IBM PC XT 256K Memory - How Does It Work?

[curiousguy23]

Hi All,

i was looking at the IBM PC XT technical reference and it states that the board can take a maximum of 256K of memory. If you look at the schematic you can then see 36 sockets for memory chips and if you see this picture of it

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

You can see that the memory chips are ud61256 256k by 1 DRAM chips. How can the board support all these chips if the max it can handle is 256k?

Thanks

 

[Borek]

640kB ought to be enough for anybody. If memory serves me well that was the limit for XT. Doesn't mean all were equipped with 640, early models (and/or cheap ones) had less.

Edit: taken from wikipedia:

There were two widely used configurations of the XT motherboard. The first could support up to 256kB on the motherboard itself (four banks of 64kB chips), with a maximum of 640kB achieved by using expansion cards. This was the configuration the XT originally shipped in. The second configuration - introduced in stock units in 1986 - could support the whole 640kB on the motherboard (two banks of 256kB chips, two banks of 64kB), had the later revision AT-compatible BIOS with a faster booting time, as well as support for 101-key keyboards and 3.5" floppy drives. The earlier configuration could be adapted to 'late' configuration after a couple of minor modifications.

 

[curiousguy23]

Not sure you got what I am saying. Even if the max was 640kB , on the photo you can clearly see 27 chips each with 256Kbits of DRAM. This would amount to 6912Kbits in total and thus 864KB. From what I can see that board is equipped with more memory than it can handle, or I might be seeing it wrongly.

 

[Borek]

Yep, I misunderstood.

 

[rcgldr]

It's probably 768KB with parity. I don't recall if the memory map of the XT and what it allowed versus the AT.