Maximum amount of memory on a 486

Discussion about old PC hardware.

Re: Maximum amount of memory on a 486

Postby feipoa » 2017-11-16 @ 13:58

BitWrangler wrote:I thought 256k 512k or a meg didn't matter it was the tag RAM that defined cacheable area.

With direct-mapped cache, normally, you scale the TAG with the cache, so 1024K of cache would use 64kx8 TAG. I'm not really sure what would happen if you ran 256K of cache with a 64kx8 TAG, but I have a feeling that it isn't going to cache all the 256 MB in WT mode.
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Re: Maximum amount of memory on a 486

Postby Vallenrod » 2017-11-17 @ 00:25

Vallenrod wrote:... across DOS/32a (replacement for dos4gw) which used a little less memory to run.


Just a small correction: it was actually PMODE/W not DOS/32a... little detail
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Re: Maximum amount of memory on a 486

Postby feipoa » 2017-11-18 @ 00:59

luckybob wrote:I have two "high end" 486 boards. Both VIP and UMC 8433 chipsets (iirc), one is a pc chips with no cache and the other has sockets for up to 1mb of cache, but i cannot for the life of me find jumper settings, and there are like 30 jumpers on that board, so just winging it is a non-starter.

my simms are edo/fpm and I never considered even attempting them in a 486.

Both motherboards are in cases and they are currently buried in a pile at the moment. I'd be happy to get them out for you though.

Any luck?
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Re: Maximum amount of memory on a 486

Postby luckybob » 2017-11-18 @ 01:05

I've been binge watching Bojack Horseman since that post. I'll try tonight. hopefully.
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Re: Maximum amount of memory on a 486

Postby feipoa » 2017-11-18 @ 13:15

or tomorrow?
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Re: Maximum amount of memory on a 486

Postby luckybob » 2017-11-19 @ 07:17

In my defense, I've had to box 6 things for ebay today, and deal with a drive in my NAS going down. Not to mention getting all the things I needed for apple pie moonshine. ^.^

Both motherboards (pc chips and mitac) only reported 256mb like so:
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I forgot to take a picture of it, but when I went to 2x sticks, the ram dropped to 128mb

here are the simms i used.
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Either my boards don't support the 128mb per slot, or just not how it is wired on the simm. I know the simms work, eight of them gave me 1gb in my BITCH'N FAST DUAL PENTIUM PRO (tm) setup.
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Re: Maximum amount of memory on a 486

Postby feipoa » 2017-11-19 @ 09:08

Interesting... and thanks for testing this. Are those one sided, or two sided SIMMs?

Sounds like these boards may not act the same as the Biostar MB-8433UUD. When I use one stick of 128 MB FPM, it reads 128 MB. When you use one stick, does the system report 64 MB or 128 MB? I am using double-sided, 16-chip FPM SIMMs. If you have single-sided, 8-chip 128 MB SIMMs, I doubt this configuration would be supported.

I will test this myself soon, but my max configuration would be 128 + 128 + 64 + 64. I am currently held captive by an annoying 386 motherboard repair job.
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Re: Maximum amount of memory on a 486

Postby luckybob » 2017-11-19 @ 09:13

2 sided. 16 total chips.

this is the 2nd motherboard i tried it on: https://i.imgur.com/8MNdvdn.jpg

im offering a bounty for proper jumper settings...
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Re: Maximum amount of memory on a 486

Postby luckybob » 2017-11-19 @ 09:18

i will have to check tomorrow about the single stick. ive got round two with my nas in the morning.
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Re: Maximum amount of memory on a 486

Postby feipoa » 2017-11-19 @ 09:20

I haven't seen this particular board before. Does it have a Phoenix BIOS?

OK, 16-chip. Same as me, except the EDO vs. FPM. Curious about a single stick of 128 MB...
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Re: Maximum amount of memory on a 486

Postby luckybob » 2017-11-19 @ 09:22

award. its in the previous post. iirc I remember running the part numbers on the ram. I remember reading they were fpm/edo compatible.

i have slept since then, so take it with a grain of salt.
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Re: Maximum amount of memory on a 486

Postby feipoa » 2017-11-29 @ 08:36

I tested more than 256 MB of RAM on a Biostar MB-8433UUD v3.0 and any amount greater than 256 MB does not get reported. Also, two sticks of 128 MB will show up as 256 MB only if the SIMM modules are not placed adjacent to each other. So, only these combinations with modules inserted in: SIMM1 & SIMM3, SIMM1 & SIMM4, or SIMM2 & SIMM4. So if you want all the 256 MB cached, you need to use 1024KB of cache in write-through mode.

