Shared Memory Interface
3-92 ADSP-21368 SHARC Processor Hardware Reference
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Only SDRAM clock (SDCLK) ratios of two and four are supported
in shared memory systems.
Bus Lock and Semaphores
To allow the processors in shared memory systems to share resources such
as memory or I/O, semaphores can be used. A semaphore is a flag that can
be read and written by any of the processors sharing the resource. The
value of the semaphore tells the processor when it can access the resource.
Semaphores are also useful to synchronize tasks being performed by differ-
ent processors in a system.
With the use of its bus lock feature, the processor has the ability to read
and modify a semaphore—a key requirement of shared memory systems.
Read-modify-write operations on semaphores can be performed if all of
the processors obey two simple rules:
1. A processor must not write to a semaphore unless it is the bus
master.
2. When attempting a read-modify-write operation on a semaphore,
the processor must have bus mastership for the duration of the
operation.
Both of these rules apply when a processor uses its bus lock feature, which
retains its mastership of the bus and prevents other processors from simul-
taneously accessing the semaphore.
Bus lock is requested by setting the
BUSLK bit in the SYSCTL register. When
this happens, the processor initiates the bus arbitration process by assert-
ing its BRx line. When the processor becomes bus master, it locks the bus
by keeping its BRx line asserted, even when it is not performing an external
read or write. When the
BUSLK bit is cleared, the processor gives up the bus
by deasserting its
BRx line.
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