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Old 03 February 2021, 11:01   #1
teppic
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Does WinUAE distinguish between 16bit and 32bit fast RAM?

For example, on an A500/A2000 a normal RAM expansion is on the 16bit bus like the onboard RAM, but almost all 68020+ accelerator cards allowed for installing 32bit RAM to match the 32bit data path of the CPU.

In WinUAE's GUI the memory isn't shown any differently so I wondered if it was emulated or if all RAM was just set to the same width as the CPU -- e.g. having a 68020 stuck on 16bit RAM would slow it down a lot on a real machine.
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Old 03 February 2021, 22:12   #2
kfasheldon
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If I am not mistaken Zorro2 memory will be set as 16bit as 68000 is a 16bit external cpu and Zorro3 32 bit - 68020+ being full 32bit, introduced on the A3000 onwards.

In the real world the 2mb Chip memory on an a1200 is 32bit but chipset access slows this down and additional memory would be fast ram and this would be 32bit 'Fast RAM' - no chip set access and thus can be accessed by the cpu around double the speed of chip ram. The ram is accessed via the Budgie chip which is 16/32 bit and so may hold back access against direct access on an accelerator card ram.

Still you can select Chip, Slow (A1000), Zorro 2 16bit, Zoro 3 32bit and CPU Ram in WinUAE so it should fall into place - not sure if any slowdown is offered in 16 bit though - seems kind of pointless unless chip ram for timing.
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Old 04 February 2021, 03:38   #3
teppic
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Zorro II just means how and where the memory is configured, rather than the bit width. Many A500/A2000 accelerators configured 32bit Zorro II memory, especially if they had the EC CPU versions which were limited to 16mb address space.

If you install RAM on supported accelerator cards like these in WinUAE it shows up as Zorro II in the RAM settings, but it looks exactly the same as 16bit Zorro II RAM, which is why I was wondering if expansion memory is just matched to the CPU rather than emulated as it would be on real hardware.
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Old 04 February 2021, 19:10   #4
Toni Wilen
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4.5 betas already made it user configurable.

"Added force 16-bit checkbox to RAM GUI panel. Affects more compatible and cycle-exact CPU modes only. If 68020+ and 16-bit: CPU always splits 32-bit accesses to two 16-bit accesses. Defaults have not changed, chip ram is 32-bit if A3000 or AGA, all other RAM is 32-bit if 68020+ CPU. Most common use case is to be able to change "slow" RAM bus width. ("Real" 16-bit "slow" ram vs 32-bit accelerator board "slow" ram)"
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Old 05 February 2021, 10:34   #5
teppic
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Good timing for the question, I had no idea this was about to be added.

So if I understand correctly, on pre A3000 machines if you add a 32bit CPU accelerator then by default any regular 16bit expansion RAM (slow or fast) is simply treated as 32bit, but in 4.5 these can be individually emulated as 16bit?

I'm not sure what 32bit accelerator slow ram is though - since this is never accessed by the chipset isn't it always fast?
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Old 05 February 2021, 21:30   #6
Toni Wilen
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Quote:
Originally Posted by teppic View Post
So if I understand correctly, on pre A3000 machines if you add a 32bit CPU accelerator then by default any regular 16bit expansion RAM (slow or fast) is simply treated as 32bit, but in 4.5 these can be individually emulated as 16bit?
Yes. Every configurable RAM type has "Force 16-bit" checkbox. Default is 32-bit if 68020+.

Quote:
I'm not sure what 32bit accelerator slow ram is though - since this is never accessed by the chipset isn't it always fast?
Yes. It would be quite stupid accelerator if it has 32-bit CPU but 16-bit RAM
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Old 06 February 2021, 13:47   #7
teppic
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Quote:
Originally Posted by Toni Wilen View Post

Yes. It would be quite stupid accelerator if it has 32-bit CPU but 16-bit RAM
I think there were a few accelerators for the A500 that had no option at all to install RAM - so people would not only be using it with 16bit RAM but their expensive new 68020 would still be waiting for memory access with chip RAM. I expect these barely ran any faster than the stock 68000 for most uses.
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