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From: Paul <
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Newsgroups: microsoft.public.windowsxp.general
Subject: Re: Running XP on a 64bit processor computer
Date: Mon, 20 Nov 2017 22:00:39 -0500
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james@nospam.com wrote:
I have a computer which originally came witt Vista. It's a 64bit
machine. I got it with XP Pro SP3 installed. I know that XP is 32bit.
I'm a little puzzled about this. Can XP (or any 32bit OS) run on a
computer that has a 64bit processor?
(I know you cant do it the other way. You cant run a 64 bit OS on a 32
bit machine).
Does running a 32 bit OS on a 64bit machine work any better or worse
than it does using an actual 32 bit computer?
The CPU has two modes.
Mixed mode supports 32-bit and 64-bit instructions at the same time.
The processor knows which are which, via the OPcode decoder. It
knows how much of each to "suck in", based on that.
32-bit and 64-bit OSes both work in mixed mode.
So yes, WinXP x32 runs on a 64-bit CPU, as long as the
CPU is in mixed mode. And it is.
*******
I have one table somewhere, which claims these CPUs also
have a "pure" 64-bit mode. Why anyone or any hardware
company would think of invoking that, is anyones guess.
As the Mixed Mode is so much nicer. There is at least
one Linux distro, that runs in pure 64-bit mode. Maybe
they did it as a bar bet or something.
*******
On Core2 processors, the x32 instructions execute faster
than the x64 instructions. This is due to a packing
mechanism the CPU uses. It has some 64 bit plumbing, which
can suck in two 32 bit things at a time. That packing
mode adds maybe 5% more performance (for certain programs)
to the CPU. When you switch to the 64-bit OS, you have
to balance any (slight) efficiency gain from 64-bit wide
data handling, versus the loss caused by the lack of packing.
More modern processors no longer have that distinction,
and then 64-bit wins as long as the data you need to
handle is really wide.
For counting loops, many counting loops use small numbers.
The 32-bit storage handles them just fine. Using a 64-bit
OS, doesn't make such counting work any better.
However, if you do arbitrary precision arithmetic with
GMP library, the 64-bit processing makes a big difference.
Code run in 64-bit mode is 70% more efficient than code
run in 32-bit mode. I ran some C code for Mersenne Primes
on top of GMP and benchmarked it. That's the most speedup
I know of, because of using a 64-bit OS. For many other
things, the 64-bit OS buys you... sweet nothing.
The main benefit of 64-bit, is allowing a program
to use more than 2GB of memory. Like the Firefox
people who want to eat all the RAM in the computer.
That's why they're pushing the 64-bit version. I sometimes
use 32-bit versions of programs, for the express
purpose of "containing" them. It's a kind of memory
quota, and prevents runaway behavior.
*******
The only thing that doesn't work, is installing a
64-bit OS on a 32-bit-only CPU. And you know that
already. The installer disc would tell you early on,
it cannot be done. For example, my AthlonXP CPU
is 32-bit only, which means half the DVDs in the
house cannot be used on it (for OS installs). A
couple of my early Pentium 4 processors are in
that predicament (32-bit only).
*******
The single biggest issue with putting WinXP on a
new machine ? The AHCI driver for the HDD interface.
If you get a blue screen and an "Inaccessible Boot Volume"
error, that's what is blocking it.
You need to press F6 early in the install and offer
a floppy diskette with the AHCI driver. WinXP
has *some* in-box drivers, so if you configure the
BIOS correctly, there won't be an issue. If you leave
it in AHCI... there will be an issue. If you have
the small Intel TXTSETUP.OEM file set, you can fix
it via F6. The prompt appears at the bottom of the
screen, early in the WinXP install process.
I think my laptop is stuck in AHCI mode, so
that's the first issue I'd have to deal with.
You can also slipstream an AHCI driver into a
WinXP installer disc, and then when installing,
you don't need to press F6 and you don't need
a floppy drive. So that's how heroic WinXP users
running newer hardware can do it.
Paul
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