Same as JD. Vista and XP both had about 3.5 gigs avail with 512 Gig GPU and dropped to about 3.2 when GPU was changed to a 1 gig GPU. Also have seen it drop to as low as 2.8 avail, it depends on how many MMIOs that require memory mapping, ie a sound card requires mapping.
There is a switch in some bios' that has to be toggled to go from 3 gigs to 4 gig (if I'm not mistaken).
There are a couple of confussion factors, such as (a) the 3 gig limit, and (B) the 2 gig limit imposed on program space - the kernal uses 2 gig which is shared between programs. Then throw in PAE.
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Memory mapped I/O and disabled RAMModern personal computers are built around a set of standards that depend on, among other things, the characteristics of the original PCI bus. The original PCI bus supported 32-bit physical addresses and 32-bit wide data transfers. PCI (and PCI Express, and AGP) devices present at least some, if not all, of their host control interfaces via a set of memory-mapped I/O locations (MMIO). The address space in which these MMIO locations appear is the same address space as that used by RAM, and while RAM can exist and be addressable above the 4 GB point, these MMIO locations decoded by I/O devices cannot be. They are limited by PCI bus specifications to addresses of 0xFFFFFFFF (232−1) and below. With 4 GB or more of RAM installed, and with RAM occupying a contiguous range of addresses starting at 0, some of the MMIO locations will overlap with RAM addresses.
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