ST20-C1 datasheet(142/205 Pages) STMICROELECTRONICS

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signal

Code: 23 F5

Description: Perform the first par t of a semaphore signal (or V) on the semaphore

pointed to by Areg. If no process is waiting then the unsigned count is incremented. If

processes are waiting then the first process is removed from the queue and its

identifier placed in Breg ready to be run. The instruction returns a boolean value in

Areg stating whether a run process is required.

This instruction should be used in the following sequence:

ld semptr; signal; cj

continue; run; continue:

Definition:

if (frontptr

= NotProcess)

-- queue empty

{

word

′[Areg @ s.Count] ← word[Areg @ s.Count] + 1

Areg

′← false

Breg

′← undefined

}

else

-- queue not empty

{

if (frontptr = backptr)

-- one process on queue

word

′[Areg @ s.Front] ← NotProcess

else

-- many processes on queue

word

′[Areg @ s.Front] ← word[ frontptr @ pw.Link ]

Areg

′← true

Breg

′← frontptr

}

Creg

′← undefined

where frontptr = word[Areg @ s.Front]

backptr = word[Areg @ s.Back]

Status Register:

No effect

Comments:

Secondar y instruction.

The increment of the count is unchecked.

This instruction may require 4 accesses to memory. The semaphore structure

should be placed in on-chip memory if interrupt latency is critical.

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