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Re: What is the maximum physical RAM for a 32bit MIPS core?

To: Hartvig Ekner <hartvige@mips.com>
Subject: Re: What is the maximum physical RAM for a 32bit MIPS core?
From: "Steven J. Hill" <sjhill@cotw.com>
Date: Tue, 05 Feb 2002 14:46:31 -0600
Cc: linux-mips@oss.sgi.com
References: <200202051747.SAA21696@copsun18.mips.com>
Reply-to: sjhill@cotw.com
Sender: owner-linux-mips@oss.sgi.com
Hartvig Ekner wrote:
> 
> You have to distinguish between physical and virtual memory. The MIPS32
> architecture supports implementations with up to 36 bits of physical
> address space, however the virtual address space in kernel and user mode
> is as you describe below.
> 
I wasn't talking about the MIP32[tm] cores specifically, I was using a
generalization of 32bit. However, this is good to know. All of the data
sheets that I just downloaded from the MIPS site for the R4k[X] cores
don't mention the 36-bit PA item. Care to elaborate?

> One note: Many MIPS32 implementations choose not to implement all 36 PA
> bits, but limit themselves to 32 bits. This saves a few bits in the TLB
> and a few address lines.
> 
So, if someone did want 36 PA bits on Linux, the TLB exception handlers
and a little of the page table construction/management code would have to
change. The userspace contraints and such would still remain. Cool.

-Steve

-- 
 Steven J. Hill - Embedded SW Engineer

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