[CentOS] 1 GB RAM CentOS 4 only sees 885 MB

Thu Apr 21 16:38:50 UTC 2005
Bryan J. Smith <b.j.smith at ieee.org>

Research the 1/3GB (32-bit) v. 4/4GB (PAE36) addressing models for Intel PAE36.
If the kernel is the former, you'll run into the limit you have.
If the kernel is the latter, you won't, but you'll take a paging hit as PAE36 swaps in/out 512MB pages from above 4GB
(yes, I know, you only have 1GB but it's complicated to explain :-).

BTW, *all* Athlons, both 32-bit and 64-bit, are actually 40-bit EV6 (Alpha 264), and not 32-bit/PAE36 addressing.
But the Athlon uses 32-bit/PAE36 paging for compatibility.
If you have a mainboard with a BIOS that has a "Linux" option for memory/IO addressing (very rare),
there is a way to use the Linux kernel to get a "flat" 64GB model,
as well as get the 32-bit Athlon's AGPgart (which is in the CPU, not the chipset - at least on the MP) to act like an I/O MMU for "safe" >4GB I/O transfers.
Not even Intel's EM64T offers that

BTW, I have *never* done this personally with a 32-bit Athlon.
I only read about the kernel hack and the few mainboard BIOSes that support it (which prevents Windows from booting when set).
Understand the 32-bit and 64-bit AMD processors are the same core design (even 48-bit/PAE52, although much is not enabled/useful on the 32-bit), including the *physical* 40-bit/1TB addressing of EV6.

Intel is still using 32-bit/4GB GTL+ *physically* on even their latest EM64T processors,
which is a major performance issue.

-----Original Message-----
From:  ryan 
Date:  4/20/05 8:27 pm
To:  CentOS mailing list 
Subj:  [CentOS] 1 GB RAM CentOS 4 only sees 885 MB

Is this a known bug? Suprisingly google didn't turn up much. I'm running
DDR RAM and a AMD XP processor on CentOS 4.

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