Life becomes a lot simpler when you don't have to worry about time and memory constraints.

But when you talk about C51 code, the problem with cooperative multitasking isn't just that each task needs to have it's own stack.

The traditional use of shared global variables for parameter passing and local variables based on call tree analysis will also be affected.

In a cooperative system, each task may have multiple calls to inform that a task switch is possible. And these calls need not be located in the outermost function, but may for example be placed in delay() calls etc.

This produces a very complex call tree - the problem is similar to a program that makes heavy use of function pointers.

That is why I say that cooperative multitasking is problematic for C51 processors.

For most other processors this wouldn't be a problem. They would keep all auto variables and parameters on the stack, so it would be enough to switch stack pointer (or swap the stack, if the processor for some reason has too little direct-mapped RAM to fit multiple stacks, but have secondary RAM that may be used as backing store).