IDT7005S/L
HIGH-SPEED 8K x 8 DUAL-PORT STATIC RAM
MILITARY AND COMMERCIAL TEMPERATURE RANGES
resource is secure. As with any powerful programming side, the left side could undo its semaphore request and
technique, if semaphores are misused or misinterpreted, a perform other tasks until it was able to write, then read a zero
software error can easily happen.
into Semaphore 1. If the right processor performs a similar
Initialization of the semaphores is not automatic and must task with Semaphore 0, this protocol would allow the two
be handled via the initialization program at power-up. Since processors to swap 4K blocks of Dual-Port RAM with each
any semaphore request flag which contains a zero must be other.
reset to a one, all semaphores on both sides should have a
The blocks do not have to be any particular size and can
one written into them at initialization from both sides to assure even be variable, depending upon the complexity of the
that they will be free when needed.
software using the semaphore flags. All eight semaphores
could be used to divide the Dual-Port RAM or other shared
resources into eight parts. Semaphores can even be as-
signed different meanings on different sides rather than being
given a common meaning as was shown in the example
above.
Semaphores are a useful form of arbitration in systems like
disk interfaces where the CPU must be locked out of a section
ofmemoryduringatransferandtheI/Odevicecannottolerate
any wait states. With the use of semaphores, once the two
deviceshasdeterminedwhichmemoryareawas“off-limits”to
the CPU, both the CPU and the I/O devices could access their
assigned portions of memory continuously without any wait
states.
Semaphores are also useful in applications where no
memory “WAIT” state is available on one or both sides. Once
a semaphore handshake has been performed, both proces-
sors can access their assigned RAM segments at full speed.
Another application is in the area of complex data struc-
tures. In this case, block arbitration is very important. For this
applicationoneprocessormayberesponsibleforbuildingand
updating a data structure. The other processor then reads
andinterpretsthatdatastructure. Iftheinterpretingprocessor
reads an incomplete data structure, a major error condition
may exist. Therefore, some sort of arbitration must be used
between the two different processors. The building processor
arbitrates for the block, locks it and then is able to go in and
update the data structure. When the update is completed, the
data structure block is released. This allows the interpreting
processortocomebackandreadthecompletedatastructure,
thereby guaranteeing a consistent data structure.
USING SEMAPHORES—SOME EXAMPLES
Perhaps the simplest application of semaphores is their
application as resource markers for the IDT7005’s Dual-Port
RAM. Say the 8K x 8 RAM was to be divided into two 4K x 8
blockswhichweretobededicatedatanyonetimetoservicing
either the left or right port. Semaphore 0 could be used to
indicate the side which would control the lower section of
memory, and Semaphore 1 could be defined as the indicator
for the upper section of memory.
To take a resource, in this example the lower 4K of
Dual-Port RAM, the processor on the left port could write and
then read a zero in to Semaphore 0. If this task were
successfully completed (a zero was read back rather than a
one), the left processor would assume control of the lower 4K.
Meanwhile the right processor was attempting to gain control
of the resource after the left processor, it would read back a
one in response to the zero it had attempted to write into
Semaphore 0. At this point, the software could choose to try
and gain control of the second 4K section by writing, then
reading a zero into Semaphore 1. If it succeeded in gaining
control, it would lock out the left side.
Once the left side was finished with its task, it would write
a one to Semaphore 0 and may then try to gain access to
Semaphore 1. If Semaphore 1 was still occupied by the right
L PORT
R PORT
SEMAPHORE
REQUEST FLIP FLOP
SEMAPHORE
REQUEST FLIP FLOP
D0
D0
D
D
Q
Q
WRITE
WRITE
SEMAPHORE
READ
SEMAPHORE
READ
2738 drw 20
Figure 4. IDT7005 Semaphore Logic
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IDT [ INTEGRATED DEVICE TECHNOLOGY ]
