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Thread: Using MDA SRAM Memory as General Purpose: Safe?

  1. #1
    Join Date
    Sep 2011
    Michigan, USA

    Default Using MDA SRAM Memory as General Purpose: Safe?

    Hi, I am working on a hardware level programming project (No BIOS) with a 5150 Motherboard and I've noticed the 4KB of memory on the MDA card that starts at B000:0000 is SRAM. It would be convenient for me to use a portion of this memory for general purpose storage to avoid the complexities of coding a RAM refresh. I know that the memory is normally used in byte pairs: Character and Attribute. I'm wondering if anyone has an idea of potential problems or damage writing the full range of bytes (0-255) to both these locations? I realize that jibberish would start showing up the screen... which actually might be kind of cool. LOL. Thanks! Michael

  2. #2
    Join Date
    Jan 2007
    Pacific Northwest, USA
    Blog Entries


    Sure, why not? It's probably going to be slower than planar memory, however.

  3. #3


    Yeah, there's no reason you couldn't do this, you're just liable to find it slow (at least on 386 systems, where it's limited to the speed of the ISA bus and probably not cacheable either - I suppose hypothetically a VESA video card on a 486 might allow faster access, although still probably not caching.) But on a PC/XT clone or other older PC, where main memory isn't appreciably faster than the ISA bus anyway, why not?
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  4. #4


    For what good it does, PC DOS 7.0 comes with UMBMONO.SYS which lets you use the 4K of RAM on an MDA card as UMBs (Upper Memory Blocks). More useful is UMBHERC.SYS to use the 64K on a Hercules card.

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