distinguish between Maxmem, realmem, physmem

[John Baldwin]

On Tuesday, June 26, 2012 3:41:10 pm Ping Chen wrote:
> Hi,
>    I am a bit confused with all these variables defined in 
freebsd(especially in freebsd 6.1): Which one of this represents the real 
memory of a system? Say we bought a system with 4G ram, which one tells me the 
RAM is 4G?
> 
>  Accordign to source code:
> 
> Maxmem ==> the highest page of phisycal address page  : if I understand 
correctly, this is the highest page number of physical memory that is usable?
> 
> realMem --> somehow get assigned by realmem = Maxmem:  this is confuing, if 
they are the same, why bother a realmem variables

I think realMem is legacy.

> physmem --> the number of usage pages : this seems the right one extract the 
memory info, however, it seems system allocate portion of memory to messge 
buffer which makes this physmem < 4G (assume RAM is 4G)

Correct.  Note that the firmware can also take up part of RAM as well (e.g. to 
hold ACPI tables).

-- 
John Baldwin