ST20-C1 datasheet(78/205 Pages) STMICROELECTRONICS

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7.1 Processes

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Multi-tasking

This chapter describes the features of the ST20-C1 core provided to suppor t multi-

tasking, and how to use them. The architecture of the ST20-C1, including the registers

and memory arrangement, are described in Chapter 3. Interrupts and traps are

described in Chapter 6. A full list of constants and data structures is given in Appendix

Support is provided in the instruction set for timeslicing, scheduling processes and

manipulating queues of processes.

7.1

Processes

A process (also known as a task or thread) is an independent unit of software with a

single thread of control, i.e. a sequential algorithm. Any number of processes may be

run. A process which has been started but not terminated may be in one of several

different states:

• executing on the CPU;

• interrupted or trapped by an exception;

• inactive, i.e. waiting for a peripheral or semaphore signal;

• waiting for CPU time.

A process that is not inactive is said to be active. A process that is not executing or

interrupted is said to be descheduled. The states and main transitions are shown in

Figure 7.1. The scheduling transitions can be trapped so that a software scheduling

kernel can modify the transitions and so change the scheduling behavior, for example

by providing a system of process priorities.

Figure 7.1 Process states and main transitions

Waiting

for CPU

time

Executing

Interrupted

Inactive

Descheduled

processes

Active

processes

Interrupt

or trap

Return

Event

Terminate

Run

Terminated

Not started

Descheduled

Timeslice

trapped

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