max 4 planes so double cas has to have 2x more bandwidth than 32-bit mode)

Not sure how it works in chipset level, perhaps some kind of double data rate transfer? (2x32)

The reason I'm asking is that I've got no A1200 to test with but I've seen that in WinUAE double CAS by itself is faster than plain 16-bit fetch, but not as fast as 32-bit fetch, and so I can't make much sense of it.

You won't get real results in AGA mode except by using copper. (bitplane dma and copper dma are the only cycle-exact things in AGA modes, they have to be or all copper effects would break)

Blitter is always only approximated in AGA mode.

[Leffmann]

Did some tests with the copper instead of the blitter, and now the results were identical when switching between 32-bit and double CAS, and as expected when using both, so I'm content But can I ask why the blitter can only be approximated in AGA mode?

[Toni Wilen]

Originally Posted by Leffmann

But can I ask why the blitter can only be approximated in AGA mode?

Permission granted


..


.


Because it isn't really the biggest problem in AGA emulation.

Problem is CPU. 68020 CPU is really slow if it needs to fetch data/instructions from chipram and other DMA sources are taking most bandwith already but CPU can also be really fast if code is already in instruction cache and there is no memory accesses. This is the biggest compatibility problem without easy fix.