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Old 08 April 2019, 16:47   #1
roondar
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Amiga 68020+ external cache use?

I've recently read that one of the features of the 68020 and later (or perhaps this started with the 68030) is the ability to use external cache memory to speed up the system.

Given that such extra cache memory was used to great effect on Intel 386 and 486+ systems, that made me wonder - were there any Amiga accelerators/CPU boards that supported or used external cache?

And if so, did they actually run faster?

The CPU command suggests this feature is at least supported (the 3.1 reference for the CPU command has the EXTERNALCACHE and NOEXTERNALCAHE options), but I can't recall ever seeing it in action.
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Old 08 April 2019, 20:08   #2
Thomas Richter
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Quote:
Originally Posted by roondar View Post
I've recently read that one of the features of the 68020 and later (or perhaps this started with the 68030) is the ability to use external cache memory to speed up the system.
This is not really CPU specific. In principle, one can design such a circuit for any CPU.


Quote:
Originally Posted by roondar View Post
Given that such extra cache memory was used to great effect on Intel 386 and 486+ systems, that made me wonder - were there any Amiga accelerators/CPU boards that supported or used external cache?
Not that I know of, but I believe something like this may have existed as an upgrade product for 68K based Macs. Since this is ages ago, my memory might be fading. You may try to find something in the internet, maybe there is even a product review available.





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Originally Posted by roondar View Post

The CPU command suggests this feature is at least supported (the 3.1 reference for the CPU command has the EXTERNALCACHE and NOEXTERNALCAHE options), but I can't recall ever seeing it in action.
The call is actually a bogus. It goes into the exec.library, but it has no idea about external caches. If a vendor would have offered such a product, it would have been necessary to supply an additional software that patches into the exec cache control functions and from there controls the cache in some proprietary way.
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Old 08 April 2019, 22:55   #3
AJCopland
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Yep Thomas you remember correctly
http://www.micromac.com/products/iic...ache_card.html
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Old 09 April 2019, 02:59   #4
Rotareneg
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The 68030 added a cache inhibit output signal to make controlling an external cache easier. Also, the '030 manual goes into a bit of detail about how to implement an external cache while the '020 manual only mentions external caches in a couple passing remarks in reference to memory access timing.
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Old 09 April 2019, 03:24   #5
AmigaHope
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The only Amiga CPU cards I can think of that included cache memory were 68000-based accelerators like the SupraTurbo28. All other devices went all-or-nothing (no ability for memory onboard, or memory onboard is discrete fast RAM)
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Old 09 April 2019, 08:28   #6
Locutus
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The Cyberstorm MKI has a connector for a L2 Cache card, but no such card was ever released.
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Old 09 April 2019, 10:33   #7
roondar
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@Thomas: now that is some fascinating trivia about the CPU command! Forward thinking, but completely unsupported functionality - sounds like Commodore allright

@Locutus: Interesting to hear there was a connector but no card for the Cyberstorm. Wonder why it didn't see a release.

@Amigahope: I was unaware of the Supra 28 using cache memory. Fascinating to see only a 68000 based accelerator actually ended up using external cache on the Amiga. More so as fast cache external memory was one of the key reasons 386/486 systems could be much faster than you'd expect.

It does make me wonder why this wasn't done. On 386/486 systems the cache ram would be significantly faster than standard RAM (though I'm not sure of the exact specs - generally it was static RAM though). It wasn't uncommon for such a system to be 25% or more faster than a system without external cache.

Was this not something Amiga cards could've used (surely a static RAM cache would be much faster than any onboard Fast RAM?) or is there another reason for the absence of such things?
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Old 09 April 2019, 14:26   #8
zipper
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Quote:
Originally Posted by roondar
I was unaware of the Supra 28 using cache memory. Fascinating to see only a 68000 based accelerator actually ended up using external cache on the Amiga. More so as fast cache external memory was one of the key reasons 386/486 systems could be much faster than you'd expect.
Supra did have "huge" 16 kB static RAM cache and did need some Fast RAM to get benefit of it. It was a nice card after all.
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Old 12 April 2019, 11:07   #9
AmigaHope
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Supra did have "huge" 16 kB static RAM cache and did need some Fast RAM to get benefit of it. It was a nice card after all.
It was pretty much a necessity for products like the SupraTurbo and ICD AdSpeed, as they were positioned as budget accelerators with no onboard fast RAM, but since the 68000 lacked an internal cache they'd be reduced to a crawl by slow memory unless they were given some sort of cache.

Budget no-fastmem 68020 and 68030 cards like the Sapphire got away with it since the 020 and 030 had their (tiny) onboard caches, but the large-cache 68000 accelerators often wound up faster for code/data that didn't fit well in 256 bytes even though the 68000 took more cycles for the instructions themselves.

They are even faster than a stock A1200 thanks to chipmem latency starving the poor 68020.
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