ST20-C1 datasheet(83/205 Pages) STMICROELECTRONICS

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7 Multi-tasking

7.7

Initializing multi-tasking

7.7.1

Initializing the scheduling queue

Before multi-tasking operations can be performed, the scheduling queue must be

initialized. The scheduling queue is described in section 7.3. The queue must be

initialized by setting it to empty, which means setting the front pointer to empty:

ld MostNeg; ld SchedulerQptr; stnl q.FPtrLoc;

7.7.2

Creating and starting a process

A process consists of code, a work space area, a process control block and the values

for the Iptr, Wptr and Tdesc. To create a process, a large enough work space is

created, and a process control block of three words. It may be convenient to put all the

process control blocks together in one area of memory. For fast context switches and

minimal interrupt latency, process control blocks should be in on-chip memory.

The entry point for the process code is written into pw.Iptr of the process control

block. The address of the top of the work space is written into pw.Wptr of the process

control block, so the process code must adjust the initial Wptr down using

ajw to

create space for local variables.

Field or bit

Value

mac_count

As in previous process.

mac_buffer

As in previous process.

mac_scale

As in previous process.

mac_mode

As in previous process.

global_interrupt_enable

As in previous process.

local_interrupt_enable

Set.

overflo w

False.

underflo w

False.

carry

False.

user_mode

True.

interrupt_mode

False.

trap_mode

False.

sleep

False.

reserved

Undefined.

start_next_task

False.

timeslice_enable

As in previous process.

timeslice_count

MaxTimesliceCount.

Table 7.3 Restarted process status register

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