ST20-GP6 View Datasheet(PDF) - STMicroelectronics

Part Name

Description

Manufacturer

ST20-GP6

GPS PROCESSOR

STMicroelectronics 

ST20-GP6

Registers

FptrReg1

BptrReg1

Areg

Breg

Creg

Wptr

Iptr

Local data

Iptr.s

P Link.s

Iptr.s

Q Link.s

Iptr.s

R

S

Program

Figure 4.2 Linked process list

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

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

cesses 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 timeslic-

ing 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, includ-

ing startp (start process) and endp (end process). When a main process executes a parallel con-

struct, 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 pro-

cesses 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 ter-

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