setting memory size with hw.physmem

[Manjunath Ranganathaiah]

On of my system with 8G of RAM running 7.0R amd64, dmesg shows memory usage as:

usable memory = 8576397312 (8179 MB)
avail memory  = 8289071104 (7905 MB)

If I set hw.physmem tunable to  8g, dmesg shows the following

usable memory = 7771095040 (7411 MB)
avail memory  = 7507386368 (7159 MB)

If I set  hw.physmem to 8950M

usable memory = 8565911552 (8169 MB)
avail memory  = 8278892544 (7895 MB)

if I set hw.physmem to 8999M

usable memory = 8576397312 (8179 MB)
avail memory  = 8289071104 (7905 MB)


Is this expected behavior?
How do I  increase or decrease physical memory size by 1MB?


Thanks,
-Manjunath