video memory and shadowing

[salah81]

Can someone please help me about how video memory and the main RAM inyteract with each other .. and about shadowing .. thanks a lot ..

[Jiggy]

Hi and welcome,
i found this on video memory/AGP.
RAM optimized for video adapters. VRAM chips have two ports so that video data-what will be displayed next-can be written to the chip at the same time the video adapter continuously reads the memory to refresh the monitors current display.
AGP port, the adapter has direct access to RAM, eliminating the need for expensive video RAM on the adapter itself to store bitconsuming file such as texture maps.
the chip set puts the AGP on the same part of the bus memory, where data transfers take place at 528MB per second.
hope that helps you.
Jiggy
http://www.PCGuide.com/ubb/eek.gif

[bunk]

The video operation programing code is in ROM. This is safe, secure and slow. Shadowing the video ROM means the programing for the video is copied from ROM and placed into main RAM. The computer will then use the RAM code instead of the ROM code. This is done since RAM is much faster to access and execute. Video ROM should be on the video card.

Video RAM is on the actual video card. It is used to hold graphic textures and to manipulate video data. Normal RAM can be used to do this but it is much more efficient to use the on card video RAM since it is closer to the video processor and usually optimized to do this work much faster and
effeciently than main RAM can handle the job. This also has the benefit of freeing up main RAM for other work.

A video card will use shadowed memory for accessing video driver code instead of video ROM if this option is enabled, for the simple fact of speed. This is separate issue from video RAM. Video RAM for manipulation of graphic data can be done by video RAM or main RAM. Video RAM is used first since it is optimized for this job and usually faster, main RAM can be used as need to back up and hold data the video RAM can't hold.

As Jiggy pointed out video RAM can read from the video RAM and write new data to it at the same time. This is one reason why it is faster. But the concept that AGP using main RAM could replace video RAM hasn't happened. This is due to the fact that video card were able to widen the data bus, and the fact that video RAM is physically closer to the video proccesor. This closeness means less physical travel time for data, over data lines that handle larger chunks of video data in one cycle still give on card video RAM a huge speed advandtage over AGP using normal main RAM. This is obvious when you compare a computer using a 64 MB video card vs an identical video card with 32 MB. When the graphic data needs get hot and heavy the video card with more video RAM will win, regardless of main RAM. AGP is still very useful since it speeds up the swap of data from video RAM and main RAM when today's programs use so much graphics data, but video RAM is used first and main RAM can supplement it.

Ever notice that video RAM points to the next generation of main RAM? http://www.PCGuide.com/ubb/smile.gif
This has been true for quite a while, what ever is the new hot technology for video RAM will end up being used for main RAM in a year or two. DDR was the most recent example of this.

[This message has been edited by bunk (edited 11-18-2001).]

[This message has been edited by bunk (edited 11-18-2001).]

[mjc]

What is actually stored on the ROM chip is the Video BIOS, which Windows ignores, that is why you need specific drivers for your video card under Windows. Also with most cards these days having Flash-ROMS (which are much faster than ordinary ROM chips and in some cases as fast as RAM) it may no longer be necessary to Shadow the Video ROM, which would free up some RAm for other things.

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mjc
Links list:Computer Links (http://www.dreamwater.org/tech/mjc/index.htm)

Celts are the men that heaven made mad, For all their battles are merry and their songs are all sad.

[kenja]

Welcome, salah81

Good questions, I've been wondering about these things.

MaximumPC magazine recommends disabling video BIOS shadowing, but they only deal with Windows. If I'm running Linux, should video BIOS shadowing be enabled for economy cards (like an S3 S540 or a TNT2 M64 chipset)?

Related question: MaximumPC also recommends disabling the system BIOS shadowing, but what would be recommended for a Linux system?

[mjc]

Not sure about the other but the S540 has a flash ROM and that I think is the key...the Flash-Rom, it is so much faster then a plain old ROM.

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mjc
Links list:Computer Links (http://www.dreamwater.org/tech/mjc/index.htm)

Celts are the men that heaven made mad, For all their battles are merry and their songs are all sad.