11-26-2012
Quote:
Originally Posted by
vistastar
I thought OS don't allocate any swap area nor physical memory to a new got virtual space
That depends on how you define "allocate". Most OSes do either a partial or a full memory reservation. This reservation is backed by available disk and ram combined
Quote:
but allocate physical pages directly when this space is written/read, and swap out when done some thing.
You are describing pagination more than allocation.
Quote:
Do you mean, OS allocate swap area to a process when it use malloc to ask for memory, but swap it into physical memory when this space is written/read?
No. I mean most Unix OSes reserve (some or all of the allocated) space and this space must exist somewhere, whether in RAM or on disk or on tape or whatever doesn't matter.
Last edited by jlliagre; 11-26-2012 at 09:17 AM..
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MEM(4) Kernel Interfaces Manual MEM(4)
NAME
mem, kmem - main memory
SYNOPSIS
major device number(s):
raw: 1
minor device encoding:
mem: 0; kmem: 1; null: 2
DESCRIPTION
Mem is a special file that is an image of the main memory of the computer. It may be used, for example, to examine (and even to patch) the
system.
Byte addresses in mem are interpreted as physical memory addresses. References to non-existent locations cause errors to be returned.
The file kmem is the same as mem except that kernel virtual memory rather than physical memory is accessed. Only kernel virtual addresses
that are mapped to memory are allowed. Examining and patching device registers is likely to lead to unexpected results when read-only or
write-only bits are present.
On PDP-11s, the I/O page begins at location 0160000 of kmem and the per-process data segment for the current process begins at 0140000 and
is USIZE clicks (64 bytes each) long.
FILES
/dev/mem
/dev/kmem
/dev/MAKEDEV script to create special files
/dev/MAKEDEV.local script to localize special files
BUGS
On PDP-11's, specifying an odd kernel or user address, or an odd transfer count is [generally] slower than using all even parameters.
On machines with ENABLE/34(tm) memory mapping boards the I/O page can be accessed only through kmem.
3rd Berkeley Distribution January 28, 1988 MEM(4)