[PATCH] Revised Documentation/bigmem.txt

Myrdraal (myrdraal@jackalz.dyn.ml.org)
Mon, 18 May 1998 09:18:43 -0400

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Thank you very much those who responded on my previous thread. Here is
an updated version of Documentation/bigmem.txt. Better?

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--- linux/Documentation/bigmem.txt.bak Sat May 16 22:48:06 1998
+++ linux/Documentation/bigmem.txt Mon May 18 09:18:37 1998
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+ -----------------------------------
+ [Mon May 18 09:18:27 EDT 1998]
+* What is the purpose of this document?
+The purpose of this document is to tell you how to set up Linux to work on
+x86 machines with more then 950MB of RAM. If you do *not* have more then
+950MB of RAM, you do not need to read any further. If you have an Alpha
+or some other 64bit architecture, then you do not need to worry about this.
+* Okay, so what do I need to change?
+What you need to do is edit include/asm-i386/page.h and arch/i386/vmlinux.lds
+and change where it has '0xC0000000' to some other value. Note that this
+is in hexadecimal.
+* What does '0xC0000000' mean?
+0xC0000000 indicates that the kernel has a virtual address space of 1GB,
+which means that the physical memory the kernel can use is limited to 950MB.
+* So, what am I supposed to change it to?
+Well, for example, if you have 2GB of physical memory, you could change
+it to 0x70000000. This would give your kernel slightly more then 2GB
+of virtual memory. This is enough to map all your physical memory, plus
+a bit more for various I/O memory mappings.
+* Why doesn't the kernel just default to letting you use 3gb physical memory?
+Basically, it is a tradeoff between physical and virtual memory. The default
+allows you to have 1gb physical memory (Slightly less, actually) and 3gb
+virtual memory. 'virtual memory' means the size per process and the amount
+of swap. If you set the maximum physical memory to 3gb, that means you only
+have 1gb of virtual space (The x86 architecture is limited to 4gb physical
+memory) - No one process could use more then 1gb of virtual memory. So you
+would need multiple processes to use all your memory. mmap()ing files counts
+as virtual memory.
+* I have more questions, who can I ask?
+You may feel free to send email to myrdraal@deathsdoor.com, and I will
+respond as soon as possible. If that email address does not work, use
+ -Myrdraal


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