Hardware Reference
In-Depth Information
addressing supported 1MB, but the upper 384K was already in use by the system hardware (BIOS,
video, adapter ROM, and so on).
Figure 6.24
shows the memory map for system using an Intel G45 chipset, which supports a maximum
of 16GB of RAM. For a 32-bit OS, the line labeled “Top of usable DRAM (32-bit OS)” is at
4,096MB. Note that the PCI memory range, FLASH, APIC (Advanced Programmable Interrupt
Controller), and Reserved areas take up a total of 770MB of the memory below 4GB. You can also
see the 384K (0.375MB) of memory below 1MB that is used by the system. This means that if you are
running a 32-bit OS, even if you have 4GB of RAM installed, the amount usable by the OS would be
4,096MB - 770MB - 0.375MB, which is 3,325.625MB (or about 3.24GB, rounded down).
Figure 6.24. Memory map for a system using an Intel G45 chipset.
Can any of that unused memory between 3GB and 4GB be reclaimed? For those running a 32-bit OS,
the answer is no. However, if you are running a 64-bit OS on a system that supports memory
remapping (primarily a function of the motherboard chipset and BIOS), the answer is yes. Most
modern motherboard chipsets have a feature that can remap the otherwise disabled RAM in the fourth
gigabyte to the fifth (or higher) gigabyte, where it will be both visible to and usable by a 64-bit OS.
Note, however, that if the motherboard doesn't support remapping, even when a 64-bit OS is being
run, the memory will be lost.
Note that the 3GB limit is not as strictly defined as it was with the 640K limit. This means that if you
