question about system memory space mapping

From: Min Wang (spamtrap_at_crayne.org)
Date: 10/15/04 

Date: Fri, 15 Oct 2004 19:53:14 +0000 (UTC)

Hi,

I've been confused by this question for a long time.

According to Intel manual "82443GX Host Bridge", the low-order system memory
space is mapped to main memory(max 2GB), in which
1) 00000-fffff is DOS compatible.
2) e0000-fffff is bios area
3) a0000-bffff is video buffer area

So is my following understanding correct?
1) During POST, bios information is copied from mainboard to bios
area(e0000-fffff).
2) In real-mode video card A/D controller reads information from video
buffer area(a0000-bffff) and output video signal to monitor. During this
process, no video memory on video card is touched.
3) DOS requires at least 1MB(00000-fffff) physical memory to run.
4) When cpu puts 00000-fffff on its address bus, the data bus will access
nothing but physical memory.
5) In protected-mode, we can't have unlimited PCI devices because only
2GB-4GB system memory space is available for mapping of AGP/PCI devices
on-board memory and registers.

Thanks,
Min

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