"Video BIOS Shadow" Enable or Disable?

jrichrds

Platinum Member
Oct 9, 1999
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Some tweaker's guides claim enabling is better. Others claim disabling is better (video BIOS has gotten fast enough).

Why does a video BIOS need to be "shadowed" in RAM anyways?
And when is the video BIOS accessed? (other than at bootup)
 

grunjee

Senior member
Jun 18, 2001
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Disable it.

I have no idea why but even recent boards will have this setting enabled by default. I don't think it'll hurt having it enabled, but it's just plain pointless.
 

jrichrds

Platinum Member
Oct 9, 1999
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That's what made me wonder...I thought video bios and other types of shadowing were remnants of the DOS/Windows 3.1 days. But newer motherboards still have the setting and even have it enabled by default.
 

Buz2b

Diamond Member
Jun 2, 2001
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You can find some excellent info on this site. To quote:
"This feature is only valid when the video BIOS is shadowed. It enables or disables the caching of the video BIOS ROM at C0000h-C7FFFh via the L2 cache. This greatly speeds up accesses to the video BIOS. However, this does not translate into better system performance because the OS bypasses the BIOS using the graphics driver to access the video card's hardware directly.
As such, it would be a waste of L2 cache bandwidth to cache the video BIOS instead of data that are more critical to the system's performance. In addition, if any program writes into this memory area, it will result in a system crash. So, it is recommended that you disable Video BIOS Cacheable for optimal system performance."
 

Mem

Lifer
Apr 23, 2000
21,476
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I always have all video shadowing off,video ram cacheable disabled,system bios cacheable disabled on all my boards for gaming and stability,works great for me .


:)
 

Peter

Elite Member
Oct 15, 1999
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Nah. The reason why this does not have any effect is that with PCI and AGP cards, the VGA ROM _always_ runs in Shadow RAM, regardless of the setting. The PCI specification demands that, and it is even technically impossible to run BIOS code directly off a ROM on an AGP or PCI card. Even further, PCI and AGP card ROMs must be written to cope with being cached.

Same btw for the system ROM.

Wasting bandwidth in the L2 cache ... now that is nonsense. The nature of how caches work is that they hold the most recently accessed stuff ... so if after the OS has booted, noone uses the system or VGA BIOS anymore, it will be retired from the caches quite quickly, to never be seen again in there.

The OS does not really bypass the BIOSes btw. VGA BIOS is consulted for power management and mode switching, and system BIOS is active all the time for power management, parts of how USB works, and a couple more things in a modern ACPI enabled system.

Bottom line: Leave that stuff enabled.

regards, Peter
 

Buz2b

Diamond Member
Jun 2, 2001
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Well, I guess everyone else is wrong and Peter is right?

Just disable it. It works.
 

teqwiz

Senior member
Sep 8, 2002
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Originally posted by: Peter


Wasting bandwidth in the L2 cache ... now that is nonsense. The nature of how caches work is that they hold the most recently accessed stuff ... so if after the OS has booted, noone uses the system or VGA BIOS anymore, it will be retired from the caches quite quickly, to never be seen again in there.
............................................................................................................................

This statement is accurate. This is a nature of caches. Shadowing reserves a block in RAM for the bios of the video card for faster access by the system if/when needed. This was a good thing in earlier Dos and Dos based games because it allowed the pif file associated with the game to access that information quickly. Now days though, the Vid bios has it's own board with cmos and maintains and manages it's own power/memory and base system functions from there. That is My understanding. Shadowing does not address the L2 cache. I don't even know if that is possible, except through the OS kernel typically.

.................................................................

The OS does not really bypass the BIOSes btw. VGA BIOS is consulted for power management and mode switching, and system BIOS is active all the time for power management, parts of how USB works, and a couple more things in a modern ACPI enabled system.

True.
.................................................................

Bottom line: Leave that stuff enabled.

Maybe? Check the Owners Manual or website for your peticular Card. My TI-4600 doesn't run in 3D with shadowing enabled. Info From Nvidia KB. So the answer here is: Onboard Video, Yes more than likely, AGP/PCI, depends.
 

Buz2b

Diamond Member
Jun 2, 2001
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Originally posted by: Jeff7
I've found that Peter seems to really know his stuff.
I totally agree and have publically said so on this very (AT) forum. However, I must disagee in this instance. From the research that I have done on BIOS settings, the conclusions have been almost totally for disabling this feature for the reasons outlined.
 

Peter

Elite Member
Oct 15, 1999
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Buz2b, now go REALLY do some research. Belief is not getting us further here. I'm a BIOS engineer you know, I handle these things every day, I know how they work, down to the very lowest level of chipset and CPU setup.

