Multitasking

Date:2017-08-09

Intro

For increased geek factor: we can connect to our clock over serial!

A clock program can definitely be the only thing that runs on the microcontroller which is the heart and brain of our clock. However, since I (and hopefully you, too) use AmForth on the controller, I do not want to give up connecting to the running clock to inspect, how things are going, or to correct/configure things. Interactivity is too nice to give it up. That being said, one more piece is needed to execute the code of the clock (keeping track of time and the periodic jobs) in the background, namely the multitasker.

If you have never used Forths cooperative multitasker, go reading up (Multitasking) in the Cookbook section. I’ll wait, no problem.

The multitasker revolves around some pieces of per-task information, residing in the task control block, and a linked list of these blocks. The function pause in a task releases the CPU just now and lets the next task in the list resume.

Design Decisions

• There will be two tasks:

  Task1 running the command loop at the serial connection, in other words the shell

  Task2 running run-masterclock, such that all work regarding the clock is done in the background and does not interfere with the shell

• More tasks are certainly possible, but not needed so far.

• for Task2 a task: named task-masterclock is defined

• this task is activated in function start-masterclock. There could be more such starter functions, so I wanted it to remain small and separate

• in starttasker all extra tasks are started (start-masterclock) and linked into the list of task control blocks (task-masterclock tib>tcb alsotask). At last the multitasker ist started (multi).

Putting it all together

This part needs a few code lines in the main program.

\ --- multitasker
#include multitask.frt
: +tasks  multi ;
: -tasks  single ;

: run-masterclock
  begin

    tick.over? if ... then
    ...

    pause
  again
;
$40 $40 0 task: task-masterclock \ create task space
: start-masterclock
  task-masterclock tib>tcb
  activate
  \ words after this line are run in new task
  run-masterclock
;
: starttasker
  task-masterclock task-init            \ create TCB in RAM
  start-masterclock                     \ activate tasks job

  onlytask                              \ make cmd loop task-1
  task-masterclock tib>tcb alsotask     \ start task-2
  multi                                 \ activate multitasking
;

At the prompt enter:

> init starttasker

and the show should start. Get a list of tasks with:

> tasks