Real and Free Memory

Daniel Rock hackers at
Wed Jul 20 17:45:56 GMT 2005

Özkan KIRIK schrieb:
> # cat /var/run/dmesg.boot | grep real
> real memory  = 268435456 (256 MB)
> # sysctl vm.vmtotal  | grep Real
> Real Memory:            (Total: 232792K Active 122448K)

Real memory output from dmesg isn't the total amount of memory in the
system, but only the highest address where memory was found. Memory holes
below may reduce the available memory:

real memory  = 6442450944 (6144 MB)
Physical memory chunk(s):
0x0000000000001000 - 0x0000000000090fff, 589824 bytes (144 pages)
0x0000000000985000 - 0x000000007ff1ffff, 2136584192 bytes (521627 pages)
0x0000000100000000 - 0x0000000174baffff, 1958412288 bytes (478128 pages)
avail memory = 4085907456 (3896 MB)

In above output you can see a very large memory hole of 2GB in total. The
machine has 4GB RAM. Between 2GB-4GB has a memory hole to map in the PCI
address space. Depending on the BIOS you will likely find a memory hole
between 640kB-1/4/16MB in your system. Just boot into verbose mode and read
the lines after "Physical memory chunk(s):"


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