That depends upon the tasks.

Every task will use the accumulator, PSW, PC and SP. So they are definitely part of the task context.

However the tasks may also use(and most of them do) the register banks. You need to save their contents if other tasks are sharing the same register bank. In such a case the registers(of the register bank) may also be treated as part of the context.

The same can be said for the B register, since it will be shared by all tasks which use the MUL and/or DIV instructions.

Any register which if fiddled with will cause the task to malfunction should be saved before the task is "swapped out" of the cpu.

Rumi