486 Cache size and speed questions

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486 Cache size and speed questions

Postby superchad » 2018-5-15 @ 03:50

I am curious about something, did some research but so far can't find concrete evidence

What difference does the cache size make with a 486

128KB vs 256KB
256KB vs 512KB
512KB vs 1MB

and what difference does cache speed make
20NS vs 15NS
15NS vs 12NS

do they make any performance difference for DOS or Windows gaming? even if very slight differences
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Re: 486 Cache size and speed questions

Postby noshutdown » 2018-5-15 @ 05:08

cache size difference has slight impact on 386 and 486 performance. the most significant importance of cache size is the range of cacheable ram: 128kb can only cache 32mb of ram, while 1mb cache can cache 256mb of ram. cacheable ram is halved if you set cache to write-back mode.
for cache speed, there are only two grades of cache timing to choose from: 2-1-1-1 and 3-2-2-2. 3-2-2-2 is much slower than 2-1-1-1.
20ns allows 2-1-1-1 up to 25fsb, anything higher than that would have to live with 3-2-2-2.
15ns allows 2-1-1-1 up to 33fsb, which is default for most 486 machines.
12ns cache is rare but shall allow 2-1-1-1 up to 40fsb.
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Re: 486 Cache size and speed questions

Postby amadeus777999 » 2018-5-15 @ 08:36

I have one board with only 128KB of ram and the speed difference is negligible - the mode is set to write through though, to yield 32 instead of 16 MB cacheable area(8K cache per 1MB ram in WB mode - 1:128).

The front side bus speed you can run at tight timings does depend not only on the chips latency(e.g. 15ns) but also strongly on the batch it came from. I have 15ns UMC srams(UM61M256K-15 / 9534) which go higher than 12ns ones clock wise. Then there's a bunch of Alliance chips which do not go beyond 33mhz and are marked at 15ns too.

The difference regarding latency values is that they should be guaranteed to run at frequency blablabla(+a certain setting) no matter what/without error. From experience this is not true for all chips(see above). Also the values can be misleading as a 15ns IC should be able to theoretically run at up to ~66mhz(1000/15). This is not likely possible due to the demand on the sram itself which I guess somebody more knowledgeable will be able to explain.

Addendum:
Best thing is to find a seller with a bigger number of ICs and buy a few with different date codes and test them in a board as a tag ram paired with a set known to work flawlessly at higher speeds.

When I have bought srams I use the LuckyStar board at 60mhz fsb(+tightest timings) as a test system. Insert new sram as tag, boot and when it makes it into DOS I add it to the "good stash" which later runs at 50mhz through memtest and Winbech a few hours.
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Re: 486 Cache size and speed questions

Postby superchad » 2018-5-15 @ 15:14

how do i set the timingss, I don't see any timing options in BIOS for either of the two boards i have

also is 60MHz FSB even possible on a 486?
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Re: 486 Cache size and speed questions

Postby amadeus777999 » 2018-5-16 @ 11:46

There must be a timings option unless it's an OEM board with a locked BIOS. Go to "Advanced Chipset Options" usually in the 3rd row, 1st colum on the BIOS screen. There should be settings regarding the cache wait-, ram wait-states and other options(maybe vga, dma, controller related).

More modern boards with a SiS496/497 or UMC8881/8886 chipset may support frequencies up to 66mhz front side bus(some crystals go even higher BUT this is more a theoretical than practical option).
Be aware that the most kinky setting in terms of speed is usually 40mhz fsb where the PCI bus and the (cache)ram when run full steam(ratio 1:1 and lowest wait states) usually deliver pretty good performance.
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Re: 486 Cache size and speed questions

Postby Intel486dx33 » 2018-5-18 @ 05:41

superchad wrote:I am curious about something, did some research but so far can't find concrete evidence

What difference does the cache size make with a 486

128KB vs 256KB
256KB vs 512KB
512KB vs 1MB

and what difference does cache speed make
20NS vs 15NS
15NS vs 12NS

do they make any performance difference for DOS or Windows gaming? even if very slight differences


I just upgraded the cache in my Gateway 486dx 33 computer with Micronics JX30G mobo.
It originally had only 128kb of cache so I had some cache laying around from 1995 and put it in.
The original cache was at -15 but the NEW cache I added was at -20
Does not seem to impact performance. The computer works fine. I just had to add the cache and reset some mobo jumpers and adjust the bios settings. Works fine with mixed speed cache. I now have 512kb of cache. It is suppose to make the computer perform better by about a factor of about 10%
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