Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uunet!sjsca4!molehill!poffen
From: poffen@molehill (Russ Poffenberger)
Newsgroups: comp.sys.ibm.pc
Subject: Re: 640K limit
Message-ID: <1990Jan15.030306.19993@sj.ate.slb.com>
Date: 15 Jan 90 03:03:06 GMT
References: <4668.25aed7f2@uwovax.uwo.ca> <1468@blackbird.afit.af.mil>
Reply-To: poffen@sj.ate.slb.com (Russ Poffenberger)
Organization: Schlumberger ATE, San Jose, CA
Lines: 43

In article <1468@blackbird.afit.af.mil> ewilliam@galaxy-43.UUCP (Edward M. Williams) writes:
>In article <4668.25aed7f2@uwovax.uwo.ca> 23031_676@uwovax.uwo.ca writes:
>>elementary question:  Like many others, I have purchased a 1 mb system (NEC 
>>286), yet it is not clear that the extra 360 can be used for anything. Modal
>>response is yes/no(!) DesQ and other software manuals speak about use of 
>>extended memory over 1 mb, but not between 640 and 1 mb. Is there any way, 
>>for any kinds of software, of getting any use out of this memory?
>
>IBM really messed things up when they designed the original PC (hindsight
>is wonderful!) - they didn't think that anyone would ever need more than
>640k of memory.  The original PC came with 64k!  They thought it would be 
>safe to put all the video screen memory mappings, ROMs, and all that 
>stuff up above the 640k line.
>
>Nowadays with memory capacities STARTING at 1M, the stuff between 640k and
>1M poses a slight problem.  With a 80286, the best that can be done is to
>find the holes that aren't being used for video memory or ROMs and a 
>program such as LOADHI (from Quarterdeck Software - it comes bundled with
>QEMM) that can put device drivers and TSRs up there so they don't take up
>much of the main 640k.
>
>The 80386 has some special hardware that can be used to 'fool' the processor
>into thinking that the memory between 640k and 1M is actually somewhere
>else - usually above 1M.  That way, programs like DESQview (with QEMM-386)
>can use it to run normal programs.
>
>Hope this clears up some of the confusion!
>

Well, I think you confused it a little more by saying only 386 systems can map
memory above 1m. The truth is that MOST modern 286 systems these days have
"Split Memory Addressing". By using some clever address mapping schemes in the
RAM addressing hardware, the memory that normally falls between 640K and 1024K
is re-mapped to appear above 1024 as extended memory. (384K worth)

If your system displays extended memory during cold boot, then this system
has this capability.

Russ Poffenberger               DOMAIN: poffen@sj.ate.slb.com
Schlumberger Technologies       UUCP:   {uunet,decwrl,amdahl}!sjsca4!poffen
1601 Technology Drive		CIS:	72401,276
San Jose, Ca. 95110
(408)437-5254