Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!hp4nl!mhres!hst
From: hst@mh_co2.mh.nl (Klaas Hemstra)
Newsgroups: comp.sys.ibm.pc
Subject: Why is VGA ROM duplicated at C000:0 & E000:0 (QEMM related)
Message-ID: <4086@mhres.mh.nl>
Date: 6 Dec 89 18:05:26 GMT
Sender: news@mhres.mh.nl
Lines: 61

I have a question concerning the VGA ROM BIOS.

On the two different systems I was able to check this ROM BIOS was located at
address C000:0000   AND (!!!) AT E000:0000.
(Systems: A Compaq 386s with Compaq VGA and a Paradise VGA card on an AT comp.)

Of course I don't care how VGA uses my memory map, as long as it works, but
this duplicated memory usage causes a problem running QEMM (386).

The QEMM program makes it possible to map some unused extended memory (using
the 386 mapping possibilities) to the memory range for A000:0000 - FFFF:FFFF.
You can then load TSR's in that high (RAM) memory and gain some valueable
memory for DOS applications.
QEMM works fine, it detects ROM areas and will not use them.

But here the VGA is eating away two chunks of memory (2x 24K to be exact).

This leaves less AND much more skattered memory for QEMM.
On the Compaq we also had a ethernet board installed using memory from 
D000:0000 - E000:0000.
So the memory map above A000:0000 contains the following:
	A000:0000 - B000:0000	: The VGA page buffer
Free	B000:0000 - B800:0000	: FREE (I Think)
	B800:0000 - C000:0000	: The VGA text mode screen memory
	C000:0000 - C600:0000	: VGA ROM BIOS
Free	C600:0000 - D000:0000	: FREE
	D000:0000 - E000:0000	: Ethernet card (DEBCA)
	E000:0000 - E600:0000	: VGA ROM BIOS Duplicate
Free	E600:0000 - F000:0000	: FREE
	F000:0000 -10000:0000	: BIOS ROM

This leaves 32K + 40K + 40K for QEMM.
This seames ok but QEMM also would like to have a 64Kbyte EMS frame buffer
(contigously in memory). Well that one it can't have. Also on of the TSR's
I would like to LOAD in HIgh memory is about 50 K (Ethernet driver).
Thats also not possible.

So I told QEMM to include the memory from C000:000 - C600:0000.
At first this seamed to work fine but later I discovered that some VGA
graphic modes did not work. Of course because there ROM BIOS was gone ?!
After some experimenting I was able to include C300:0000 - C600:0000, but
I'm still not sure about all VGA modes (haven't tested most of them).

I could reconfigure the Ethernet board to some other memory-locations but
these would not solve all problems.

Biggest questions: Why does VGA duplicates it's BIOS,
		   Is it always the case ?
		   Can it be disabled ?

Further any remarks about QEMM usage and memory skattering would be welcome.
If you find it interesting for more people post it, if not maybe I will
post a summary.

Thanks for your attention,
					Klaas Hemstra

Klaas Hemstra  (hst@mh.nl)                   |    /  / ,~~~  ~~/~~
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