distinguish between Maxmem, realmem, physmem

[Ping Chen]

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

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)

Could someone explains more?

Thanks

Ping