I was actually hoping with this thread that there might be some server type systems which sport 512 MB, perhaps those with dual CPU riser cards, or similar.
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Re: Maximum amount of memory on a 486

Postby luckybob » 2017-11-29 @ 08:48

hell, even the IBM 9595 systems only allow up to 256mb. (Side note, if anyone has 8x 32's let me know)
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Re: Maximum amount of memory on a 486

Postby FFXIhealer » 2017-11-29 @ 15:26

I'm just happy with my 4 sticks of 16MB each in my Pentium Windows 95 build. 64MB of memory in Windows 95 feels like an insane amount - more than games or the system could ever use. Back in the old DOS days of my father's 80486, we first ran with 8MB, then with 16MB. That upgrade felt huge. LOTS of space to play Star Wars: X-Wing, Doom, and Carmen Sandiego games, none of which ever required more than the original 8MB.
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Re: Maximum amount of memory on a 486

Postby Anonymous Coward » 2017-11-29 @ 23:53

feipoa wrote:
BitWrangler wrote:I thought 256k 512k or a meg didn't matter it was the tag RAM that defined cacheable area.

With direct-mapped cache, normally, you scale the TAG with the cache, so 1024K of cache would use 64kx8 TAG. I'm not really sure what would happen if you ran 256K of cache with a 64kx8 TAG, but I have a feeling that it isn't going to cache all the 256 MB in WT mode.


I think using a 64kx8 TAG with 256kb cache is possible, but it has to be explicitly supported by the chipset. I think it should be able to cache the full 256MB of RAM, but with a lower hit rate.

I have a 386SX board with 32KB cache. Most boards need 64kb to cache the full 16MB, but this one has some extra tag pieces, and seems to cache all 16MB too.
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Re: Maximum amount of memory on a 486

Postby feipoa » 2017-11-30 @ 02:50

Many of the AWARD 486 BIOSes have a TAG width option of 7+1 dirty, or 8+0. At least one manual I read recommends 7+1 when using L2 write-back, and 8+0 for write-through. You think if I use 8+0 with, say, only 512K I can cache all 256 MB?
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Re: Maximum amount of memory on a 486

Postby oeuvre » 2017-11-30 @ 14:13

In a perfect world, we'd have 486s with 32GB RAM.
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Re: Maximum amount of memory on a 486

Postby BitWrangler » 2017-11-30 @ 16:28

Heh, SSDs on PCI are probably as fast as most 486 memory access, get up to 133MB/sec vs what was typical 90MB/sec?
Basement full of ancient PC stuff, starting to go through it. Most recently toyed with DOS era stuff 15 years ago, so memory might be rusty. So what's this BitWrangler guy's deal ??? >>> http://www.vogons.org/viewtopic.php?f=25&t=56382
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Re: Maximum amount of memory on a 486

Postby Anonymous Coward » 2017-12-01 @ 14:29

I doubt that any PCI card on a 486 can even get close to 133MB/sec. I would guess more like 20MB/sec at best.
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Re: Maximum amount of memory on a 486

Postby FFXIhealer » 2017-12-01 @ 14:45

I wouldn't know. I've personally never had to use an IDE or SATA riser card via PCI or ISA. All of my computers always had IDE or SATA controllers built into the motherboard.

But on my Windows 98SE system, I did to a Crystal Disk test and I think the board supports ATA/33, but I'm only pulling around 25MB/s read speeds (19.5MB/s write) on my Western Digital Caviar 40GB hard disk drive... for what it's worth. I guess that means the HDD is being limited by the interface. The 3TB Seagate drive running on my 2016 gaming rig is pushing 170MB/s sequential read and 175MB/s write speeds (SATA III, or 6Gbps). And my 850 EVO is pushing 557MB/s read and 534MB/s write on the same bus.
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