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PAE has compatibility problems with some drivers. PAE was actually supported in the original release of XP, but was disabled by SP2 because it just didn't work reliably.

Is this really an issue any more?

The distinction is so miniscule as to be irrelevant.

Any computer a CUSTOMER bought for their desktop at home, or in their office, was likely to be XP, or Vista 32 bit, or WIndows 7 32 bit.

In late 2003, I put together a machine with 8 GBytes of RAM. The idea was to run a database management system in it to do backtesting. And to get that to work fast enough, I wanted as much of the database to reside in memory as I could manage. Realistically, keeping the whole database in RAM was out of the question, but I set it up so that most of the indices would be there.

The 8 GBtes of RAM were, in fact, all used. Each individual process was limited to 4 GBYTES. There were two Xeon processors in there each with PAE. The operating system's kernel (Red Hat Enterprise Linux 3 at the beginning) could access all of RAM if it wanted because it controlled the memory mapping registers (inaccessable to user application prorams). What the OS did was take a few megabytes for itself, assigned bits of the rest to the applications, and used the rest as a big cache for data. Anything read in from disk was just kept in a cache and the least recently used was discarded (unless modified) when RAM was needed for something more important. That was all routine for a Linux OS; I did not have to do anything special. That was all available in 2003, if not a little before. Now maybe Windows XP (current at the time) could not manage it well, but Linux sure did. Thus, the RAM was usually loaded up to about 95% of that available.

In those days, 64-bit processors were not routine in the Intel i*86 world. But some users needed more than 4 GBytes RAM, and there was hardware and software available to do that. The only reason I built that machine was because I could not get one like that with six small fast SCSI hard drives "off the shelf" from any of the manufacturers. So I would have been a CUSTOMER if that had been an option.

Even now, with 64-bit processors more common, things are not really as great as the name implies. My brand new machine, nominally 64 bits, uses an Intel Four Core XEON E5-2603 processor (the M.B. can accept two of these, but for what I am doing now, one is enough) but that processor can handle only 2^46 bytes of RAM, not the full 2^64 bytes. There are only RAM sockets for 8 memory modules for each processor, so if 4 GByte modules are the largest there are, I can get only 32 GBytes in there unless I put in the other processor for a total of 64 GBytes of RAM. Since I am no longer doing that database work, I imagine I will never do even that.

But this is all ancient history, by computer standards. When I bought my most recent machine about a month ago, companies like DELL were not even offering 32-bit desktops anymore. I suppose their idea is that anone who even wants a desktop wants heavier iron than a 32-bit machine. Perhaps the laptops are still available in 32-bit.
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