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Extended memory card in an XT-286

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richo132

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May 19, 2007
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I have an ibm 65x5770 16bit extended memory adaptor that also has one serial and one parallel port.
It is a full length card with 2 banks of 6 sockets for 60 pin simm modules.
As obtained, it has 3 pairs of simms that are reported on boot as 1408kb. This appears to be 256 * 6 = 1536 - 128kb for some reason.
My first guess about this was 512kb on the M/B, with the first 128kb on the adapter being used for top up to 640kb. However, the XT-286 posts with 640kb without the adapter, so there is something else going on.
There is a bank of 8 DIP switches on the card - presumably for setting the memory config.
Can anyone provide info on this card ?
Cheers.
 
The IBM XT-286 is sure to duplicate the 5170, in that the memory count shown at startup is the sum of base and extended memory. You know that the card isn't contributing to the 640K base memory and is providing all extended memory, so:

640 KB base
768 KB extended, provided by subject card
=======
1408 KB


768 KB / 6 SIMMS = 128 KB per SIMM
 
Thanks for the responses - very helpful.
I must apologise for misleading you in my original note.
The 6 SIMMS are 256kb and the memory check during POST reports 2048Kb.
I used the AT advanced setup disk to set extended memory to 1408Kb.
Any other memory size setting results in errors.
I suppose this is:
640 Kb Base
1408 Kb extended
====
2048 Kb reported during POST

What puzzles me is why 6 * 256Kb = 1536Kb is reported as 1408Kb - where did the other 128Kb go ?
Cheers.
 
My guess is that the card is setup to backfill to the 512k level and thus 2 sets of RAM chips are handling the 513-640 area. A number of the memory expansion cards I had defaulted to that. Jumpers should let you redefine the memory starting address.

You can even increase the amount of RAM backfilled. This makes task switching a lot faster with Desqview and Windows 286 when coupled with Expanded Memory.
 
As far as I know XT 286 motherboards should have 640k on the board. 128kb in DIP, and 512kb in SIMM. Your extended memory board should not have to backfill conventional memory as it is already present on the board. Another guess would be that perhaps the missing 128kb of memory is configured as expanded instead.
 
Spot-on!
I pulled the card and checked the dip switch setting thanks to earlier link to stason.org
Switches were set for 512Kb on the M/B - after setting correctly, memory is now fully available.
Cheers.
 
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