Path: utzoo!attcan!uunet!lll-winken!ames!ncar!gatech!rutgers!ucla-cs!wales
From: wales@valeria.cs.ucla.edu (Rich Wales)
Newsgroups: comp.sys.ibm.pc
Subject: Re: ramdrive above 640K limit
Keywords: ramdrive
Message-ID: <19586@shemp.CS.UCLA.EDU>
Date: 17 Jan 89 06:43:00 GMT
References: <4616@sfsup.UUCP> <1212@bucket.UUCP>
Sender: news@CS.UCLA.EDU
Reply-To: wales@CS.UCLA.EDU (Rich Wales)
Organization: UCLA CS Department, Los Angeles
Lines: 28

My system (a Wugo PC-II-AD, a Taiwanese turbo XT clone) has 1 meg on the
motherboard, and the top 384K *is* accessible as a RAM disk.

What Wugo did was implement a band-switching hack in their hardware, in
which the extra RAM (640K-1Meg) can share the same address space with
128K-512K.  This is under control of I/O port 0E0H (which is unused and
reserved in the "true blue" IBM PC).  If the value 0FFH is output to
port 0E0H, the alternate RAM is accessed; regular memory mapping is
restored by outputting a zero to port 0E0H.

My system came with a special RAM disk device driver that knows about
this odd hardware.  As it turned out, the RAM disk driver had a serious
bug in it -- there were some "windows" during which the extra memory
was "mapped in" while hardware interrupts were enabled.  If any hardware
interrupt handler resided even partially between 128K and 512K, needless
to say, this was a sure recipe for utter chaos and disaster.  I eventu-
ally found the bug (by disassembling the driver) and managed to fix it
with DEBUG; the RAM disk works just fine now.  I sent a letter to Wugo's
US office describing the bug and the fix; hopefully they'll do something
about it in their later releases.

Presumably, one could write a TSR that would use this extra memory as a
disk cache.  Maybe I will one of these days, if I get really ambitious.

-- Rich Wales // UCLA Computer Science Department // +1 (213) 825-5683
   3531 Boelter Hall // Los Angeles, California 90024-1596 // USA
   wales@CS.UCLA.EDU      ...!(uunet,ucbvax,rutgers)!cs.ucla.edu!wales
"Now, if you do see me again today, I want you to report it to me immediately."