Memory exhausted in AIX 4.2..

Hi,
    Recently, I tried to run Squid in our new ibm rs/6000 machine.
    Everything seems well, except core dumped every half hour. The message
    is "FATAL: xmalloc: Unable to allocate 4096 bytes!". I don't know why
    because this machine has 1G real memory, how can it be not enough for
    it?
    So, after several core dumped, I found that all have this kind of info:

        Memory Usage: rss 523536 KB (almost 5XXXXX KB)

    Then I know that it is true that it really finished all the memory.
    If I use "top" to monitor the system process, something look like this,

 Memory: 1536M Total. Real: 1024M, 229M Free, 629M Buffers. Virtual: 512M, 386M
                                   ^^^^^^^^^
    The 'Free' memory become less and less and finally, Squid died. So, can
    anybody tell me how to solve it?
    BTW, there is a core dump exist, but too small (about 24k) and I can't
    use debugger to check it out (another core dump..:(), show me something
    like 'Illegal instruction (core dumped)'.

--
Alex Lau