Weird ram problem

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te_lanus
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Weird ram problem

Post by te_lanus » Fri 21 Jun, 2019 6:28 am

Hi

I'm compiling PCem on Kubuntu 18.04.02, and noticed with my self compiled version (commit 91640c0) that most systems from the 386 line and up have very restricted ram options. EG: 386 only having 16mb, 486 only having 64mb and the socket 7 cpu's only having 128mb ram. Is this a problem with my compile, in such a case how do I fix it to have more ram available

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SarahWalker
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Re: Weird ram problem

Post by SarahWalker » Fri 21 Jun, 2019 6:29 am

That's normal. PCem generally limits RAM to what would be possible on the real system you've selected.

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omarsis81
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Re: Weird ram problem

Post by omarsis81 » Fri 21 Jun, 2019 1:14 pm

Like Sarah said, PCem tries to emulate real behavior of the emulated hardware, but for instance in the 386DX AMI clone you can selected up to 256 MB RAM - that is possible with very uncommon RAM sticks and expansion adapters

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ppgrainbow
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Re: Weird ram problem

Post by ppgrainbow » Fri 21 Jun, 2019 1:55 pm

orarsis81, On a 386 which PCem emulates, the motherboard which uses the OPTi-495SX chipset, the RAM is actually limited to 32 MB, not 256 MB. The AMI BIOS (dated June 6, 1992) will NOT even recognise more than 64 MB and will not even POST if the RAM is manually set to anything higher than 655,296 KB.

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omarsis81
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Re: Weird ram problem

Post by omarsis81 » Fri 21 Jun, 2019 3:31 pm

ppgrainbow wrote:
Fri 21 Jun, 2019 1:55 pm
orarsis81, On a 386 which PCem emulates, the motherboard which uses the OPTi-495SX chipset, the RAM is actually limited to 32 MB, not 256 MB. The AMI BIOS (dated June 6, 1992) will NOT even recognise more than 64 MB and will not even POST if the RAM is manually set to anything higher than 655,296 KB.
You're right. If I select 256MB the initial POST screen will count the RAM OK, but then when the OS boots I won't recognize more than 64
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ppgrainbow
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Re: Weird ram problem

Post by ppgrainbow » Fri 21 Jun, 2019 3:46 pm

omarsis81 wrote:
Fri 21 Jun, 2019 3:31 pm
ppgrainbow wrote:
Fri 21 Jun, 2019 1:55 pm
orarsis81, On a 386 which PCem emulates, the motherboard which uses the OPTi-495SX chipset, the RAM is actually limited to 32 MB, not 256 MB. The AMI BIOS (dated June 6, 1992) will NOT even recognise more than 64 MB and will not even POST if the RAM is manually set to anything higher than 655,296 KB.
You're right. If I select 256MB the initial POST screen will count the RAM OK, but then when the OS boots I won't recognize more than 64
If you manually use a text editor to up the RAM to 640 MB, that's the absolute maximum that the AMIBIOS can handle. In order to get the AMIBIOS to recognise more than 64 MB, the BIOS must be dated August 1994 or later.

As far as I know, no ISA-based 386 or 486 machine can ever handle 256 MB or even as much as 640 MB at the bare maximum. Any attempt to set more than 640 MB of RAM and the BIOS will not post:
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That's the case when I set the mem_size variable to 786432 (768 MB). Try to set the mem_size to 1048576 (1 GB or higher) and PCem will crash. The only PCs that can handle up to 256 MB of RAM are the late PCI-based 486s from 1995 to 1996.

In retrospective, checking the README.TXT file, the RAM for the OPTi-495SX based 386/485 and the ALi1429-based 486 are limited to 32 MB, not 64 MB, 256 MB or even 640 MB.

tk421
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Re: Weird ram problem

Post by tk421 » Tue 02 Jul, 2019 2:28 am

I am not sure what the effect of allocating massive amounts of RAM to an old 386 or 486 would have on performance. Typically I only allocate the same amount of RAM that these old systems would have had back in 1992 or even 1995. It was normal for a Win95 pc to have 16mb of RAM.

I know that adding more RAM should actually increase the performance of these older systems.

Even for the Pentium MMX PCs, I rarely ever go above 32mb of RAM. I admit newer games like Star Trek Armada work best with 64mb of RAM, so I set the RAM to 64mb, but I have yet to see a need for 128mb unless the program or game in question runs best with that much RAM.

May I ask if I should be allocating much more RAM to my PCem emulated PCs? Would setting 128mb of RAM slow down PCem?

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ppgrainbow
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Re: Weird ram problem

Post by ppgrainbow » Tue 02 Jul, 2019 3:00 am

It was not until at least 1999 or 2000 when Pentium III came out and 32 MB to 128 MB of RAM became more common and by 2003, 256 MB to 512 MB became more comment also.

If you're using the DataExpert OPTi-495SX-based AMI 386 or a ALi-1492-based AMI 486, anything more than 64 MB will not be detected by the AMI BIOS and the emulated chipsets that is emulated on the motherboard won't even accept more than 32 MB of RAM For Pentium MMX-based PCs, 16 MB to 32 MB were common and they can be maxed out anywhere between 512 MB to as much as 768 MB.

Anything more than 512 MB of RAM is too much for Windows 95, Windows 98 and Windows Me even on emulated motherboards that PCem v15 emulates.

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