Page 261 - Embedded Microprocessor Systems Real World Design

Keeping Track of Tasks

The RTOS keeps track of tasks with a tusk control block (TCB). This is where the
RTOS saves information about tasks. One TCB entry is made for every task
managed by the RTOS. The TCB must store the following:

Task ID. This is typically the task number. Depending on the RTOS and the
processor it is running on, there may be a maximum of 128 tasks, 256 tasks,
32,768 tasks, and so on. The maximum number of tasks usually is what can be
identified with a byte/word/doubleword or whatever the word width of the
processor registers.
Task State. Ready, blocked, and so on.
Task Priority. The priority level of the task; a numerical value, usually 0 to 127,
0 to 32,767, and so on.
Task Address. Where in memory the code for the task is located.
Task Stack Pointer. The microprocessor stack is used to pass variables and store
the context for subroutine calls and interrupts. Each task needs to be able to
perform subroutine calls and service interrupts (or at least save the return
address for an interrupt). For this purpose, each task has its own stack. The TCB
includes the value of the stack pointer when the task last executed (or the top
of the task stack the first time the task executes).

The task stack is stored on the microprocessor stuck. As each task is given control,
the microprocessor stack pointer is modified to point to the stack for that task. Each
task must be allocated sufficient stack space to save the processor context, any
dynamic/temporary variables stored on the stack, and any information stored to
the maximum depth of subroutine calls. The processor context also may need to
include things such as the context (registers) of a floating-point coprocessor or
something similar. When a task stops running for any reason, the RTOS stores the
stack pointer for the task in the TCB.
Once a task is ready to run again, the processor context needed to resume
execution where it left off is stored on the task stack. The RTOS must get the stack
pointer from the TCB, put that value into the microprocessor stack pointer, and
return control to the task.
Depending on the RTOS, the TCB may contain additional information such as
environment information for dynamically allocated tasks and the like. In addition
to a stack for each task, the RTOS will have a kernel stack for use by the RTOS
itself.

242 Embedded Microprocessor Systems