[MacNala]

I think it is about time I found out about 64bit computing.
To that end I am considering purchasing a new PC.
From what I have read most, if not all, new PCs have the ability to run 64bit OSs.

My question: Is it possible to run both 64bit and 32bit OSs?

Obviously not at the same time but using a removable drive, one with 32bit OS and one with 64bit OS installed. I have a number of apps based on 32bit versions of Windows XP which I currently run in virtual machines on a WinXP host. I assume that this could continue but it is the native 64bit support for 32bit apps/OSs which puzzles me.

[floppybootstomp]

The short answer to your main question is: Yes it is possible to run a 32 bit system and a 64 bit system on a 64 bit machine, either by dual booting or by booting from removable/swappable hard drives.

You see, there is a common scientific law about 32 bit and 64 bit systems similar to that age old maxim:

'A dinosaur is very very small at one end, much much much bigger in the middle and then smaller again at the other end'

A sound theory - yes?

The theory for different computer operating systems is thus:

'A 64 bit system has twice as many bits as a 32 bit system'

No arguing with that eh?

[muckshifter]

The x64 versions of Windows Vista adopt virtually none of the negative quirks of their XP x64 predecessor, bringing new levels of hardware and software compatibility to the mix.

I strongly recommend W7 64bit with any compatible 64bit PC ... you also need to know what other 64bit hardware, apart from the CPU, is needed.

64bit CPUs are relatively new, AMD having the first as opposed to Intel's software run 64bit first offerings, however, the other important component is the motherboard.

The motherboard needs to be "fully" 64bit compatible, not so much a worry these days, and not just "can be fitted with a 64 bit CPU but not really work at 64 bit 'cos they cheated" syndrome.

When it come to apps, software, there then lays a whole new ballgame.

As the soon to be Windows 7 is near release, and as I said, strongly recommend you use it and not XP, I'll shed what little I do know about it.

Under Windows 7 / Vista 64, 32-bit applications run on top of an emulation of a 32 bit operating system that is called Windows on Windows 64, or WOW64 for short. WOW64 intercepts all operating system calls made by a 32 bit application.

For each operating system call made, WOW64 generates native 64 bit system calls, converting 32 bit data structures into 64 bit aligned structures. The appropriate native 64 bit system call is passed to the operating system kernel, and any output data from the 64 bit system call is converted into a format appropriate for the calling application before being passed back.

Like 32 bit applications, WOW64 runs in user mode so any errors that occur in translating an operating system call will only occur at that level. The 64 bit operating system kernel cannot be affected.

Since WOW64 runs in user mode, all 32 bit application code must also run in user mode. This explains why 32 bit kernel mode device drivers and applications that rely on them, will not work under Windows 7 / Vista 64.

We found out the hard way in the early days that although WoW64 is implemented in WinXP, it's pretty crap without being written specifically and have 32Mb ram fitted.

The old adage of "drivers" has been pretty much eliminated.


Bottom line is ...

Most 32 bit applications will run quite happily under Windows 7 / Vista 64. The main exceptions are:

1. 32 bit device drivers.
2. Applications that cannot function without the 32 bit device drivers that they use. Prime examples are antivirus, antimalware and firewall applications.
3. Applications that rely on code injection.

Some applications may work with reduced functionality. These include uninstallers, registry cleaners and tweaking programs, amongst others, since they only have access to that part of the Registry made visible to them by WOW64


ref;

http://msdn.microsoft.com