Dma Transfers To External Memory - Analog Devices ADSP-2106x SHARC User Manual

7.5.2.2 DMA Transfers To External Memory

The ADSP-2106x's DMA controller can also be used to transfer data
directly to external memory. The external handshake mode for external
port DMA channel 7 or 8 will provide the
handshaking for this type of transfer. Again, this is not possible when
a host processor has gained control of the bus.
7.6 BUS LOCK & SEMAPHORES
Semaphores can be used in multiprocessor systems to allow the
processors to share resources such as memory or I/O. 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 for synchronizing the
tasks being performed by different processors in a multiprocessing
system.
With the use of its bus lock feature, the ADSP-2106x has the ability to
read and modify a semaphore in a single indivisible operation—a key
requirement of multiprocessing systems.
Because both external memory and each ADSP-2106x's internal
memory (and IOP registers) are accessible by every other ADSP-2106x,
semaphores can be located almost anywhere. Read-modify-write
operations on semaphores can be performed if all of the ADSP-2106xs
obey two simple rules:
1. An ADSP-2106x must not write to a semaphore unless it is the bus
master. This is especially important if the semaphore is located in
the ADSP-2106x's own internal memory or IOP registers.
2. When attempting a read-modify-write operation on a semaphore,
the ADSP-2106x must have bus mastership for the duration of the
operation.
Both of these rules are adhered to when an ADSP-2106x uses its bus
lock feature, which "locks in" its mastership of the bus and prevents
the other processors from simultaneously accessing the semaphore.
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Multiprocessing
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