Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!cs.utexas.edu!sun-barr!olivea!oliveb!veritas!amdcad!brahms!phil
From: phil@brahms.amd.com (Phil Ngai)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: hard 80386 questions
Keywords: 80386 (80286)
Message-ID: <1991Mar30.012050.25932@amd.com>
Date: 30 Mar 91 01:20:50 GMT
References: <5468@archive.BBN.COM> <1991Mar29.091522.18681@mailer.cc.fsu.edu> <1991Mar29.182908.14124@cbnewsl.att.com>
Sender: usenet@amd.com (NNTP Posting)
Distribution: na
Organization: Advanced Micro Devices, Inc; Sunnyvale, CA
Lines: 42

In article <1991Mar29.182908.14124@cbnewsl.att.com> feg@moss.ATT.COM (Forrest Gehrke) writes:
|I am not sure--but your explanation sounds as though shadow ram is still
|in the 384k upper memory.  But so is the ROM bios.  When using shadow ram,
|is the address range of the bios still the same as the ROM?  How is
|this address substitution trick accomplished?

Simply put, there are (at least) two address spaces: the processor's
address space and the memory address space. In between the two is the
hardware which can do almost any mapping that can be thought of.
Logically, many motherboards map the first 640K straight through.  The
next 384K of the processor address space goes to ROM BIOS type things.
If shadow RAM is enabled, portions of that 384K are mapped to RAM
instead of ROM. The size of these portions are probably in the 4-16K
range.

If you do not have shadow RAM turned on, some motherboards will remap
the 384K of RAM up beyond the 1 meg boundary in the processor address
space. More and more motherboards simply waste the RAM if you don't
enable shadow RAM as the delay involed in the mapping process can make
it difficult to avoid wait states.

|tests show that crt write speed is nearly double the rate of
|the SETSYS shadow ram bios.  Obviously, something is different and I
|am wondering if somewhow the memory cache (I have 64kb of sram)
|is now getting into the act?  But, this raises the question
|why the SETSYS shadow ram doesn't.

I don't know why but I don't think the cache has anything to do with it.

|BTW, in spite of all this speedup of write speed, I don't see 
|this in real life with real programs.  Is this because most
|programs are writing directly to the screen and not through

Yes.

|bios?  Also, can you name any popular applications which do
|not write directly?  What about Windows?

Windows in particular likes to go straight to the hardware.

--
Gun control is elitist.