OK, here's how to prove yourself wrong:

Do a chipset register dump. Now go disable Video Shadowing. Do another chipset register dump, and compare the two. You won't see a single change. Then do a register dump of the VGA card, and see where its ROM got mapped (register offset 0x30). You won't find the legacy VGA BIOS address 0x000C0000 there, but a very high adress. BIOS copies the ROM contents from up there into RAM at the legacy VGA BIOS location Cxxxxh, and runs it from there. This is shadowing. With an AGP card, non-shadowed operation is even more impossible, since you're on a bridged bus there ... which implies you cannot map stuff to legacy addresses AT ALL.

BIOS manuals, as well as web site information, and even card vendor FAQs, are incredibly outdated. Basically everyone is cutting and pasting the paragraphs from each other since the early nineties, where we still had ISA VGA cards.

As for that TI4600 not working with shadowing enabled, now here's an excellent example of my last sentence. The code contained in PCI and AGP card ROMs, be they VGA, SCSI or RAID, cannot even work unless it's copied to and then run in RAM. That's an essential part of how PCI works. See above on how to find out where the actual physical ROM really lies, and you'll see that no PCI card ever has theirs mapped into the valid extension ROM space C0000..EFFFF. It's all copied into Shadow RAM.

If you doubt what I said about how caches work, I recommend you read a book on this. Does LRU mean anything to you?

regards, Peter
 

Buz2b

Diamond Member
Jun 2, 2001
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Peter,
See, I know better than to disagree with you. :eek: But in this instance, I'm going to stick with my guns. I guess you can lead a horse to water but can't make him (me) drink. I've no doubt that as a BIOS engineer you know more about them than I will probably know in a lifetime. The bottom line is that, whether enabling or disabling Video BIOS shadowing there can be a difference and usually it is better with it off. It could very well be that the BIOS shadowing is effecting something else when we do this and not what we think it is doing. You said, "With an AGP card, non-shadowed operation is even more impossible". I'm assuming you meant it is copied no matter what we set it at but I have also ALWAYS turned off Video BIOS shadowing on all my AGP card systems for a couple of years now. They run well and in some instances better than with it turned on. Why? I don't know exactly (at least not at your level) but there was a difference in some cases.
As for the little "project" to prove myself (wrong) where the various bits of data are stored, I'll take your word for it again. What the heck else should I do? No way I can compete (pardon the obvious pun, I couldn't resist) with you. You are saying that it makes no difference whether it is on or off but then you recommend we leave it on. Why?
As for LRU, if I remember it has something to do with FIFO in a way (First In, First Out). Like Less recently used or something. Don't laugh if I've gotten it slightly off. Anyway it probably has something to do with info moving out of the cache areas.
 

Peter

Elite Member
Oct 15, 1999
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Now how stubborn can one be? You can turn the BIOS setting On or Off as you please - YOU ARE BEING IGNORED. These settings do zip unless you have an ISA card there. If you see an improvement from either setting, then it's all placebo. I recommended you leave it alone, which means On for a modern BIOS. But if you like it better if the screen says Off, so be it, it'll be On anyway. At least, stop recommending your placebos to others.

What can you do? Use simple utility programs like WCPREDIT to see for yourself that my descriptions of the ongoings are correct. No room for belief here.

LRU, exactly, Last Recently Used. This is the mechanism by which stuff that hasn't been accessed for a while gets retired from cache, to be replaced by newly accessed data. Hence, setting something to "cacheable" that then doesn't get accessed does of course NOT lead to wasted cache. This is the one absolutely essential idea behind caches - have a small and fast memory hold copies of the most recently (and most often) used locations in a much bigger and much slower main storage. If you don't access it, it won't be in the cache.

regards, Peter

 

Buz2b

Diamond Member
Jun 2, 2001
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Now how stubborn can one be?
Sometimes it even amazes me!
You can turn the BIOS setting On or Off as you please - YOU ARE BEING IGNORED. These settings do zip unless you have an ISA card there.
Ok, so I was right in my translation when I said in my last reply,"I'm assuming you meant it is copied no matter what we set it at". As to it having no effect, can you then explain why there does seem to be an "effect" with system video performance when it is enabled? Not a silly question; an honest one, searching for knowledge. And why "teqwiz"'s card won't run properly with shadowing on? Does this setting do anything on a different level or could a particular card, or cards have something in their BIOS that is effected?
At least, stop recommending your placebos to others.
Hang on their Peter, don't get "pissy" with me now. You may be frustrated by my questions and lack of knowledge on your level but I am asking questions about, expressing concern about and trying to learn about this setting. I am also "recommending" the same placebo that many, many others do; right or wrong. Also not done on a "whim" but through some (at least I thought) reasonable research from (again what I thought) were knowledgable sources. The BIOS guide I linked to above is well respected. Could it have some missimformation on it? Certainly; we all make mistakes. Could it have some info on it that is taken from other sources that could in your terms be "cut and pasted" info that is outdated? Actually, he is very forthcoming about where he gets a lot of his info. That site is also very educational for some of us and at least gives us a better working knowledge of our BIOS.
Why don't you do an FAQ on BIOS settings? Seems like a natural for you considering your vocation. But try to be more patient with us "dummies". Call it the "lemming effect" if you want but you can't stop the tide by chastising them. Rather by taking the time to convince them of their folly. Up to this point you were trying. I hope you will continue to do so.
Actually, could you go back to the end of the first paragraph of this reply and answer the questions I posed? That might help.
 

