ST20-C1 datasheet(163/205 Pages) STMICROELECTRONICS

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8 Instruction Set Reference

wait

Code: 23 F6

Description: Perform the first par t of a semaphore wait (or P) on the semaphore

pointed to by Areg. If the semaphore count is non-zero then the count is decremented

and the process continues; otherwise the current process is added to the back of the

semaphore list ready for the current process to be descheduled. The instruction

returns a boolean value in Areg stating whether a process deschedule is required.

The instruction should be used in the following sequence:

ld semptr; wait; cj continue; stop; continue:

Definition:

if (word[Areg @ s.Count]

≠ 0)

{

word

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

Areg

′← false

}

else

{

if (frontptr

= NotProcess)

-- queue empty so add to queue

{

word

′[Areg @ s.Front ]

← Tdesc

word

′[Areg @ s.Back ]

← Tdesc

}

else

-- queue not empty so add to back of queue

{

word

′[backptr @ pw.Link]

← Tdesc

word

′[Areg @ s.Back ]

← Tdesc

}

Areg

′← true

}

Breg

′← undefined

Creg

′← undefined

where frontptr = word[Areg @ s.Front]

backptr = word[Areg @ s.Back]

Status Register:No effect

Comments:

Secondary instruction.

Areg must be word-aligned.

The semaphore structure should be in fast memory if interrupt latency is critical.

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