From: jim@ferkel.ucsb.edu (Jim Lick)
Subject: Re: SCO SYSV386 3.2r2.0 ODT 1.1 80486-33 outperforms SUN SPARC 4/260
Organization: University of California, Santa Barbara
Date: Thu, 9 Apr 1992 01:59:09 GMT

In <1992Apr7.212615.18372@ukw.uucp> lupe@ukw.uucp (Lupe Christoph) writes:
>X11 on a VGA framebuffer is bounded by the performance of just this framebuffer.
>The trouble with VGA is that you need at least the 800x600 mode for a decent
>display. If you use bit plane organisation, i.e. the 4 bit depth, rendering
>is a pest. If you use chunky organisation, i.e. the 8 bit modes, you use
>twice as much address space. And address space is the most precious commodity
>on present days' PCs.

Yes, and even 800x600 can be limiting on an X-window session.  Fortunately,
the TSENG ET4000 type cards which have 1 meg memory can do 1152x900 or
1024x1024 using the free X386 server (for unix), assuming your monitor
is capable.

As for limited address space, the deal is this:  The original PC could
address 1 meg total.  So the original PC design had 640k of memory space,
and the rest was left for the ROM-BIOS, display adaptor, and other devices.
The AT system can address 16 megabytes.  Since the 640-1024k range already
had devices stuck in it, that range was left to them to be compatible.
But the AT could only address 16 megs in it's protected mode, but in that
mode it wasn't 8086 compatible, so it could no longer run DOS.  So there
was still only 640k of memory until various bank switching schemes were
devised to get access to this higher memory (and not all that efficiently).
Since the PC bus could only address 1 meg, they redesigned the bus to the
'ISA' bus which is on most systems today.  But since the 286 only addressed
at most 16 megs, the DMA was designed to only address 16 megs as well.

The 386 and 486 come along, and now they can address 32 bits, or 4 gigabytes
of address space.  Plenty of room for expansion here, but if you have an
ISA bus, forget it, you can't use anything over 16 megs with the DMA,
although there are tricks to get at higher memory, but only on the mother
board or a special expansion card.  EISA expands addressing to (I think)
the full 4 gigabytes adressable to the 386/486 processors, and is a
faster bus as well.  But up until recently, it was about $1000 (or more) 
extra for this bus over ISA system prices, so it is not widespread.  And
although ISA cards operate on this bus as well, it takes special EISA cards
to see full potential of the bus.  Not many are available, and those that
are tend to be expensive.

>I'm no expert on PC framebuffers, but I believe you can't map the full frame
>buffer into your address space (is MS-Windows different?), so your rendering
>code has to check for clipping by the address space all the time.

No, MS-Windows is no different.  This is a hardware limitation because of
PC Compatibility.  If it can be mapped above the 1 meg mark, PC systems
won't be able to see it at all, and DOS AT systems will only see it with
tricks, which is no better than the current arrangement.  The current
arrangement is that the video card has just less than 128k to work in, and
part of this is for mono compatibility, and part is for its ROM.  So all
in all you don't have a whole lot of room to work with here, and you
have up to 1 meg of memory on the board in many cases.

>The German Unix magazine iX has a test of three X servers for MS-Windows.
>All three benchmarked in the 4000~5000 xstone range. A good xterminal
>is approaching (exceeding?) the 100,000 xstone limit these days.
>The same is true for fast workstations with graphics accelerators.

The free X386 server is a bit better.  I forget the figures, but they
are still pretty low compared to even low end workstations.

                            Jim Lick		       
Work: University of California	| Play: 6657 El Colegio #24
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