Shadowing Video Bios

Shadowing Video Bios

Post by Jon M. Tayl » Thu, 07 Dec 1995 04:00:00





Quote:>On my 486-50 I usually enable the shadowing of the video bios, because
>without it text under DOS is as slow as a sleeping snail. I was told by a
>friend of mine that Linux doesn't need it and if I try to use it I'll run
>myself into a lot of problems. However, he's a bit hyper and everytime that
>I forgot to disable it when running Linux I never had a consistent problem.
>So what *is* the problem with shadowing the video bios?

        AKAIK, there isn't a *problem* with doing this under Linux (I do
and haven't noticed any in over a year of heavy linux use), but I don't
think you get any benefit from it under Linux.  Except for some stuff
during the initial startup phase, Linux hits all the hardware directly
and doesn't do any BIOS calls, negating any benefit from copying the ROMs
to RAM.

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Shadowing Video Bios

Post by Angus C. Mar » Thu, 07 Dec 1995 04:00:00


On my 486-50 I usually enable the shadowing of the video bios, because
without it text under DOS is as slow as a sleeping snail. I was told by a
friend of mine that Linux doesn't need it and if I try to use it I'll run
myself into a lot of problems. However, he's a bit hyper and everytime that
I forgot to disable it when running Linux I never had a consistent problem.
So what *is* the problem with shadowing the video bios?

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Shadowing Video Bios

Post by Bory » Sat, 09 Dec 1995 04:00:00





> >On my 486-50 I usually enable the shadowing of the video bios, because
>    AKAIK, there isn't a *problem* with doing this under Linux (I do
> and haven't noticed any in over a year of heavy linux use), but I don't
> think you get any benefit from it under Linux.  Except for some stuff
> during the initial startup phase, Linux hits all the hardware directly
> and doesn't do any BIOS calls, negating any benefit from copying the ROMs
> to RAM.

        So, by not shadowing those ROMS, you're freeing up memory.  That
may not be such a big thing(especially if it's only something like 16k or
32k), but on a 4MB system like mine, I'll take whatever I can get :)

Trannie

 
 
 

Shadowing Video Bios

Post by Jason Mcmull » Thu, 14 Dec 1995 04:00:00



: This reminded me of something I've been wanting to ask about here.  When
: I boot Linux, and it reports on memory, it always says there are 384K
: "reserved".

: In my motherboard manual, it has a detailed map of the 384K section
: between the end of the first 640K and the beginning of Extended memory.

: The question is, does Linux simply ignore this 384K of memory?  I know
: DOS can utilize it with various UMBs and loading DOS "high", etc.  Can
: anybody take the time out to tell me what Linux uses this reserved area for?

: Or, point me to a FAQ that deals with this issue?

  Yep. This is a FAQ.  But I'll answer it anyway...

The 384k that is "reserved" is not used for two primary reasons:
  1) The device drivers need that space (video, some ethercards, etc)
  2) Each motherboard chipset has a _COMPLETELY_DIFFERENT_ way
     of making that memory space read/write, so Linux just
     ignores it, instead of putting in 300k (and many hundred hours
     of debugging) of motherboard drivers for 384k of memory that
     it can't use a good 160k of anyway. (BIOS, Video space, etc.)

As to how DOS does it: a really cruft hack.  EMM386 first throws
the CPU into V86 mode, then performs some * page table mapping
to get that memory to work. Result: a lot of early 32bit games
(and the first couple of revs of DJGPP) couldn't run in v86 mode
so they would kindly ask you to remove the EMM386 line from your
CONFIG.SYS. Thus, Linux - which NEVER uses v86 mode (except in DOSemu
and Wine) - can't use that 384k directly.

Blame the designers of the IBM PC

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Shadowing Video Bios

Post by HighB » Fri, 15 Dec 1995 04:00:00





: > >On my 486-50 I usually enable the shadowing of the video bios, because
: >  AKAIK, there isn't a *problem* with doing this under Linux (I do
: > and haven't noticed any in over a year of heavy linux use), but I don't
: > think you get any benefit from it under Linux.  Except for some stuff
: > during the initial startup phase, Linux hits all the hardware directly
: > and doesn't do any BIOS calls, negating any benefit from copying the ROMs
: > to RAM.
:
:       So, by not shadowing those ROMS, you're freeing up memory.  That
: may not be such a big thing(especially if it's only something like 16k or
: 32k), but on a 4MB system like mine, I'll take whatever I can get :)
:

nope...they're shadowed into the memory between 640k and 1meg, which linux
doesn't use anyways.  i've heard of patches to take advantage of that 384k,
but its rather shakey from what i've heard.

 
 
 

Shadowing Video Bios

Post by Patrick Qui » Fri, 15 Dec 1995 04:00:00




>         So, by not shadowing those ROMS, you're freeing up memory.  That
> may not be such a big thing(especially if it's only something like 16k or
> 32k), but on a 4MB system like mine, I'll take whatever I can get :)

On various AMI-BIOS motherboards I've had the shaodowing seems to come
only in 128k chunks, so by shadowing video BIOS at the bottom end of the
ROMS and systems BIOS at the top end, I lose 256KB to shadow 64KB (?) of
ROM. Shadowing other ROMS within the 128KB spaces doesn't lose more
memory.

I still shadow because I use MSDOS/MSWindows quite a lot and the
performance hit is pretty huge if you don't shadow then.

Boggle

 
 
 

1. Turning off BIOS shadowing helps

Some time ago I posted about problems I was having getting the X
to work after booting from a compressed kernel image on my
system.  A recent posting mentioned that it's a good idea to
disable all BIOS shadowing, memory write protection, and the like
when running Linux.

Today I went through my system's BIOS configuration and found
that there was a 64K write-protected area at 0C000h and that
video BIOS ROM shadowing was enabled.  Now that I've turned this
off, I have no problems with running X after booting from the
compressed kernel image.
--

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Symptoms include lack of traffic sense, slow rate of travel, and the
classic, easily recognized behavior known as walking."

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