ST20-GP1 datasheet(20/116 Pages) STMICROELECTRONICS

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ST20-GP1

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Figure 4.2 Linked process list

Each process runs until it has completed its action or is descheduled. In order for several

processes to operate in parallel, a low priority process is only permitted to execute for a maximum

of two timeslice periods. After this, the machine deschedules the current process at the next

timeslicing point, adds it to the end of the low priority scheduling list and instead executes the next

active process. The timeslice period is 1ms.

There are only certain instructions at which a process may be descheduled. These are known as

descheduling points. A process may only be timesliced at certain descheduling points. These are

known as timeslicing points and are deﬁned in such a way that the operand stack is always empty.

This removes the need for saving the operand stack when timeslicing. As a result, an expression

evaluation can be guaranteed to execute without the process being timesliced part way through.

Whenever a process is unable to proceed, its instruction pointer is saved in the process workspace

and the next process taken from the list.

The processor core provides a number of special instructions to support the process model,

including startp (start process) and endp (end process). When a main process executes a parallel

construct,

startp is used to create the necessary additional concurrent processes. A startp

instruction creates a new process by adding a new workspace to the end of the scheduling list,

enabling the new concurrent process to be executed together with the ones already being

executed. When a process is made active it is always added to the end of the list, and thus cannot

pre-empt processes already on the same list.

The correct termination of a parallel construct is assured by use of the

endp instruction. This uses

a data structure that includes a counter of the parallel construct components which have still to

terminate. The counter is initialized to the number of components before the processes are started.

Each component ends with an

endp instruction which decrements and tests the counter. For all but

Function

High priority

Low priority

Pointer to front of active process list

FptrReg0

FptrReg1

Pointer to back of active process list

BptrReg0

BptrReg1

Table 4.1 Priority queue control registers

FptrReg1

Local Data

Registers

Program

BptrReg1

pw.Iptr

pw.Link

pw.Iptr

pw.Link

pw.Iptr

Wptr

IptrReg

Areg

Breg

Creg

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