No, sorry, you're getting confused between /PAE, which enables Processor Address Extensions in the processor and therefore adds more physical memory address lines, and /3GB, which changes the virtual address space division between user and kernel mode (normally 2GB each). The former makes more physical memory available to the system as a whole; the latter allows more virtual address space for processes which need a lot, often for buffering disk contents (e.g. databases, email servers). You're correct that just throwing the /3GB switch isn't enough: processes have to be aware that the address space can be bigger than 2GB, and therefore that addresses of memory objects may have the high bit set. Mistreating such addresses - e.g. casting them to signed integers - can cause problems. Therefore Windows caps the address space at 2GB for any process that doesn't have the Large Address Aware bit set (link with /LARGEADDRESSAWARE). Microsoft Exchange Information Store is notoriously bad at managing its virtual address space and their recommendation is that if you have more than 1GB of physical memory fitted, you should boot with the /3GB switch and also use /USERVA to tweak it further (3030MB, if memory serves). Otherwise it will run out of virtual address space before it runs out of physical memory. Exchange Server 2007 avoids this problem by requiring that you run the 64-bit OS for a production system. In addition they recommend not fitting more than 4GB of RAM. The extra RAM can't be used by Exchange anyway, and the data structures required to manage it swallow up large chunks of the smaller system address space, leaving even less space for drivers and TCP connections. The effect of /PAE is to change the page table entries' physical address field from being 32-bits wide to 64-bits wide. In the original Athlon 64, AMD changed the meaning of the top bit to 'No Execute'. Hardware Data Execution Prevention (DEP) sets this bit to tell the processor that execution isn't allowed on this page, so the processor will generate an access fault if execution is attempted. There wasn't room to put this on the 32-bit Page Table Entry - all the bits were already in use. If you're on a system which supports hardware DEP, Windows will put the processor in PAE mode automatically to enable this feature. However, it won't use any physical memory above 4GB. Why? Because when testing this addition in XP SP2, many devices and drivers were found to fail when trying to do DMA from addresses above 4GB. PAE mode has been around for ye
