6.42
IDT7015S/L
High-Speed 8K x 9 Dual-Port Static RAM Industrial and Commercial Temperature Ranges
18
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
semaphore request latch.
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.
Using Semaphores—Some Examples
Perhaps the simplest application of semaphores is their application as
resource markers for the IDT7015’s Dual-Port RAM. Say the 8K x 9 RAM
was to be divided into two 4K x 9 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.
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 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
4K blocks of Dual-Port RAM with each other.
The blocks do not have to be any particular size and can even be
variable, depending upon the complexity of the software using the
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.
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 IDT7015 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, OE, and
R/W) 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 A0 – 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 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 inter
processor communications. (A thorough discussing 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) 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.
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
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 a subsequent read, the processor will verify
that it has written successfully to that location and will assume control over
the resource in question. Meanwhile, if a processor on the right side
attempts to write a zero to the same semaphore flag it will fail, as will be
verified by the fact that a one will be read from that semaphore on the right
side during subsequent read. Had a sequence of READ/WRITE been
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
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
