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IBM PC/XT DRAM memory calculation

jammy2012

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Dec 27, 2012
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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/wikipedi...ainboard01.jpg

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

Thanks in advance.
 
Think there's a mix up of bits and bytes. Each bank is 9 chips giving memory addressible in units of 8-bits and one parity bit per byte, generated and checked by hardware (invisible to the OS, the hardware just generates NMI if there's an error).

On the 5160 there are 4 banks, and the max RAM is 1MB although you need a custom decoder PROM as has been discussed on here in great detail (see also here).
 
I understood what you are saying but if each chip has a memory capacity of 256kbits as stated in the datasheet (UD61256 = 256K x 1 DRAM ), how come we have 27 DRAM ICs and this system board can only support 256Kbits between all the 27 IC slots available.
 
I understood what you are saying but if each chip has a memory capacity of 256kbits as stated in the datasheet (UD61256 = 256K x 1 DRAM ), how come we have 27 DRAM ICs and this system board can only support 256Kbits between all the 27 IC slots available.
The datasheet you referenced is not for this board.
 
The datasheet provided is of UD61256 which is the same IC used in the picture of the motherboard of wikipedia to implement the memory.
I referred to the wrong part of your message, sorry.


... how come we have 27 DRAM ICs and this system board can only support 256Kbits between all the 27 IC slots available.
How do you know that ‘this system board can only support 256Kbits between all the 27 IC slots available’?
 
I referred to the wrong part of your message, sorry.


How do you know that ‘this system board can only support 256Kbits between all the 27 IC slots available’?


ic, becuase I ignored the fact that there should be certain amounts of memory reserved. thanks :)
 
I understood what you are saying but if each chip has a memory capacity of 256kbits as stated in the datasheet (UD61256 = 256K x 1 DRAM ), how come we have 27 DRAM ICs and this system board can only support 256Kbits between all the 27 IC slots available.

The board is lacking 9 ICs to have all it's memory installed. Look at the missing IC's at U48 and above, all those should be RAM, making the absolute total number of DRAM chips 36 and not 27. The thing is that this board just don't have them all installed.
 
The board is lacking 9 ICs to have all it's memory installed. Look at the missing IC's at U48 and above, all those should be RAM, making the absolute total number of DRAM chips 36 and not 27. The thing is that this board just don't have them all installed.
It's the sockets that are missing, not the ICs. There were never any sockets in those positions -- that's how this board was designed. So, nothing is missing for what this board was designed.
 
It's the sockets that are missing, not the ICs. There were never any sockets in those positions -- that's how this board was designed. So, nothing is missing for what this board was designed.

There were never any sockets or ICs there, yes, but that doesn't mean that they can't be added. This board may have been manufactured to only support 3 banks of RAM, but that's a configuration, not a part of it's design. The board is clearly designed to support a configuration where all the 4 banks of RAM can be used.

I see that I may have been missunderstanding the first coupple of messages, and I kindof assumed that all banks were 64KB and the "missing" 4th bank was where the confusion came from.
 
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You might be correct about that but unless we can determine what this board actually is circuit-wise it's not going to be conclusive. Maybe somebody would like to examine the visible traces on this board in comparison with an XT board. Even that probably wouldn't be conclusive, I guess. :)
 
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