Re: question about system memory space mapping

From: Herman Dullink (spamtrap_at_crayne.org)
Date: 10/16/04 

Date: Fri, 15 Oct 2004 22:54:14 +0000 (UTC)

> So is my following understanding correct?
> 1) During POST, bios information is copied from mainboard to bios
> area(e0000-fffff).
Actually, a copy of a run-time BIOS is placed to that area.
BIOS does many things, such as POST. It also has many functions
that are used by the OS and programs.
During POST, the part that is used by the OS is copied from slow ROM
to much faster RAM.

> 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.
No, for DOS compatibility, RAM normally in this area is disabled, and
(a part) of the memory on the video card is placed here instead.

> 3) DOS requires at least 1MB(00000-fffff) physical memory to run.
No, depending on the version DOS requires between 64KB and 256KB to run.
Programs need more.
As DOS was designed for a very old CPU (8088) with a physical limit of 1MB,
the standard DOS memory manager will only manage up to 1MB.
Various extensions (such as Himem.sys) will manage the rest.

> 4) When cpu puts 00000-fffff on its address bus, the data bus will access
> nothing but physical memory.
Depends, it'll access whatever the address decoding logic has placed/mapped
in that area.
A typical DOS machine configuration will have 640KB physical memory,
128KB for video memory, 192KB for expansion ROMs and memory
mapped devices and 64KB of ROM in that area.
Systems that are not (or do not have to be) DOS compatible can have nothing
but
physical memory in that area. That's up to the system and/or OS designer.

> 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.
That has nothing to do with protected mode, but address decoding. You can
have as many PCI devices as your address range allows.

H

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