Simple cooperative multitasking

Cooperative multitasking based on eForth and hForth ideas. A task uses three variables and has own parameter and return stacks.

The user pointer contains the address of the task specific data area

variable up

Address of the action to take when activating the task. At this point, only the wake action is defined.

: status        up @ ;

Address of the status word of the next task.

: follower      up @ cell + ;

Top of the parameter stack.

: tos           up @ 2 cells + ;

Suspend the current task and perform the action (wake for now) of the next task:

• Push the top of the return stack

• Store the top of the parameter stack in the task’s user variable

• Fetch the address of the next task, which is also the address of the task’s action, and perform the action

  : pause rp@ sp@ tos ! follower @ dup @ execute ;

Default action for a task:

• Restore the user pointer

• Restore the top of the stack and drop the top element

• Restore the top of the return stack, which will then have as the top element the instruction pointer a the time pause was called the last time

  : wake up ! tos @ sp! drop rp! ;