Analog Devices ADSP-BF535 Blackfin, Hardware Reference Manual

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ADSP-BF535 Blackfin Processor Hardware Reference 19-7
System Design
The
TESTSET
instruction can be used to implement binary semaphores or
any other type of mutual exclusion method (for example, counting
semaphores).
TESTSET
represents a system-level requirement for a multicy-
cle bus lock mechanism.
The ADSP-BF535 processor restricts use of the
TESTSET
instruction to the
L2 memory and external memory regions only. Use of the
TESTSET
instruction to address any other area of the ADSP-BF535 memory map
may result in unreliable behavior.
Example Code for Query Semaphore
Listing 19-1 provides an example of a query semaphore that checks the
availability of a shared resource.
Listing 19-1. Query Semaphore
/* Query semaphore. Denotes "Busy" if its value is non-zero. Wait
until free (or re-schedule thread-- see note below). P0 holds
address of semaphore. */
QUERY:
TESTSET ( P0 ) ;
IF !CC JUMP QUERY ;
/* At this point, semaphore has been granted to current thread,
and all other contending threads are postponed because semaphore
value at [P0] is non-zero. Current thread could write thread_id
to semaphore location to indicate current owner of resource. */
R0.L = THREAD_ID ;
B[P0]=R0 ;
/* When done using shared resource, write a zero-byte to [P0] */
R0 = 0 ;
B[P0] = R0 ;
SSYNC ;