Peter

Elite Member
Oct 15, 1999
9,640
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teqwiz was just quoting his card's manual. It's obviously working, despite the BIOS setting doing nothing.

The absolute only BIOS setting that makes a difference is the rarely seen "Video _memory_ cacheable" setting (sometimes also "VGA frame buffer cacheable"). This addresses the RAM on the card.

As for your sources, yes, that BIOS guide page, while well written and certainly based on solid research, is inaccurate in manier spots, especially when going into detail on the inner workings. (I'd certainly help them get it right, but that's a solid amount of work I wouldn't be doing for free, and that would have to be cross checked against the numerous non-disclosure agreements that my day job involves.) By the sheer length of their VGA ROM shadowing article, you see there is a lot of myth and little knowledge.

Most BIOS manuals where much of the information obviously comes from are much much worse. If you want to see careless cut&paste work of stuff dating back to the early eighties, read a BIOS manual. If you had the opportunity to compare the average BIOS manual with the source code of the very same BIOS on your screen, you'd have a third arm implanted just so you can slap your forehead all the time without interrupting your typing work.

Even the card makers, including big names like Matrox, ATi and whoever else, have enough clueless support workers and FAQ pages that still recommend tweaking these useless settings. I got into a fight with Matrox support staff once, after they told me I should try exactly all those useless BIOS settings. I took the time to lay out the tech proof that they are wasting my time, and insisted they try to find the real problem. Then I got two mails from them - one from their forum admin announcing I have been blocked out for violating their forum rules (which seem to forbid negative feedback on their product), and another from their developper staff trying to bait me into a job with them. The problem then actually disappeared three (!) driver updates later ...

regards, Peter
 

jrichrds

Platinum Member
Oct 9, 1999
2,531
3
81
I think Peter has answered at least half of the questions that I've asked on these forums. Thanks. :)

Although I didn't know any of the details myself, from reading their descriptions, I had a feeling the authors of all those tweaking guides you find on the net didn't really know how it worked either. So I'm glad I asked here!

So as a recap, for PCI/AGP cards, "Video BIOS shadow" is merely a cosmetic setting in the BIOS menu.
 

teqwiz

Senior member
Sep 8, 2002
603
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Originally posted by: Peter


"teqwiz was just quoting his card's manual."

Hey, I don't even have the manual any more. I don't need to " cut and paste ". I build these things because they're there. But research is the cure for ignorance in any and all situations.

..........................................

"It's obviously working, despite the BIOS setting doing nothing."

It's working because the shadowing is turned O F F .

..........................................

"The absolute only BIOS setting that makes a difference is the rarely seen "Video _memory_ cacheable" setting (sometimes also "VGA frame buffer cacheable"). This addresses the RAM on the card."

Ahhhh, the rare and elusive "Video_memory_cacheable", I've heard of those.

..........................................

"I got into a fight with Matrox support staff once, after they told me I should try exactly all those useless BIOS settings"
regards, Peter
Now this really suprises me?

This has been the greatest thread. Thanks, has to go to JRICHRDS for bring us this thread. I have'nt laughed so hard in a long time.
It isn't every day that I get to use Elusive and Cacheable in the same sentence.



:D
 

Peter

Elite Member
Oct 15, 1999
9,640
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Originally posted by: badthad
If it does nothing, then it doesn't hurt to disable it either. Correct Peter?
Said that already ... no matter whether you choose Enabled or Disabled in BIOS setup, it'll always be shadowed. The setup control is only regarded for ISA cards. So do as you please, just don't believe you've actually changed something.
 

Peter

Elite Member
Oct 15, 1999
9,640
1
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teqwiz, go try setting yours back to Enabled. You'll be surprised about how your 3D will still work ...
 

teqwiz

Senior member
Sep 8, 2002
603
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I have always shadowed my vid. I never had a problem until I bought this card. My MX400 never had 1 problem at all. When I did install the offending card, it suddenly would Crash to the BSOD every time I got into a game, and never right away. Random crashing some times at different places in the game, and other times, right up to the same scene in a game. I have not stated that any one should do anything but investigate they're own situation in this, the matter at hand. The website for PNY stated a short listing of recommended BIOS setting for this board. One of which was to disable That dam setting. I will however testify that I have since completed NWN, Dungeon Siege and Ghost Recon Since the resetting of the ambiquous setting (hereafter refered to as "TDS" for That Dam Setting). Futhermore: It is obvious that conceding the point is the only avenue of escape in re: TDS, So you are right. Have a nice day


P.S I got my new Soyo Dragon KT 400 today, You'll never guess which setting it has in the BIOS!? One guess, that's all you get.
 

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