486 cache upgrade curiosity \ VOGONS

486 cache upgrade curiosity

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First post, by c0keb0ttle

Posted on 2017-04-07, 22:22

c0keb0ttle Offline

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Joined: 2016-10-23, 22:07

I have a 486 DX 33 MHz which I've just upgraded from 128 KB cache to 256 KB.

4x 32k8 + 8k8 TAG -> 8x 32k8 + 32k8 TAG.

One bank is 20 ns and the other bank (plus TAG) is 15 ns.

Everything seems to be working fine, and I got a ~10% boost in FPS in DOOM timedemo from Phil's benchmark pack.

One thing is a bit odd though. If I run CACHECHK it says I only have one type of cache installed, and it no longer sees my 8 KB of L1 cache.

This is what is looked like with 128 KB installed:

1 This machine seems to have both L1 and L2 cache. [reading]
2    L1 cache is    8KB--  34.5 MB/s  30.4 ns/byte (310%) (141%)      3.9 clks
3    L2 cache is  128KB--  24.3 MB/s  43.2 ns/byte (218%) (100%)      5.5 clks
4    Main memory speed --  11.1 MB/s  94.4 ns/byte (100%) [reading]  12.0 clks
5    Effective RAM access time (read ) is 377ns (a RAM bank is 4 bytes wide). 
6    Effective RAM access time (write) is 120ns (a RAM bank is 4 bytes wide). 
7    486 Clocked at 33.4 MHz.  Cache ENABLED.

And this is what it looks like with 256 KB installed:

1This machine seems to have one cache!? [reading]
2    !! cache is  256KB--  29.4 MB/s  35.7 ns/byte (264%)         
3 >>>> If you think you do have L2 cache, you might have FAKE CACHE chips! <<<<
4    4.5 clks
5    Main memory speed --  11.1 MB/s  94.4 ns/byte (100%) [reading]  12.0 clks
6    Effective RAM access time (read ) is 377ns (a RAM bank is 4 bytes wide). 
7    Effective RAM access time (write) is 120ns (a RAM bank is 4 bytes wide). 
8    486 Clocked at 33.4 MHz.  Cache ENABLED.

The CHKCPU utility still says I have L1 cache enabled.

1 CPU Identification utility v1.25                 (c) 1997-2016 Jan Steunebrink
2 ------------------------------------------------------------------------------
3 CPU Vendor and Model: 'classic' 486
4 Internal CPU speed  : 33.6 MHz
5 CPU-ID Vendor string: - No CPUID -
6 Current CPU mode    : Real
7 Internal (L1) cache : Enabled in Write-Through mode

Anyone have any idea what is going on with CACHECHK? Am I missing something?

Reply 1 of 3, by SarahWalker

Posted on 2017-04-08, 08:28

SarahWalker Offline

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Joined: 2007-08-19, 10:51

Going to 256 kb probably means your motherboard is using interleaved access to cache, boosting L2 cache throughput to the point that CACHECHK can't reliably determine the difference between L1 and L2 cache - the difference is below the threshold CACHECHK uses to determine a new cache level.

Put in a DX2/66 and it will boost L1 performance enough for it to detect both levels again 😀

Reply 2 of 3, by c0keb0ttle

Posted on 2017-04-08, 08:54

c0keb0ttle Offline

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Posts: 182
Joined: 2016-10-23, 22:07

Thanks for the tip! I'll try that and see what happens.

Reply 3 of 3, by c0keb0ttle

Posted on 2017-04-08, 16:29

c0keb0ttle Offline

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Rank: Member
Posts: 182
Joined: 2016-10-23, 22:07

The DX2 66 MHz is indeed reported with 8 KB L1 cache as you suspected, SarahWalker. Thanks again!

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