Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!blake!ogccse!husc6!cfa!ward
From: ward@cfa.harvard.EDU (Steve Ward)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Problem of breaking DOS 640K RAM limit?
Summary: PC/XT memory map, beyond 640K
Message-ID: <1431@cfa.cfa.harvard.EDU>
Date: 9 Mar 89 20:59:58 GMT
References: <74032HJW2@PSUVM> <15373@cup.portal.com> <1345@ccncsu.ColoState.EDU> <1429@cfa.cfa.harvard.EDU>
Organization: Harvard-Smithsonian
Lines: 61

In article <1429@cfa.cfa.harvard.EDU>, ward@cfa.harvard.EDU (Steve Ward) writes:
> 
> MSDOS does NOT impose the 640K limit commonly encountered on IBM and
> clone PC's.
> 

[stuff deleted on how BIOS/MSDOS work together to size and set MSDOS memory
limits and suggestions on squeezing out/in more MSDOS memory]

> A careful check of memory space use by all your I/O boards should be
> made to make sure that any memory you install will not clash in the
> memory address space.
> 
 
Note that I DID issue the above warning about checking for memory
conflicts!  This is to avoid 2000 followup postings pointing out that
in typical PC's video display buffer memory exists between addresses
A0000h - C0000h where the exact video buffer size and location will
vary with the video display type.  Yes, it is there, no, it doesn't
change the facts as I described them except that it is difficult to
relocate the video buffer memory, mostly because a lot of software
does not use the [inadequate] video BIOS and directly reads/writes
the display video buffer.

Here is a PC memory map I found, which shows there is available memory
address space, but not useful without rearranging the memory map
locations of either the video board or BIOS extension ROM's (shadow
ROM's) or both.

I never said it would be easy!

00000h - 9FFFFh    msdos memory
A0000h - AFFFFh    EGA display buffer
B0000h - B7FFFh     MONO display buffer
B8000h - B8FFFh    CGA display buffer 
C0000h - C7FFFh    EGA BIOS ROM
C8000h - CFFFFh    hard disk BIOS ROM
D0000h - DFFFFh    user space
E0000h - EFFFFh    user space on XT, on AT may be used for BIOS ROM
F0000h - FFFFFh    BIOS ROM, though usually only top 2k-8k

Most XT and AT clones have BIOS in upper 8K.  TRUE BLUE IBM machines
may fill up the top 64K with BIOS + Basic in ROM.  There are probably
even more variations out there.  Usually, only one of the displays
is installed so there is room to expand msdos memory, but sadly
it is hard to do because the display and ROM's are scattered about
in the memory map rather than being packed at the very top of memory.

So, in more gory detail, to squeeze in more MSDOS memory your software
will have to be well behaved and use BIOS calls and/or device drivers
(especially the video) and of course, your BIOS and device drivers
will have to accomodate relocation of the video display hardware to
higher memory locations, as will the video board itself.  Too bad this
wasn't done in the beginning since this makes it too big a pain to
attempt doing for most people.  

steve w.  ward@cfa.harvard.edu