What is the PCI bus memory map on 4 GB+ machine?

NTDEV
1

I was pondering a question here on PCI access and was wondering what the
memory map for machines with more than 4 GByte of RAM is like. On less
memory machines, physical memory is mapped to PCI addresses 0 to (~size of
ram). This suggests that on 4GB+ machines there would be NO space to map
32-bit PCI targets? Am I correct to assume that 4 GB+ machines have a hole
in their memory map. Like 0-3 GB is actual ram, and 3GB-4GB is reserved for
32-bit PCI target’s, and 4GB to (size of ram) is the rest of memory? I
assume there are some other holes for ROM and APIC and such.

This would be from the point of view of PCI masters. The processor will
always work in virtual addresses so physical memory above 4 GB could
virtually be anyplace.

  • Jan

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2

I don’t remember the exact details, but there is some mapping in the chipset
just for this purpose. This is not your normal memory maps, rather something
at the physical address level.

Its my belief that this avoids the hole in physical memory just below 4G, but
I wouldn’t bet my life on it.
-DH
----- Original Message -----
From: “Jan Bottorff”
To: “NT Developers Interest List”
Sent: Thursday, April 05, 2001 5:44 PM
Subject: [ntdev] What is the PCI bus memory map on 4 GB+ machine?

> I was pondering a question here on PCI access and was wondering what the
> memory map for machines with more than 4 GByte of RAM is like. On less
> memory machines, physical memory is mapped to PCI addresses 0 to (~size of
> ram). This suggests that on 4GB+ machines there would be NO space to map
> 32-bit PCI targets? Am I correct to assume that 4 GB+ machines have a hole
> in their memory map. Like 0-3 GB is actual ram, and 3GB-4GB is reserved for
> 32-bit PCI target’s, and 4GB to (size of ram) is the rest of memory? I
> assume there are some other holes for ROM and APIC and such.
>
> This would be from the point of view of PCI masters. The processor will
> always work in virtual addresses so physical memory above 4 GB could
> virtually be anyplace.
>
> - Jan
>
>
> —
> You are currently subscribed to ntdev as: xxxxx@syssoftsol.com
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