debian 3.1 4GB内存为啥TOP只显示3GB

人面兽心

top - 03:14:38 up 38 min, 2 users, load average: 1.85, 0.59, 0.31
Tasks: 59 total, 1 running, 58 sleeping, 0 stopped, 0 zombie
Cpu(s): 1.8% us, 20.3% sy, 0.0% ni, 0.0% id, 77.6% wa, 0.2% hi, 0.2% si

Mem: 3336812k total, 1984568k used, 1352244k free, 78288k buffers
Swap: 2650684k total, 0k used, 2650684k free, 1757304k cached

物理内存是4GB，已经装上支持4GB大内存的内核kernel-2.6.8-3smp

dmesg 信息

Memory: 3331248k/4194304k available (1666k kernel code, 41596k reserved, 769k data, 172k init, 2456512k highmem)

这是为啥，到底怎么设置才能有4GB？何解？？

qtdszws

dmesg
看看下面的的几个参数是多少
zone(0): 4096 pages.
zone(1): 126960 pages.
zone(2): 0 pages.
我这个是512M