ST20-C1 datasheet(159/205 Pages) STMICROELECTRONICS

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8 Instruction Set Reference

timeslice

Code: FE

Description: Deschedules the current process and starts the next task if a timeslice

is due and timeslicing is enabled. This instruction is implemented in two stages to

protect interrupt latency. The first stage deschedules the current process (even if the

queue is empty), adds it to the back of the scheduling queue and sets the

start_next_task status bit. The second stage clears the start_next_task status bit

and starts the next process from the scheduling queue. An interrupt may occur

between the two stages.

Definition:

if ( Statustimeslicing enabled and Statustimeslice_count=0 )

{

if (

timeslice trap installed)

take timeslice trap

else

{

-- save state of current process

word

′[Tdesc @ pw.Iptr]

← next instruction

word

′[Tdesc @ pw.Wptr]

← Wptr

if (frontptr

≠ NotProcess)

--queue not empty

{

-- add current process to scheduling queue

word

′[backptr @ pw.Link]

← Tdesc

word

′[SchedulerQptr @ q.BPtrLoc] ← Tdesc

-- remove front process from queue

word

′[SchedulerQptr @ q.FPtrLoc] ← word[frontptr @ pw.Link]

-- start next process from scheduling queue

Tdesc

′← frontptr

Wptr

′← word [frontptr @ pw.Wptr]

Iptr

′← word [frontptr @ pw.Iptr]

}

Status

′

local_interrupt_enable

← set

Status

′

overflow

← clear

Status

′

undeflo w

← clear

Status

′

carry

← clear

Status

′

user_mode

← set

Status

′

interrupt_mode

← clear

Status

′

trap_mode

← clear

Status

′

sleep

← clear

Status

′

start_next_task

← clear

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