CS452 - Real-Time Programming - Spring 2017

Lecture 7a - Create (continued)

Public Service Annoucements

  1. Due date for kernel 1: Friday, 26 May, 2017


There are two important issues for scheduling

  1. When do we reschedule?
  2. Who do we activate when we schedule

When to schedule

Every time we are in the kernel, so the question is `When do we enter the kernel?'
The answer is: whever user code executes SWI.

Who to Schedule

Whoever is needed to meet all the deadlines

Because this is not an easy problem, we don't want to solve it within the kernel. What the kernel does should be fast (=constant time) and not resource constrained.

Scheduling algorithm

  1. Find the highest priority non-empty ready queue. A ready queue can be as simple as a linked list of pointers to task descriptors.
  2. The task found is removed from its queue and becomes the active task. (Until this point active has pointed to the TD of the previously active task.)
  3. When a task is made ready it is put at the end of its ready queue.Thus, all tasks oat the same priority get equal chances of running.

Implementation Comments

The main data structure is usually an array of ready queues, one for each priority.

Each ready queue is a list with a head pointer (for extraction)and a tail pointer (for insertion).



  1. How many priorities
  2. Which task should have which priority
  3. What to do when there is no ready task

General structure

Array of ready queues, one for each priority.

Each ready queue is a list with a head pointer (for extraction)and a tail pointer (for insertion).

Implementing lists without memory allocation

You are probably used to implementing lists (and similar data structures) like this

      struct element { struct element *next;
		       struct whatever *content;
      struct list { struct element *head;
		    struct element *tail;
      void insert( list *l, content *c ) {
	struct element *e;
	e = malloc( element );
	e->content = c;
	e->next = null;
	l->tail->next = e;
	l->tail = e;
We don't like this because it requires allocating and freeing memory.

Here's the most common way to do this without allocating memory

      typedef struct task-descr { ...
				  struct task-desc *rdy-next;
      } TD;
All the allocation is done when the task descriptors are declared.

Of course, because allocating and freeing constant sized pieces of data can be done in constant time, you could allocate a pool of list elements when you initialize, and manage it using a free list.

Static/Global Variables.

There are four ways we can use memory for storing data

  1. in the text section with the instructions, constant data only,, visible only to tasks that can see the instructions,
  2. in the bss section, uninitialized data only, visible by any task running the same code,
  3. in the data section, data used for initialization of any type of variable, constant, initialized variables are somewhere else,
  4. on the stack, separate tasks with the same code have their own stack.

C storage classes are not cleanly mapped onto these categories.

  1. auto, on the stack
  2. register, put it in a register if possible, not guaranteed
  3. static
  4. extern, gives visibility to program components compiled in other files.

Creating a Task

In creating a task you have to do two things

  1. Get and initialize resources needed by the task
  2. Make the task look as if it had just entered the kernel

Things you need to do

Get an unused TD and memory for its stack. To do this you must decide what will be the maximum number of tasks in your system.

A few implementation details.

Most of the work in creating a task fill in fields in the TD.

  1. task id
  2. stack pointer
  3. SPSR.
    Forgetting to save this, or doing it incorrectly is unlikely to crash kernel 1. But it's worth getting right because it can cause hard to fix bugs later in the course.
  4. Link register
    Remember that there are two link registers: the one the kernel uses when it's time to restart the task that called Create, the other the one that is used to return from the Create function.
  5. Parent tid
  6. return value
  7. State, which is READY
  8. Priority, needed to install the task into its ready queue

Must also initialize the stack

The following implementation decisions are up to you.

Initializing the Kernel

Set up the Hardware

RedBoot gives you the processor with

Then there are a few things you need to do. While RedBoot gives you the system as described above, there may be things done by a previous student that changed what RedBoot thinks it is giving you.
  1. Initialize busy-wait I/O.
  2. Initialize low memory.
  3. Turn off interrupts in the ICU etc.
  4. As a debugging aid I sometimes put distinct bit patterns in the registers in order to get some easy information about what is going where.

Prepare the Kernel Data Structures

Where is the kernel's stack pointer, right now? What does the stack look like?

The kernel data structures. At the very least you need

  1. an array of empty ready queues
  2. a pointer to the TD of the active task
  3. an array of TDs
  4. a free list of pointers to free TDs. This might take the form of bits set in a couple of words.

Prepare the Memory to be Used by Tasks

  1. task memory

Create the First User Task

Can run with hardware interrupts turned off for now. But when hardware interrupts are turned on in kernel 3 interrupts in user tasks must be turned on, though they stay off in the kernel.

Reminder. The place where the kernel starts executing has the global name main, which cannot be re-used.

Other Primitives

These primitives exist mostly so that we, which includes you, can ensure that task creation and scheduling are working when there is not much else implemented.

Tid MyTid( )


A question, to which there is a correct answer, or more specifically, a correct (answer, reason) pair.

Tid MyParentTid( )


Where is the parent Tid, and how does the kernel find it?

void Pass( )

Doesn't block: task calling Pass( ) makes a state transition from ACTIVE to READY.

Does reschedule.

When is Pass( ) a NOP?

void Exit( )

Calling task is removed from all queues, but its resources are not reclaimed or reused.

That is, the task goes into a zombie state, and will never become active or ready, but continues to own all its resources.

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