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Curious memory issue with different DOS versions

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First post, by scruit

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Working on "Uncle Sherman", my Unisys-branded Mitsubishi MP-3200 386.

Continuing to make progress, and I just got a keyboard added and tested the 3.5" disk drive. If I try to boot from a DOS 6.22 disk I get to the "Starting MS-DOS" message, then a few seconds later the screen blanks and I get a system message "MAIN RAM FAIL !!!!!!" I can boot into DOS 3.3 just fine.

I'm suspecting I have a bad memory chip in a memory range that 6.22 is interacting with but 3.3 is not. Before I embark on finding/writing a memory test program (or removing and testing all these chips in a Dramarduino) I was wondering if anyone else had a similar experience? Or maybe dos 6.22 is just not fit for a 386?

Reply 1 of 4, by kixs

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Maybe DOS 6.22 is also loading himem.sys - it will do a quick mem check. Otherwise 6.22 is perfectly fine for 386.

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Reply 2 of 4, by scruit

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kixs wrote on 2022-04-21, 11:53:

Maybe DOS 6.22 is also loading himem.sys - it will do a quick mem check. Otherwise 6.22 is perfectly fine for 386.

Thanks for your response.

I could see that, for sure. I have a bootable image memory tester that I will try (https://www.memtest.org/) and if that shows memory issues I'll build and run a dramarduino. The memory chips in this machine a M5M4257 which, per the datasheet, appear to be compatible with 41256's that the dramarduino can test. Most of the memory is socketed, so that helps.

https://www.hackster.io/news/the-dramarduino- … ps-5dca70984023

Reply 3 of 4, by Babasha

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If all the memory chips socketed - just reseat them and clean all contacts on chip and in sockets.

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Reply 4 of 4, by scruit

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I got a copy of Checkit1.1 and ran that. Curious results...

Checkit reveals there is clearly a problem with memory...

- "System Hardware Configuration" shows:
--- Base Memory: 639K, 448K available
--- Expanded Memory: none
--- Extended Memory: -12160K, 0K available
- Base memory test passes
- During the extended memory test "protected mode" the PC crashes immediately upon starting the test.
- During the CPU test the PC crashes immediately after saying "Testing protected mode...".

I'm not sure if the problem is with the memory chips themselves, or with the cache ram. Could also be the 74-series logic ics or the board itself. I have already buzzed out all the power/ground/data/address lines from the daughterboard edge connector to each of the chips and the only strange thing I found is that the soldered ram (base) all has common address/data lines, and so does the socketed ram (extended?) but the common address lines for the soldered / socketed are isolated from each other. (all socketed see each other, all soldered see each other, socketed and soldered cannot see each other) Dunno if this is an issue, so I will have to trace those lines to see if they SHOULD be common.

I'm also going to build the dramarduino and test the socketed chips. Seems like a relatively easy and benign test to try. This would also give an opportunity to deoxit and reseat.

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