Hi

My question:
if i use a lot of local variables in a taskA - i.e. some of them are in CPU registers and some are put on the stack (thats how memory normally are allocated).

Now a higher priority taskB preemtives taskA and do some job and returns control to taskA.

When taskA runs again are all local variables valid or do i have to declare them static - i asume them in Cpu regs. are restored but what about the local vars. which where stored on the stack in taskA ???????

yes, all registers are saved on the stack before a context switch and restored afterwards, so your vars will be intact..