CS452 - Real-Time Programming - Winter 2016

Lecture 7a - Create (continued)

Public Service Annoucements

  1. Due date for kernel 1: 28 January, 2016

Static/Global Variables.

Global variables

TD tdarray[64];
void main( ) {
  ....
}
int Create( int priority, ( *code( ) ) ) {
  ....
}
tdarray is equally accessible to main and Create. With suitable use of extern it can be made available to code in other files as well. Using your linker script you can have it put anywhere in memory you like.

This kind of global variable is universally recognized as bad programming practice, which doesn't mean that it isn't common in commercial software!

Static variables You are used to lines of code like

printf( "Hello, world.\n" );
How does printf get its argument? Near the end -- usually -- of the function containing printf are 14 bytes containing the ASCII representation of the string. A pointer to it is put into the code by the link editor.

Variables stored like this are called static in C. They preserve their values across function calls. There are three reasons why you might prefer not to make them writable.

  1. You must make the part of memory containing the instructions writable, which leaves you open to a whole lot of widely known exploits.
  2. If you reuse code in different tasks you may find static variables changing value unexpectedly and unpredictably.
  3. If you do something like this
     static TD* tdarray; 
    then read and write the array, strange and amazing things might happen. (Use lint or equivalent.)


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( )

Self-explanatory

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

Tid MyParentTid( )

Self-explanatory

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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