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Low Priority Taks

April 26, 2009

I’ve now implemented two tasking queues (actually three). There is a queue of runnable tasks, one of blocked tasks, and a low-priority queue. Actually, the last is not a real queue but just a pointer to a single task, which is guaranteed always to be runnable. The task-switch code will only pick this low-priority task if no other task is ready to run. The separation into queues of runnable and non-runnable tasks makes it slightly easier, and more efficient, to find the next runnable task.

The work involved in doing this took longer than I expected; it’s amazing how many unforeseen bugs turn up when making such a change. The main things that I now realize I have to be a bit more careful about are checking that any registers that may be altered by a routine are preserved, and making sure that any variables are initialized before they are used. Both these may seem obvious, but it is very easy to overlook things when calling a C routine from assembler code.

What becomes more and more obvious when trying to find these sort of bugs is just how useful it is being able to run the Operating System within a debugger (SimNow). I can’t imagine how people found these sort of errors without debuggers. I think the next move is to carefully review the current code with these points in view before looking to implement more features. Once I’ve done that I’ll post the new version and document it on the main site.

From → IanOS

2 Comments
  1. Is there anyway for you to enlarge the txt for this internet site? I’ve reading difficulties and think it is a lot easier to read larger fonts

    • admin permalink

      I’m sorry that you are having difficulties, but most – if not all – web browsers have the facility to change font sizes or enlarge everything. I think this would be a better solution to your individual problem than making changes that might adversely affect other users.

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