IBM PC/XT Memory

That picture isn't an IBM motherboard, it's a Compaq. It has a total of 768KBytes of memory installed.

The IBM motherboard could accept 256KBytes of memory, your error is that you stated the motherboard could hold 256Kbits. It would have 4 banks of 9 64Kbits chips for a max of 256kBytes.

The computers back then were 8-bit with an additional bit for Parity (a rudimentary method of error checking). So it took 9 64kbits chips to fill a bank of 64KBytes.

An 8-bit machine could handle 1 MB of RAM but typically only 640KB was usable by the OS (DOS) while the rest was reserved by the system for video and such. There were utilities available that would allow you to use part of that reserved memory space to be able to use that 768KB.
 
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Twiki said:
IIRC the XT addresses 512 K and the AT addresses 640 K.
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No, even the lowly PCjr could be tweaked to access more than 700KB.

The AT, with its a 16-bit 80286 processor, could access 16MB of RAM. The problem was that DOS could only use about 640K of that RAM for programs so any more RAM installed beyond that was either wasted, used as a cache or as a RAM disk. If you wanted to use that additional RAM to run programs, you needed to use a different OS such as Windows.
 
Twiki said:
True but memory beyond 1 meg is extended memory. I was talking about base memory.
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Addressable base memory was the same for PC/XT/AT. The PC/XT could use expanded memory, the AT could use both expanded and extended. Regardless, once you exceeded 640K or so you needed some tweaks or a different OS than DOS.

Please don't tell the PCjr, 2 PCs and the XT in my collection that they can't address 640K. I've got them convinced they can. :D
 
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