ST20-C1 datasheet(84/205 Pages) STMICROELECTRONICS

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7.8 Scheduling kernels

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The following code would set up a process control block:

ld entry_point; ld control_block; stnl pw.Iptr;

ld work_space_top; arot; stnl pw.Wptr;

The process is then inactive, so it can be added to the back of the scheduling queue

using

run, exactly as in section 7.5.

7.8

Scheduling kernels

A scheduling kernel can be written to override the default scheduling behavior of the

ST20-C1, but still using the very fast micro-code scheduling. The basis of such a

scheduler would be trap handlers to trap the system exceptions el_idle_trap,

el_run_trap, el_stop_trap, el_timeslice_trap and el_schedule_exception_trap.

When these traps are taken, a scheduling event is due or the processor has become

idle. The trap replaces the default scheduling action. When the trap handler returns,

the CPU will continue with the next instruction. Traps are described in Chapter 6.

Additional system calls may be implemented by user-defined trap handlers, called

using

ecall.

7.9

Semaphores

Semaphores are a mechanism for managing access to shared resources within a

multi-tasking environment. The semaphore operations are provided by the instructions

listed in Table 7.4.

The semaphore instructions act on a semaphore control block, defined in Table 7.5.

Each semaphore has a semaphore control block, which implements a linked list of

waiting processes and a count of free resources. The count should be initialized to the

total number of available resources (usually one), and the front list pointer should be

initialized to the empty value

NotProcess. For fast signalling and minimal interrupt

latency, semaphore control blocks should be in fast memory, preferably on-chip.

Mnemonic

Name

signal

wait

stop

stop process

run

run process

Table 7.4 Semaphore instructions

Word offset

Slot name

Purpose

s.Back

The back of the semaphore waiting list.

s.Front

The front of the semaphore waiting list.

s.Count

The unsigned number of extraprocesses that the semaphore will allow

to continue running on a

wait request.

Table 7.5 Semaphore control block

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