IBM PC/XT DRAM memory calculation

[jammy2012]

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.

 

[256bits]

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.