21
IDT70V659/58/57S
High-Speed 3.3V 128/64/32K x 36 Asynchronous Dual-Port Static RAM Industrial and Commercial Temperature Ranges
The semaphore flags are active LOW. A token is requested by writing
a zero into a semaphore latch and is released when the same side writes
a one to that latch.
The eight semaphore flags reside within the IDT70V659/58/57 in a
separate memory space from the Dual-Port RAM. This address space is
accessed by placing a low input on the SEM pin (which acts as a chip select
for the semaphore flags) and using the other control pins (Address, CE,
R/W and BEo) as they would be used in accessing a standard Static RAM.
Each of the flags has a unique address which can be accessed by either
side through address pins A
0 – A2. When accessing the semaphores, none
of the other address pins has any effect.
When writing to a semaphore, only data pin D0 is used. If a low level
is written into an unused semaphore location, that flag will be set to a zero
on that side and a one on the other side (see Truth Table V). That
semaphore can now only be modified by the side showing the zero. When
a one is written into the same location from the same side, the flag will be
set to a one for both sides (unless a semaphore request from the other side
is pending) and then can be written to by both sides. The fact that the side
which is able to write a zero into a semaphore subsequently locks out writes
from the other side is what makes semaphore flags useful in interprocessor
communications. (A thorough discussion on the use of this feature follows
shortly.) A zero written into the same location from the other side will be
stored in the semaphore request latch for that side until the semaphore is
freed by the first side.
When a semaphore flag is read, its value is spread into all data bits so
that a flag that is a one reads as a one in all data bits and a flag containing
a zero reads as all zeros. The read value is latched into one side’s output
register when that side's semaphore select (SEM, BEn) and output enable
(OE) signals go active. This serves to disallow the semaphore from
changing state in the middle of a read cycle due to a write cycle from the
other side. Because of this latch, a repeated read of a semaphore in a test
loop must cause either signal (SEM or OE) to go inactive or the output will
never change. However, during reads BEn functions only as an output
for semaphore. It does not have any influence on the semaphore control
logic.
A sequence WRITE/READ must be used by the semaphore in order
to guarantee that no system level contention will occur. A processor
requests access to shared resources by attempting to write a zero into a
semaphore location. If the semaphore is already in use, the semaphore
reason for this is easily understood by looking at the simple logic diagram
of the semaphore flag in Figure 4. Two semaphore request latches feed
into a semaphore flag. Whichever latch is first to present a zero to the
semaphore flag will force its side of the semaphore flag LOW and the other
side HIGH. This condition will continue until a one is written to the same
semaphore request latch. Should the other side’s semaphore request latch
have been written to a zero in the meantime, the semaphore flag will flip
over to the other side as soon as a one is written into the first side’s request
latch. The second side’s flag will now stay LOW until its semaphore request
latch 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
The critical case of semaphore timing is when both sides request 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 simulta-
neous 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.
One caution that should be noted when using semaphores is that
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.
Initialization of the semaphores is not automatic and must be handled
via the initialization program at power-up. Since any semaphore 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.
How the Semaphore Flags Work
The semaphore logic is a set of eight latches which are independent
of the Dual-Port RAM. These latches can be used to pass a flag, or token,
from one port to the other to indicate that a shared resource
is in use. The semaphores provide a hardware assist for a use assignment
method called “Token Passing Allocation.” In this method, the state of a
semaphore latch is used as a token indicating that a shared resource is
in use. If the left processor wants to use this resource, it requests the token
by setting the latch. This processor then verifies its success in setting the
latch by reading it. If it was successful, it proceeds to assume control over
the shared resource. If it was not successful in setting the latch, it determines
that the right side processorhas set the latch first, has the token and is using
the shared resource. The left processor can then either repeatedly request
that semaphore’s status or remove its request for that semaphore to
perform another task and occasionally attempt again to gain control of the
token via the set and test sequence. Once the right side has relinquished
the token, the left side should succeed in gaining control.
Figure 4. IDT70V659/58/57 Semaphore Logic
D
4869 drw 1
0
D
Q
WRITE
D
0
D
Q
WRITE
SEMAPHORE
REQUEST FLIP FLOP
SEMAPHORE
REQUEST FLIP FLOP
LPORT
RPORT
SEMAPHORE
READ
SEMAPHORE
READ
request latch will contain a zero, yet the semaphore flag will appear as one,
a fact which the processor will verify by the subsequent read (see Table
V). As an example, assume a processor writes a zero to the left port at a
free semaphore location. On used instead, system contention problems
could have occurred during the gap between the read and write cycles.
It is important to note that a failed semaphore request must be followed
by either repeated reads or by writing a one into the same location. The
semaphore request latch.
