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Groups > comp.os.msdos.programmer > #3227

Re: Loading DOS high on a XT

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On Sat, 30 Mar 2019 15:17:37 +0000
src153 <src153@protocol.invalid> wrote:

> On Sat, 30 Mar 2019 08:04:59 +0100, "R.Wieser" wrote:

> >And by the way, in that XP timeframe the addressing is "set up" by
> >the processor and other hardware on the motherboard.   The "BIOS" is
> >nothing more than a collection of on-board and extension-card ROMs,
> >exposing a set of basic APIs and initialisation routines (in short:
> >software).   Nothing in there can dictate how the addressing happens
> >(other than to "pretty please" the available hardware).  
> 
> With 286+ and MMU, physical RAM is mapped in two pieces, 0-640k, and
> 1M+. The virtual address space between 640k to 1M is partially taken
> by ROMs, the rest unused.
> 
> 286+ can map physical RAM into UMB space, call it shadow RAM and copy
> ROM into it. But other UMB space is empty and unmapped, typically D000
> and E000 space.
> 
> The BIOS sets up shadow RAM. UMBPCI is like an extension of BIOS, it
> uses knowledge of a motherboard chipset to map physical RAM into UMB
> space.
> 

About the only things I still remember about my XT was that the MFM
drives kept failing, and it had a memory expansion card with many DIP
RAM chips.  So, I don't recall if you could load DOS high or not.

See "IBM 64KB Memory Expansion Option" and "IBM 64/256KB Memory
Expansion Option" and "IBM 256KB Memory Expansion Option" sections
here:
http://www.minuszerodegrees.net/5150_5160/cards/5150_5160_cards.htm

One of the card descriptions above links to a page of manuals in pdf
format, one of which is supposed to have a schematic of the expansion
card.

The descriptions above say the memory can be installed as long as it
doesn't create an address conflict.  From what I remember of the IBM
schematic, i.e., not that much anymore ..., that seems to be correct.

However, one of those manuals, probably the Technical Reference, should
have the full schematic of the XT (or PC or AT respectively), as I
once possessed copies of the PC schematic many decades ago.

So, you'll likely see two things on the schematic 1) some high address
lines not connected to the main motherboard memory but going to decoder
circuitry to select the memory chips, and 2) a full set of address
lines connected to the expansion connector to allow for RAM/ROM on
expansion cards.

Sorry, I didn't download and check this to confirm.  That exercise will
be left for the reader, today.


Rod Pemberton
-- 
The lesson for Boeing.  You push the nose of a plane down, and planes
go down.

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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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