Why does my system lose up to 1GB of physical memory?
If a system is configured with 4GB of memory or higher, a large chunk of memory (memory hole) will be lost. This memory region is reserved for the following purposes:
- System BIOS (including motherboard, add-on cards, etc.)
- Motherboards resources
- Memory mapped I/O
- Configuration for AGP/PCI-Ex/PCI
- Memory allocations for other PCI devices
This is a legacy limitation of the PC architecture that affects both Intel and AMD platforms, whether they are running under 32-bit or 64-bit mode. The details of the memory hole can be viewed here.
http://techfiles.de/dmelanchthon/files/memory_hole.pdf
ftp://download.intel.com/support/motherboards/server/sb/ta71902.pdf
As a workaround, most motherboards implement the BIOS option "software memory hole" that is used to reclaim the lost memory region. Unfortunately, this is a hit and miss solution. Often the implementation is buggy. That leads to poor performance, system instability, crashes and problems with device drivers.
http://lists.opensuse.org/opensuse-amd64/2005-07/msg00147.html
http://marc.info/?l=linux-kernel&m=112213584216800&w=2
http://marc.info/?l=linux-kernel&m=117520560707409&w=2
Running ASL internal stress test, we also encounter the same stability issue. Thus software memory hole is always disabled by default at the factory.