shared memory - progress

OK,
I can assign and manipulate shared memory in BSD/OS3.0 notes to follow.
Now the problem is that as I vary the number of shared buffers and amount of
shared memory I vary between two kinds of errors. If shared mem is too low
that generates an error - 'out of buffers', if too high, I get palloc
errors - memory exhausted.
So, now I need to lay my hands on a basic tutorial that describes in general
terms how BSD divides its total memory resource among shared/kernel etc. and
a second tutorial on how postgres uses different types of memory ie. why
does one query create a palloc error and another run out of shmem.
Thanks so much for hanging with me through this.
John