64MB ram

Hi,
when my MINI6440 boots up linux it shows about 16Mb ram, why is this only
16mb when they are 64mb ram chips?

Good question.  Maybe, show us your complete boot log to that point.

Hi,
Is there any sample logs floating about , I can compare to?
Just I'm running an important app just now.

Well, I searched this site to see if I saw any 16M in various boot logs
people have submitted and saw none.

Everything I saw said 64M.

I wouldn't be able to help you fix it, I was just prompting you for what I
thought was a reasonable next step that maybe someone else would recognise.

Good luck.

Does anyone know where in the kernel code the memory size is set?

Not the kernel detects the memory size. It is the job of bootloader. It
initializes the memory controller and forwards the memory size with a so
called ATAG to the kernel.
You can try to overwrite the memory size with the 'mem=64M' kernel
parameter. From "documentation/kernel-parameters.txt":

mem=nn[KMG] [KNL,BOOT] Force usage of a specific amount of memory
            Amount of memory to be used when the kernel is not able
            to see the whole system memory or for test.