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Groups > comp.os.msdos.programmer > #3205

Re: Loading DOS high on a XT

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src153,

> It takes BIOS, UMBPCI, or EMM386 to map physical
> memory into UMB space, so how did he make it work on
> an XT?

Because it doesn't need any of that.

All you need is a latch that will remember the bits you send to it, and feed
its outputs to the address lines "above" the ones that are available for
that extension card (going to the RAM on that card).   Change the latches
contents (normally by writing to a specific address in the cards address
space) and you look at another block of physical memory.

Ofcourse, a small driver will be needed.  Which is normally loaded (or 
available in ROM) in the lower range of the extension card (leaving one or 
more adresses at its top free to write to the above mentioned latch - or in 
the case of ROM, overlapping), leaving the upper range free for the RAM 
block switching

And by the way: EMM does not "map physical memory into UMB space", it simply
/copies/ a block from beyond the 1M border into UMB space - and back again
when another block is requested.   If you ask it nicely it will as easily
copy in/out of convential memory.

Regards,
Rudy Wieser

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Thread

Loading DOS high on a XT Ruud <Ruud.Baltissen@apg.nl> - 2012-07-15 23:16 -0700
  Re: Loading DOS high on a XT Ross Ridge <rridge@csclub.uwaterloo.ca> - 2012-07-16 02:58 -0400
  Re: Loading DOS high on a XT "Rod Pemberton" <do_not_have@notemailnot.cmm> - 2012-07-16 05:10 -0400
    Re: Loading DOS high on a XT Ruud <Ruud.Baltissen@apg.nl> - 2012-07-17 01:37 -0700
      Re: Loading DOS high on a XT Jim Leonard <mobygamer@gmail.com> - 2012-07-17 14:11 -0700
        Re: Loading DOS high on a XT Ruud <Ruud.Baltissen@apg.nl> - 2012-07-17 22:36 -0700
  Re: Loading DOS high on a XT T. Ment <t.ment@protocol.invalid> - 2019-03-30 04:58 +0000
    Re: Loading DOS high on a XT "R.Wieser" <address@not.available> - 2019-03-30 08:04 +0100
      Re: Loading DOS high on a XT src153 <src153@protocol.invalid> - 2019-03-30 15:17 +0000
        Re: Loading DOS high on a XT "R.Wieser" <address@not.available> - 2019-03-30 19:30 +0100
          Re: Loading DOS high on a XT src153 <src153@protocol.invalid> - 2019-03-30 18:56 +0000
            Re: Loading DOS high on a XT "R.Wieser" <address@not.available> - 2019-03-30 21:18 +0100
              Re: Loading DOS high on a XT src153 <src153@protocol.invalid> - 2019-03-30 21:22 +0000
                Re: Loading DOS high on a XT src153 <src153@protocol.invalid> - 2019-03-30 23:09 +0000
                Re: Loading DOS high on a XT src153 <src153@protocol.invalid> - 2019-03-30 23:34 +0000
                Re: Loading DOS high on a XT "R.Wieser" <address@not.available> - 2019-03-31 10:25 +0200
        Re: Loading DOS high on a XT Rod Pemberton <invalid@lkntrgzxc.com> - 2019-04-02 18:05 -0400

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