ST20-C1 datasheet(80/205 Pages) STMICROELECTRONICS

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7.3 Queues

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The structure of the process descriptor block is shown in Table 7.1. When the process

is not executing, it contains the saved work space pointer and instruction pointer of the

process, plus a queue link if the process is in a queue.

Figure 7.3 illustrates a descheduled process.

Figure 7.3 A descheduled process

7.3

Queues

There may be any number of processes waiting for execution, so a queue (i.e. a linked

list) of waiting processes is formed, called the

scheduling queue. This is an example

of a general queue supported by the instruction set for queueing waiting processes.

A queue is a linked list of process control blocks, formed by links included in the

process control blocks. Each link points to the control block of the next process in the

queue unless it is the last in the queue, which is undefined.

The front and back pointers of a queue are held in a queue control block, as shown in

Table 7.2. The queue control block is held in memory, and the address of the block is

the identifier of the queue .

A complete queue is illustrated in Figure 7.4. In the case of the scheduling queue, the

control block is stored at the reserved address called

SchedulerQptr (which has the

Word offset

Slot name

Purpose

q.BPtrLoc

The back of the queue.

q.FPtrLoc

The front of the queue.

Table 7.2 Queue control block

Process

work space

(stack)

Work space pointer

Link

Next process

Task descriptor

Code

local

Iptr

Process descriptor block

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