[mythtv-users] I've lost 1 gig of ram

Josh Mastronarde jmastron at gmail.com
Wed Jan 21 06:14:09 UTC 2009


On Tue, Jan 20, 2009 at 9:43 PM, Claude Gélinas <claude at phyto.qc.ca> wrote:

> So now where is my missing 1 gig of ram...

I can help with that -- I work on memory controller chipsets for, urm,
let's just say your CPU manufacturer's major competitor :-)

Many system devices (PCI cards, PCI devices integrated on the
motherboard, BIOS, etc) require address space below 4GB (addressable
with a 32-bit address, 2^32=4GB).  When you have 3GB or less DRAM,
there's no conflict, but with 4GB+, a "hole" must be carved out,
generally from 3GB-4GB.  The BIOS sets that up and reports the memory
ranges to the OS (via ACPI tables, usually).

DRAM is generally addressed in a fixed fashion relative to the CPU, so
by default the DRAM "behind" the hole is simply not used.  Many of our
chipsets, and I assume some of those for AMD also, have a feature
called "remap", "reclaim", or similar, that allows that "hole" DRAM to
be visible at a new location above the top of real DRAM.  So, CPU
0-3GB = DRAM 0-3GB, CPU 4-8GB = DRAM 4-8GB, and CPU 8-9GB = DRAM 3-4GB
(the last one only if remapping is enabled, giving 8GB total,
otherwise 7GB).  There's often a slight performance penalty due to the
extra decode/subtraction required.

In any case, look for a BIOS option to enable remapping; your
motherboard may or may not support it.

Josh


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