MATRA MHS
M 67025
The semaphore must use a WRITE/READ sequence in Semaphore initialization is not automatic and must
order to ensure that no system level conflict will occur. A therefore be incorporated in the power up initialization
processor requests access to shared resources by procedures. Since any semaphore flag containing a zero
attempting to write a zero to a semaphore location. If the must be reset to one, initialization should write a one to
semaphore is already in use, the semaphore request latch all request flags from both sides to ensure that they will
will contain a zero, yet the semaphore flag will appear as be available when required.
a one, and the processor will detect this status in the
subsequent read (see table 5). For example, assume a
Using Semaphores - Some Examples
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 concerned. If a
processor on the right side then attempts to write a zero
to the same semaphore flag it will fail, as will be verified
by a subsequent read returning a one from the semaphore
location on the right side has a READ/WRITE sequence
been used instead, system conflict problems could have
occurred during the interval between the read and write
cycles.
Perhaps the simplest application of semaphores is their
use as resource markers for the M 67025’s dual-port
RAM. If it is necessary to split the 8 k × 16 RAM into two
4 K × 16 blocks which are to be dedicated to serving either
the left or right port at any one time. Semaphore 0 can be
used to indicate which side is controlling the lower
segment of memory and semaphore 1 can be defined as
indicating the upper segment of memory.
To take control of a resource, in this case the lower 4 k of
It must be noted that a failed semaphore request needs to a dual-port RAM, the left port processor would then write
be followed by either repeated reads or by writing a one a zero into semaphore flag 0 and then read it back. If
to the same location. The simple logic diagram for the successful in taking the token (reading back a zero rather
semaphore flag in figure 2 illusrates the reason for this than a one), the left processor could then take control of
quite clearly. Two semaphore request latches feed into a the lower 4 k of RAM. If the right processor attempts to
semaphore flag. The first latch to send a zero to the perform the same function to take control of the resource
semaphore flag will force its side of the semaphore flag after the left processor has already done so, it will read
low and other side high. This status will be maintained back a one in response to the attempted write of a zero into
until a one is written to the same semaphore request latch. semaphore 0. At this point the software may choose to
Sould a zero be written to the other side’s semaphore attempt to gain control of the second 4 k segment of RAM
request latch in the meantime, the semaphore flag will flip by writing and then reading a zero in semaphore 1. If
over to this second side as soon as a one is written to the successful, it will lock out the left processor.
first side’s request latch. The second side’s flag will now
Once the left side has completed its task it will write a one
stay low until its semaphore request latch is changed to a
to semaphore 0 and may then attempt to access
one. Thus, clearly, if a semaphore flag is requested and the
semaphore 1. If semaphore 1 is still occupied by the right
processor requesting it no longer requires access to the
side, the left side may abandon its semaphore request and
resource, the entire system can hang up until a one is
perform other operations until it is able to write and then
written to the semaphore request latch concerned.
read a zero in semaphore 1. If the right processor
Semaphore timing becomes critical when both sides
request the same token by attempting to write a zero to it
at the same time. Semaphore logic is specially conceived
to resolve this problem. The logic ensures that only one
side will receive the token if simultaneous requests are
made. The first side to make a request will receive the
token where request do not arrive at the same time. Where
they do arrive at the same time, the logic will assign the
token arbitrarily to one of the ports. It should be noted,
however, that semaphores alone do not guarantee that
access to a resource is secure. As with any powerful
programming technique, errors can be introduced if
performs the same operation with semaphore 0, this
protocol would then allow the two processes to swap 4 k
blocks of dual-port RAM between one another.
The blocks do not have to be any particular size, and may
even be of variable size depending on the complexity of
the 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 assigned different meanings on each side, rather
than having a common meaning as is described in the
above example.
semaphores are misused or misinterpreted. Code integrity Semaphores are a useful form of arbitration in systems
is of the utmost performance when semaphores are being such as disk interfaces where the CPU must be locked out
used instead of slower, more restrictive hardware-intensive of a segment of memory during a data transfer operation,
systems.
and the I/0 device cannot tolerate any wait states. If
Rev. D (29/09/95)
7
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