Re: question about system memory space mapping

From: Min Wang (spamtrap_at_crayne.org)
Date: 10/18/04 

Date: Mon, 18 Oct 2004 19:55:57 +0000 (UTC)

Thanks a lot, Herman, Dirk and Hutch. Your answers clarified some puzzles
confusing me for several years... I was a pure GUI software programmer and
now I want to know something low-level :)

Now I know that if OS programs the chipset properly, almost no system
resource would be wasted/ignored. When I tried to understand this, more and
more questions came into my mind. Is there any book/tutorial which details
how various chipsets/controllers are programmed during POST and how does
host bridge recognize all the other bridges/devices and assign address
ranges to them? I think this is mostly a hardware/bios issue and linux OS
tutorial can't help. I'm studing NT device driver programming and I need to
understand the translation for "cpu virtual address -> cpu logical
address -> cpu physical address -> bus logical address -> device logical
address". I already have some idea how cpu address translation works. Now I
need to know how information on cpu address bus reaches device address bus.

Can the video adapter work under both graphic mode and text mode at the same
time? Suppose I have a PCI video adapter and the frame buffer base address
is mapped to physical address 0xe1000000. When I put data to this physical
address, the screen would be overwritten. But if at the same time I write to
physical address a0000-bffff, what would happen?

Min

"Herman Dullink" <spamtrap@crayne.org> 写入消息新闻:ckqiio$kg5$1@reader10.wxs.nl...
>> Several more questions:
>> 1) If I have 1M physical memory in my system and I'm using DOS 6.0,
>> a0000-bffff of physical memory is disabled and therefore never
>> used/wasted, right?
> Depends on your hardware. The address decoders (usually in the chipset)
> can map the normally unused memory in that area (the Upper Memory Area;
> or UMA) to an address above 1MB. This way, of that 1MB, 640KB is used
> for conventional DOS memory, 256KB is used for Extended memory and
> 128KB is used for ROM shadowing (for holding a copy of the BIOS in RAM).
>
>> 2) For a typical PC system, such a way of mapping of video buffer and
>> bios is "hard-coded" by mainboard manufacturer. There is no programming
>> controller that Software/OS can use to change the way of mapping.
> It often _is_ programmable. It's controlled by the chipset, so when
> altering
> some chipset registers, you can change the way of mapping.
> These chipset registers are very, very specific to that chipset. You
> cannot (easily)
> create a piece of software that will work on all systems. But you can make
> it
> run on your 82444GX.
>
>> For example if I have 2M physical, I can't map 1M to 00000-fffff and the
>> other 1M to 300000-3fffff.
> For that answer, you have to look at your chipset registers.
>
>
>> How can DOS runs with 64K memory?
> DOS v1 was very, very small :-)
>
>> How is bios, video buffer and physical memory mapped(addressed by cpu) in
>> this case?
> That 64KB would be RAM, BIOS would stay in ROM (in older PC's, ROM isn't
> that
> much slower than RAM), and the video buffer would still be somewhere in
> the a0000-bffff
> area. A video adapter has it's own memory.
>
>
> H
>
>

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