IBM PC/XT DRAM memory calculation

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SUMMARY

The IBM PC/XT motherboard originally supported a maximum memory of 256 kilobytes (kB), with later revisions allowing for up to 640 kB. The confusion arises from the presence of 27 DRAM chips, each rated at 256 kilobits (Kb), totaling 6912 Kb, which can lead to misunderstandings about the board's capabilities. The motherboard may not be an authentic IBM model but a clone, and the actual memory configuration can vary based on the specific motherboard design. The 9 chips in each bank include one for parity, contributing to the overall memory architecture.

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Hardware enthusiasts, vintage computer restorers, and anyone interested in the technical specifications and memory configurations of the IBM PC/XT motherboard.

jammy2012
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Hi,

with regards to the DRAM of the IBM PC/XT motherboard, how come the maximum memory of the motherboard is 256Kbits and on the link below, it shows that there are 27 DRAMS of 256Kbits each, which therefore makes a total of 6912 Kbits (27*256) ?

http://upload.wikimedia.org/wikipedia/de/1/11/PCXTMainboard01.jpg

IS the board equipped with more memory that it can handle ?

Thanks in advance.
 
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You motherboard is probably not an IBM motherboard but a clone. IBM used the Intel 8088 microprocessor and the one on the board is the NEC V20.

The XT went through a few revisions. Originally 256kbytes was offered, and a later motherboard upgrade allowed 640kbytes.

I cannot make out the type of memory chip on the board, so you could have 3 x 64kbyes or 192kbytes on board, or 3 x 256kbytes on board = 768kybes with 640kbyes as main ram and 128kbytes as extended.

The reason you have 9 chips in each bank is one bit of the 9th chip is used for parity and a bit from each of the other 8 chips to make an 8 bit byte of memory.
The actual configuration would be motherboard dependent.

You wrote "maximum memory of the motherboard is 256Kbits " when you actually meant 256kbyes.
 

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