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Thread: IBM Portable 5155 with 640kb mod

  1. #1

    Default IBM Portable 5155 with 640kb mod

    Hi all,

    I have come across a 5155 with the 640kb mod to the mobo but I notice unlike my other XT machines it does not do the memory test when it powers on. Is this a normal side effect from the mod? It sees all the memory on the computer in DOS but I just wondered if this step gets disabled because of the modification.

    Any answers would be helpful, thanks.

  2. #2
    Join Date
    Apr 2010
    Albany, OR USA
    Blog Entries


    No, it should definitely count the memory during POST. The 5155 came with the same Mother Board as the early 5160. The 640k Mod ads a Multiplexer to the originally vacant U84 socket, ads a jumper and replaces half of the RAM dips with 256k chips to increase on board memory to 640k. Unless the BIOS is modified, the memory count will still show up. Are you certain that it is a true IBM board? Maybe the Motherboard was swapped out with a clone at some point. Some Clones enabled 640k on board before IBM did "Officially"did.


  3. #3


    I'm 99% sure it is an original board I will open it up and verify later I just wondered myself why it never does the memory count test, it's possible the bios may be modded I will look for signs of that. Would there be a reason someone would mod the bios?

    Anyway thanks I'll update when I have a look.

  4. #4


    It looks like the BIOS was changed, so thats the reason why, I will replace it with a stock BIOS I have. Thanks all.

    Also is there a good advantage to having the board with 640kb vs having something like an AST Six Pak Plus? Is there any performance differences?

    Thanks again.


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