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SubjectRe: [PATCH] 2.4.4 kernel reports wrong amount of physical memory
On Mon, 14 May 2001, Rik van Riel wrote:

> It would be cool if one of you two could update the docs ;)

OK, here is my attempt, as a patch to Configure.help in 2.4.5-pre1. I
hope it is clear, accurate, and not too long-winded, and that my mailer
does not munge patches.

Cheers,
Wayne

--- linux-2.4.5-pre1-lcd-via-3.23/Documentation/Configure.help.orig Sat Apr 28 00:27:57 2001
+++ linux-2.4.5-pre1-lcd-via-3.23/Documentation/Configure.help Mon May 14 21:10:30 2001
@@ -198,38 +198,39 @@

High Memory support
CONFIG_NOHIGHMEM
- Linux can use up to 64 Gigabytes of physical memory on x86 systems.
- However, the address space of 32-bit x86 processors is only 4
- Gigabytes large. That means that, if you have a large amount of
- physical memory, not all of it can be "permanently mapped" by the
- kernel. The physical memory that's not permanently mapped is called
- "high memory".
+ Linux can use up to 64GB (64 Gigabytes) of physical memory on x86
+ systems. However, the address space of 32-bit x86 processors is only
+ 4GB large. As Linux allocates 3GB of this address space to user
+ space, and the kernel reserves 128MB of address space for its needs,
+ only 896MB of address space is available for mapping physical
+ memory. This Configure option controls how the kernel uses this
+ 896MB window to access physical memory.

- If you are compiling a kernel which will never run on a machine with
- more than 1 Gigabyte total physical RAM, answer "off" here (default
- choice and suitable for most users). This will result in a "3GB/1GB"
- split: 3GB are mapped so that each process sees a 3GB virtual memory
- space and the remaining part of the 4GB virtual memory space is used
- by the kernel to permanently map as much physical memory as
- possible.
+ The default choice, "off", causes the kernel to permanently map
+ physical memory into this 896MB window. The kernel will be able
+ to use at most 896MB of physical memory, but this is fine for most
+ users.

- If the machine has between 1 and 4 Gigabytes physical RAM, then
- answer "4GB" here.
+ The "4GB" option allows the kernel to access up to 4GB of physical
+ memory. This is done by dynamically mapping the physical memory
+ into the 896MB window as necessary; such dynamically mapped memory
+ is known as "high memory".

- If more than 4 Gigabytes is used then answer "64GB" here. This
- selection turns Intel PAE (Physical Address Extension) mode on.
- PAE implements 3-level paging on IA32 processors. PAE is fully
- supported by Linux, PAE mode is implemented on all recent Intel
- processors (Pentium Pro and better). NOTE: If you say "64GB" here,
- then the kernel will not boot on CPUs that don't support PAE!
+ The "64GB" option is necessary to support more than 4GB of physical
+ memory. This selection turns Intel PAE (Physical Address Extension)
+ mode on. PAE implements 3-level paging on IA32 processors. PAE is
+ fully supported by Linux and PAE mode is implemented on all recent
+ Intel processors (Pentium Pro and better). NOTE: If you say "64GB"
+ here, then the kernel will not boot on CPUs that don't support PAE!

- The actual amount of total physical memory will either be
- auto detected or can be forced by using a kernel command line option
+ The actual amount of total physical memory will either be auto
+ detected or can be forced by using a kernel command line option
such as "mem=256M". (Try "man bootparam" or see the documentation of
your boot loader (lilo or loadlin) about how to pass options to the
kernel at boot time.)

- If unsure, say "off".
+ If unsure, say "off" if you have 896MB or less of physical memory;
+ say "4GB" otherwise.

Normal PC floppy disk support
CONFIG_BLK_DEV_FD
-
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