mikerofone
Experienced Member
Hi all,
Edit: I found the missing 128KB, they are right next to the RAM sockets:
I guess the chips themselves or something on their path is bad, and I need to debug this. Leaving the rest of my post for posterity - thanks everyone for being my rubber ducks. :D
Any suggestions on how to debug the broken chips welcome.
Cheers
mikerofone
Edit: I found the missing 128KB, they are right next to the RAM sockets:
I guess the chips themselves or something on their path is bad, and I need to debug this. Leaving the rest of my post for posterity - thanks everyone for being my rubber ducks. :D
Any suggestions on how to debug the broken chips welcome.
Cheers
mikerofone
I cannot wrap my head around how one would get 640KB of base memory in a Compaq Portable III, even though that seems to have been the default config. I cannot find any information online about how base memory works on that machine, only on extended/expanded memory (see last page in this pdf which claims that 640KB are on the mainboard). Does anyone have a manual? I could only find Compaq Portable 386 documentation online, hardly anything on the III.
Mine came with the switches etc set for 640KB of base RAM, but it complains about memory errors unless I set it to 512KB. Digging into the configuration options, I do not understand how this machine would be configured for 640KB, unless 320KB RAM modules are a thing?
The mainboard takes two 30-pin SIMM RAM modules, in the bottom middle:
These are populated with 2x256KB modules (I googled the markings on the chips). Here's a closeup:
So it makes perfect sense that only 512KB work - there are simply 128KB missing. There is an additional way to add memory, a Compaq memory expansion card that can be configured for XMS or "Compaq Expanded Memory":
It was populated with 2x256KB when I got it, but I replaced them with the 1MB SIMMs I had lying around. These work perfectly fine in both XMS and Compaq EMS mode.
However, that still leaves the hole between 512-1024KB; I couldn't make any of that RAM be used for base memory.
There's a third memory card that came with the computer, but it was a regular ISA card that was installed in the external ISA extension box. However, the ISA extension module seems to have a short that I have yet to debug, so I haven't been able to see what happens when that card is plugged in.
This is the card, holding what seems to be 2MB of RAM in 18x1MBit chips:
Given that it has to be installed in the optional, bulky ISA extension box, I highly doubt that the "default" of 640KB base RAM requires such a card to be present.
The configuration stickers in the case don't bring any clarity for me:
- The "No Extended Memory / Select Address Bank XXX" jumpers in the left image control how much of the RAM on the expansion board (RAM2) are configured as XMS. One can have none, some, or all banks configured for XMS; the rest becomes EMS. I've tried various combinations, nothing added more base.
- I'm not 100% sure what DIP switches 3 and 4 control exactly, since I don't know what "640KB + XXX" is supposed to mean. I've stuffed 2 1MB SIMMs into the mainboard sockets and tried various combinations of these, but I never got it to recognize more than 512KB.
If I don't install any RAM at all, the computer immediately throws an error before even attempting to POST. I was curious whether there should be 128KB soldered onto the mainboard, but that doesn't seem to be the case.
Any suggestions welcome! Only having 512KB RAM available in DOS is quite annoying.
Cheers
mikerofone
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