7

M67025E

4146J–AERO–06/03

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 semaphores are mis-

used or misinterpreted. Code integrity is of the utmost performance when semaphores

are being used instead of slower, more restrictive hardware-intensive systems.

Semaphore initialization is not automatic and must therefore be incorporated in the

power up initialization procedures. Since any semaphore flag containing a zero must be

reset to one, initialization should write a one to all request flags from both sides to

ensure that they will be available when required.

Using Semaphores –

Some Examples

Perhaps the simplest application of semaphores is their use as resource markers for the

M67025E’s dual-port RAM. If it is necessary to split the 8 Kb

× 16 RAM into two 4 Kb × 16

blocks which are to be dedicated to serving either the left or right port at any one time. Sema-

phore 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 Kb of a dual-port RAM, the left port

processor would then write a zero into semaphore flag 0 and then read it back. If suc-

cessful in taking the token (reading back a zero rather than a one), the left processor

could then take control of the lower 4 Kb of RAM. If the right processor attempts to per-

form the same function to take control of the resource after the left processor has

already done so, it will read back a one in response to the attempted write of a zero into

semaphore 0. At this point the software may choose to attempt to gain control of the

second 4 Kb segment of RAM by writing and then reading a zero in semaphore 1. If suc-

cessful, it will lock out the left processor.

Once the left side has completed its task it will write a one to semaphore 0 and may then

attempt to access semaphore 1. If semaphore 1 is still occupied by the right side, the left

side may abandon its semaphore request and perform other operations until it is able to

write and then read a zero in semaphore 1. If the right processor performs the same

operation with semaphore 0, this protocol would then allow the two processes to swap

4 Kb 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 sema-

phores 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 a useful form of arbitration in systems such as disk interfaces where

the CPU must be locked out of a segment of memory during a data transfer operation,

and the I/0 device cannot tolerate any wait states. If semaphores are used, both the

CPU and the I/0 device can access assigned memory segments, without the need for

wait states, once the two devices have determined which memory area is barred to the

CPU.

Semaphores are also useful in applications where no memory WAIT state is available

on one or both sides. On a semaphore handshake has been performed, both proces-

sors can access their assigned RAM segments at full speed.

Another application is in complex data structures. Block arbitration is very important in

this case, since one processor may be responsible for building and updating a data

structure whilst the other processor reads and interprets it. A major error condition may

be created if the interpreting processor reads an incomplete data structure. Some sort of

arbitration between the two different processors is therefore necessary. The building

processor requests access to the block, locks it and is then able to enter the block to