Page 248 - Embedded Microprocessor Systems Real World Design

even though the LOCK signal was supposed to prevent it. It was supposed to work
like this:

CPU 1 reads the semaphore, asserting the LOCK signal
CPU 2 requests access to memory and is put in a wait state due to the LOCK
CPU 1 writes to the semaphore.
CPU 2 is released from the wait state, reads the semaphore, finds the value
that CPU 1 wrote.
It actually did this:

CPU 1 reads the semaphore, asserting the LOCK signal.
CPU 2 requests access to memory, gets memory, reads the semaphore, finds it
is 0.
CPU 1 writes to the semaphore.
CPU 2 writes to the semaphore, overwriting the CPU 1 value.
The problem showed up when it did because the firmware change altered the
relative timing of the two processors so that they occasionally conflicted in access-
ing the memory. Although this problem occurred because of a flaw in handling the
LOCK signal, a similar circumstance can occur any time that two (or more) proces-
sors try to write to a single RAM location. The following are some guidelines for
using multiport RAM:

Wherever possible, do not have two processors that write to one memory loca-
tion. As mentioned earlier in the chapter, segregate buffers so that one proces-
sor writes to a buffer and the other one reads.
Never have a situation in which two processors can simultaneously check a
memory location (such as a semaphore) and then write to it. This is a sure way
to get contention. Have one CPU write a flag location to indicate that data are
in the buffer, have the other CPU write the location only when it has taken the
data. Then the two CPUs do not contend for the location at the same time.
If you must have a resource (such as a buffer) that is shared between multiple
processors, have a two-step arbitration protocol. In this scheme, each CPU writes
a unique code to a semaphore to indicate it wants the resource (whatever the
resource is). Then each CPU checks the semaphore (preferably twice) to ensure
that its own code is written there. If a conflict occurs, whichever CPU’s code is
left in the semaphore wins.
The last guideline works like this (in this example, a nonzero value in the flag
location indicates that the buffer is in use; a zero value indicates that the buffer is
free) :

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