IDT70V07 Integrated Device Technology, IDT70V07 Datasheet - Page 17

• Manufacturer Part Number: IDT70V07
• Description: 32k X 8 3.3v Dual-port
• Manufacturer: Integrated Device Technology

is written to a one. From this it is easy to understand that, if a
semaphore is requested and the processor which requested it no
longer needs the resource, the entire system can hang up until a one
is written into that semaphore request latch.
a single token by attempting to write a zero into it at the same time. The
semaphore logic is specially designed to resolve this problem. If
simultaneous requests are made, the logic guarantees that only one
side receives the token. If one side is earlier than the other in making
the request, the first side to make the request will receive the token. If
both requests arrive at the same time, the assignment will be arbitrarily
made to one port or the other.
semaphores alone do not guarantee that access to a resource is
secure. As with any powerful programming technique, if semaphores
are misused or misinterpreted, a software error can easily happen.
handled via the initialization program at power-up. Since any sema-
phore request flag which contains a zero must be reset to a one, all
semaphores on both sides should have a one written into them at
initialization from both sides to assure that they will be free when
needed.
Using Semaphores—Some Examples
tion as resource markers for the IDT70V07’s Dual-Port SRAM. Say the
32K x 8 SRAM was to be divided into two 16K x 8 blocks which were
to be dedicated at any one time to servicing 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.
SRAM, 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 16K. 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 16K section by writing, then reading a zero into
IDT70V07S/L
High-Speed 32K x 8 Dual-Port Static RAM
The critical case of semaphore timing is when both sides request
One caution that should be noted when using semaphores is that
Initialization of the semaphores is not automatic and must be
Perhaps the simplest application of semaphores is their applica-
To take a resource, in this example the lower 16K of Dual-Port
17
Semaphore 1. If it succeeded in gaining control, it would lock out the
left side.
Semaphore 0 and may then try to gain access to Semaphore 1. If
Semaphore 1 was still occupied by the right side, the left side could
undo its semaphore request and perform other tasks until it was able
to write, then read a zero into Semaphore 1. If the right processor
performs a similar task with Semaphore 0, this protocol would allow the
two processors to swap 16K blocks of Dual-Port SRAM with each
other.
variable, depending upon the complexity of the software using the
semaphore flags. All eight semaphores could be used to divide the
Dual-Port SRAM or other shared resources into eight parts. Sema-
phores can even be assigned different meanings on different sides
rather than being given a common meaning as was shown in the
example above.
interfaces where the CPU must be locked out of a section of memory
during a transfer and the I/O device cannot tolerate any wait states.
With the use of semaphores, once the two devices has determined
which memory area was “off-limits” to the CPU, both the CPU and the
I/O devices could access their assigned portions of memory continu-
ously without any wait states.
“WAIT” state is available on one or both sides. Once a semaphore
handshake has been performed, both processors can access their
assigned SRAM segments at full speed.
case, block arbitration is very important. For this application one
processor may be responsible for building and updating a data
structure. The other processor then reads and interprets that data
structure. If the interpreting processor 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 processor
to come back and read the complete data structure, thereby guaran-
teeing a consistent data structure.
Once the left side was finished with its task, it would write a one to
The blocks do not have to be any particular size and can even be
Semaphores are a useful form of arbitration in systems like disk
Semaphores are also useful in applications where no memory
Another application is in the area of complex data structures. In this
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