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Re: [midatlanticretro] 704k??? And boards, boards, boards...

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  • Chris M
    ... I assume you re talking about the interrupt vector table. I was aware that you could interrogate certain locations to determine how much memory was
    Message 1 of 6 , Jan 23, 2006
      > However, it could be set to anything that the chip
      > can address using a simple
      > program to twiddle the appropriate bits in DOS low
      > memory.

      I assume you're talking about the interrupt vector
      table. I was aware that you could interrogate certain
      locations to determine how much memory was installed
      (which initially was determined by dos? then put there
      I guess). I didn't realize it was that easy to bump up
      the memory. Very interesing...
      A good book to learn some of this stuph is "Inside
      the IBM PC" by Peter Norton. There were a couple of
      different versions. Back in the day I owned the
      larger, revised copy I guess (looked great too).
      Somewhere I have an earlier, smaller version.

      > The program I used to set the memory limit was
      > intended for the PCjr where it
      > was fairly common to stuff in over 800K. The jr
      > used system RAM for the video
      > buffer, so the memory 'adjustment' program also
      > moved the frame buffer up and
      > out of the way. If there are no ROMs in any side
      > cars (ie: no hard drive
      > adapter, etc.) then a jr can hold 896K RAM using at
      > least one third party RAM
      > expansion side car. Subtracting 32K for the video
      > frame buffer leaves 864K
      > for DOS.

      Right. The Peanut used "VGA" circuitry to vary the
      location of video ram, yet making it appear to be
      where standard CGA memory was supposed to be. VGA here
      meaning video gate array, some funky logic, not the
      later video standard we've all come to know and love.
      ANOTHER GOOD BOOK ;) is "the IBM Personal Computer
      from the Inside Out" by Sergent and Shoemaker (not
      sure of exact spellings or title). The first edition
      goes into the Peanut, later versions either supplanted
      coverage of the AT or later the PS/2's.

      > This biggest limit to this technique is that DOS
      > memory must be a single
      > block of RAM. The video frame buffer generally sets
      > the upper limit to this
      > game, however option ROMs can also get in the way.

      Not personally aware of any aftermarket expansion
      into the A and B blocks, but it's an interesting
      trivia question. Can't see any outfit making stuph to
      go into the B (CGA & MDA) area, unless it was a video
      card to replace the others altogether. Still haven't
      found the time to play with my Vermont Microsystems
      graphics card. It's a dual card similar I guess to
      IBM's Professional Graphics Adapter (PGA). Mine has an
      onboard 80188, the PGA had an 8088 I believe. Wish I
      had one of those bad boys. No, VM is not what you'd
      call a famous player in the PC marker, but if you do a
      search, you'll find info about them taking Autodesk to
      court over the use of some funky triangulation
      algorithm, brought there via sneakernet (i.e. they
      hired a former employee of VM). They won too.

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