user manual

AN806: Memory Upgrade Resource Guide Page 3
bytes, which equates to 4GB. However, part of this 4GB „address space‟ must be reserved
for devices that require MMIO (Memory-Mapped Input Output).
As an example, one such device is the graphics card, which requires an amount of address
space equal to its frame buffer size (the amount of memory installed on the card) to be
„reserved‟ for such data exchanges. This reserved address space is therefore not available
to Windows as accessible memory.
Memory usage in 64-bit operating systems
With 64-bits available to represent the locations of bytes of memory the maximum
theoretical memory address size increases to 2
64
bytes, which equates to 16,384GB (yes,
gigabytes!). In reality the 64-bit versions of Windows Vista support up to 128GB, or even
more. Therefore, for 64-bit systems, the address ranges for devices that require Memory
Mapped Input Output can be set well above the 4GB address line.
In summary, a 64-bit system with a 64-bit version of Windows Vista or XP will be able to
utilise the full 4GB of installed memory. Users with a 32-bit operating system will also see a
significant increase in useable memory of up to 75%.
So, what does all this mean to a person purchasing memory? In short, with a 32 bit OS, a
system cannot fully address all of the installed system memory. On current motherboards,
the PCI architecture will need about 512MB of address space reserved in the MMIO “office
block” outlined above. Also, other devices with onboard memory can reserve space in
addition to the 512MB PCI block. Video cards, sounds cards with onboard memory, tuner
cards, et al can reserve additional space in this upper register area.
Real Life Example
Let‟s take the computer I use daily as an example. I built it using the following set of
components:
 Asus P5K Deluxe motherboard
 Windows XP 32bit, Service Pack 2
 2 x 2GB of Corsair PC6400C5
DHX
 Abit X600 Pro with 256MB of
Onboard Video Memory
If you look at system properties, you can see
that I can use 3.25GB of the 4GB of system
memory. This is because of the MMIO
reservation of 512MB of address space for
the PCI devices and the 256MB reservation