3.9.3 Semaphore locking (mcs51/ds390)
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Some architectures (mcs51/ds390) have an atomic bit
test and clear instruction. These type of instructions
are typically used in preemptive multitasking systems, where a routine
f.e. claims the use of a data structure ('acquires a lock
on it'), makes some modifications and then releases the lock when
the data structure is consistent again. The instruction may also be
used if interrupt and non-interrupt code have to compete for a resource.
With the atomic bit test and clear instruction interrupts
don't have to be disabled for the locking operation.
SDCC generates this instruction if the source follows this pattern:
volatile bit resource_is_free;
if (resource_is_free)
{
resource_is_free=0;
...
resource_is_free=1;
}
Note, mcs51 and ds390 support only an atomic bit test
and clear instruction (as opposed to atomic bit test and set).
2008-02-29
