ST20-C1 datasheet(79/205 Pages) STMICROELECTRONICS

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7 Multi-tasking

A process state is held in memory and the CPU registers. Sufficient of the register

state must be saved when the process is interrupted or a context switch occurs, so

that the process can be reloaded and continue execution at a later time. The register

state consists of:

• the instruction pointer register;

• the work space pointer register;

• the task descriptor register;

• the evaluation stack registers;

• the status register.

In order to save memory space and context switch time, processes are only desched-

uled when the evaluation stack and status register are empty. This is achieved by only

allowing processes to deschedule at certain instructions, called deschedule instruc-

tions, after which the final values in the evaluation stack are undefined and the status

stop and timeslice.

Table 7.2 lists the multi-tasking instructions.

7.2

Descheduled processes

If the process is waiting for a peripheral or a semaphore or descheduled by a timeslice

then the evaluation stack is not saved. The instruction pointer and Wptr are saved in

the

process descriptor block. The task descriptor is the address of the process

descriptor block. It therefore identifies the waiting process and points to its saved

state. The task descriptor is a fixed address for each process, unlike the Wptr which

changes as the code executes. When the process is running, the task descriptor is

held in the Tdesc register.

Mnemonic

Name

run

Run process

stop

Stop process

timeslice

Timeslice

ldtdesc

Load task descriptor

enqueue

Enqueue a process

dequeue

Dequeue a process

Table 7.2 Multi-tasking instructions

Word offset

Slot name

Purpose

pw.Iptr

The process saved instruction pointer.

pw.Wptr

The process saved work space pointer.

pw.Link

The link to the next process in the queue.

Table 7.1 Process descriptor block

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