well;
you can do that.in fact saving the entire context on stack and preserving stack pointer is normally used with systems with large stack.in 8051 the built in stack may not be sufficient.you shall have to limit the no.of tasks you can run.saving context frame on built in stack is natural and fastest way.probably keil does same thing in
"fast task"and achieve 70-100 microsec switching times.a keil user may tell you better.cmx switch time of 200-300 micro is
a definite indication of stack swap which i mentioned earlier.
dont forget to save the register set,not only program addresses.
pranav