Memory map with onboard video

[Candy]

Does anybody know how a memory map looks like (E820) with a computer with onboard video? Does it reserve some bits of memory or something?

TIA, Candy

[Tim]

The same as a computer with video on a separate card: the video RAM will show up in the PCI registers for the card, and is generally a long way above the top of actual RAM. Regardless of whether the video chipset is on the motherboard or on a separate card, it's still accessed over the PCI bus and is electronically the same.

[Candy]

The same as a computer with video on a separate card: the video RAM will show up in the PCI registers for the card, and is generally a long way above the top of actual RAM. Regardless of whether the video chipset is on the motherboard or on a separate card, it's still accessed over the PCI bus and is electronically the same.

ok, question was asked slightly wrong. Those with shared memory, how do you know where their memory starts and how do you prevent yourself from fucking it up, and on the other hand, how do you map it if you only use 256k ?

[Tim]

Again, the shared memory is considered to be VRAM, so it's not accessible as normal memory. So it doesn't come into the memory map scheme of things. The sharing is done at the hardware level. If this were not the case, then any protected-mode OS written before shared video memory came into use would break -- it wouldn't be able to access the screen reliably, and there would be a chunk of memory whose contents changed every time it wrote to the screen.

[Candy]

Again, the shared memory is considered to be VRAM, so it's not accessible as normal memory. So it doesn't come into the memory map scheme of things. The sharing is done at the hardware level. If this were not the case, then any protected-mode OS written before shared video memory came into use would break -- it wouldn't be able to access the screen reliably, and there would be a chunk of memory whose contents changed every time it wrote to the screen.

I thought it might be marked with some new type code in the normal memory map, so you would not usually use it but knew that it was there. Are you sure it is not reported normally?

[Tim]

Well, one way to find out is to get hold of a computer with a graphics card that uses RAM as VRAM and look at the memory map you get. But I'm confident that the memory map will omit video RAM completely, and report the memory that the video card isn't using as normal RAM.