Re: Using virtual memory and/or disk to save reduce memory footprint
On Mar 11, 3:28 pm, Pawel Dziepak <pdzie...@quarnos.org> wrote:
James Kanze wrote:
(Also, FWIW: I just checked the Intel manuals, and the
linear address space is 36 bits, not 32. So there's
absolutely no argument in favor of taking part of the users
32 bit address space for the system.)
Volume 3, 3.4: "A linear address is a 32-bit address in the
processor's linear address space. Like the physical address
space, the linear address space is flat (unsegmented),
2^32-byte address space with addresses ranging from 0 to
FFFFFFFFh."
I don't see anything that can make me believe that linear
address space is 36 bits. Virtual addresses are always 32-bit,
hence virtual address space is 4GB.
Volume 3, 3.3: "the IA-32 architecture also supports an
extension of the physical address space to 2^36 bytes (64
GBytes);"
However, it is impossible to directly use physical address
space and that's why programs are given no more than 4GB
virtual address space.
Let's see if I understand what you're saying. Intel has
provided some additional addressing modes which it is impossible
to use.
There are also logical addresses (used with segmentation)
which are 48-bit.
I think that we shouldn't argue what is the size of address
space, because that is said in Intel manual. The problem is
that only segmentation (logical addresses) makes simultaneous
access to more than 4GB possible, and even then it is highly
inefficient and (IMHO) rather useless.
You need segmentation for security reasons; a client process
will not use the same segments as the OS. And the processor has
six segment registers---you can access up to six segments
simultaneously, and at no real additional cost. (Loading a
segment is fairly expensive, but typically, you have to do that
when changing to system mode anyway, for security reasons.)
Segmentation lacks few features of paging, that make using it
harder from the kernel's point of view.
Segmentation and paging are more or less orthogonal. And with
an Intel 32 bit processor, you always use segmentation, "There
is no mode bit to disable segmentatin" (A direct quote from the
Intel manual.) Some things (particularly security issues) are
best handled by segments, others (paging) by virtual memory. A
good OS for the Intel will use both.
An example may be dealing with concurrent memory allocation
and noncontinuous pages. Of course it is always possible to
use it together with paging but then you have two similar
mechanisms doing almost the same.
I don't think you understand segmentation, at least not as it is
implemented in the Intel. It's very different from paging,
almost orthogonal. And it can't be turned off.
The next problem is portability, paging is supported by a
large number of architectures, segmentation - not really.
That's true. And the few others that support segmentation
define it very differently---what IBM calls a segment on its
mainframes has almost nothing to do with what Intel calls a
segment. But then, the addressing architecture of the Intel
*is* different than that of most other processors. As is a
number of other things. Any time you port an OS to a new
architecture (or even a new implementation of an existing
architecture), there are a number of things that have to be
adapted: things like context swapping and memory management tend
to be very processor specific.
When most of OSes were designed 4GB was so big number of
memory, that nobody worried about it.
I don't know about "most OSes", but both Linux and Windows NT
are fairly recent; DEC was already feeling the crunch of only 32
bit addressing in the VAX when Dave Cutler and his group left
DEC for Microsoft to implement Windows NT, and Torvalds started
Linux even later. Most desktop machines couldn't support 4GB,
but larger machines could. And of course, everyone is aware of
Moore's law.
That was a mistake, but IMHO when you have to use some strange
techniques to access more memory on a certain architecture
that's a sign that you have to change to 64-bit hardware, what
undoubtedly would be a better solution.
Using a 64 bit architecture is certainly a better solution. If
only because even 4 GB isn't really enough for todays
processors.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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