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Half of Windows 7 Machines Running 64-Bit Version
nk497 writes "Microsoft has said that nearly half of machines running Windows 7 are using the 64-bit version, up from just 11% of PCs running Vista. The 32-bit version is limited to 4GB RAM, while the 64-bit version allows 192GB, as well as added security and virtualization capabilities. While Microsoft is pushing 64-bit as a way to gain performance in the OS, it earlier this year advised users to install the 32-bit version of Office 2010, 'because currently many common add-ins for Office will not function in the 64-bit edition.'"
Is there a good technical reason for 32-bit Windows 7 not supporting more than 4 GB of RAM, period? PAE has been in use for a long time now, and while you can't have a single process that exceeds 3 GB in Linux (tunable, I'm given to understand, can also be a 2 GB per process limit in some installations), you can definitely go past 4 GB of total system memory.
PAE can break badly written drivers, which are more common on desktop versions of the OS than they are on server versions.
Everything I've ever thrown at 64 bit windows runs just fine, and usually somewhat smoother than 32 bit. Even some really old stuff. The only software I ever found that don't run on 64-bit are some really old dos games and utilities, but then they didn't even run under 32-bit XP either.
It boggles my mind why so many people with 64-bit hardware would still install a 32-bit version of windows. I wonder how much of this is actually ignorance and/or just force of habit rather than actual knowledge that they have something that actually doesn't work under 64 bit.
You try accessing more than 2GB of RAM (or 3GB of RAM with the /3GB switch in boot.ini) in a single process. What you end up having to do is (firstly) your own memory management (Which sucks) and having to manage multiple 2GB "windows" so if you want to read data you have to swap in an out of these "windows" to be able to read them as the kernel itself is only 32bit and can only directly address 4GB of RAM.
So you end up coding in what is known in Windows as "Addressable Window Extensions" and they are a pain in the arse. Doing this on SQL server and Oracle was basically a necessity, and when PAE was first thought of, this is exactly what was being thought about, database systems. They have been able to use PAE in VMware etc and other places as they give the upper and lower limits for memory address directly to the operating system (Windows, Linux whatever else is actually running in the VM) and then they address via the hypervisor that memory address space, meaning that the hypervisor doesn't have to do a lot of memory management (Certainly nothing like protected memory)
So in effect, the biggest reason for 32bit Windows 7 not supporting more than 4GB of RAM is because the kernel itself is a 32bit app and doesn't have the 64bit address space to directly address more than 4GB of RAM itself.
In the long term it's just too hard, and it's easier to code for 64bit than to deal with what are effectively kludges to make this work.
If you need to know more about this, I would suggest Mark Russinovich's Windows Internals book.
Curiosity was framed; ignorance killed the cat. -- Author unknown
But yes, it is an artificial limit. Their lower end products are limited on purpose. For example Home Premium is limited to 16GB which is as much as you are likely to see in a home system, but pros might want more. Hence 7 pro (and ultimate) has a higher limit.
As to why they choose to limit it to 192GB? No idea.
The actual limit for The Windows NT 6.1 setup is 2TB. I don't remember the particular technical reasons for that, but they are there. There is no reason to process addressing for the full 64-bits of memory when no system exists that can take it. You'll also find that CPUs have memory limits lower than the 64-bit cap. They don't have all 64 address lines because it is not needed. I don't know what it is currently, but it is still below 64-bit. Again, no system could possibly have that much (never mind space, a memory controller couldn't handle the electrical load) so no sense in adding hardware you don't need.
Thus are ARE real limits below the actual 64-bit space but you are correct, 192GB is not one of them. That is an artificial limit and I don't know why they chose it. Doesn't really matter, I do not see people using more than that in a desktop system (144GB is the most I've seen workstation hardware support) and they can always up the limit.
However you are correct that it seems odd.