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Critical Sections
The unit of measure for the measured time is in CPU_TS (timestamp) units. It is necessary to
find out the resolution of the timer used to measure these timestamps. For example, if the
timer used for the timestamp is incremented at 1 MHz then the resolution of CPU_TS is
1 microsecond.
Measuring the interrupt disable time obviously adds measurement artifacts and thus
increases the amount of time the interrupts are disabled. However, as far as the
measurement is concerned, measurement overhead is accounted for and the measured
value represents the actual interrupt disable time as if the measurement was not present.
Interrupt disable time is obviously greatly affected by the speed at which the processor
accesses instructions and thus, the memory access speed. In this case, the hardware
designer might have introduced wait states to memory accesses, which affects overall
performance of the system. This may show up as unusually long interrupt disable times.
4-2 LOCKING THE SCHEDULER
When setting OS_CFG_ISR_POST_DEFERRED_EN to 1, μC/OS-III locks the scheduler before
entering a critical section and unlocks the scheduler when leaving the critical section.
OS_CRITICAL_ENTER() simply increments OSSchedLockNestingCtr to lock the scheduler.
This is the variable the scheduler uses to determine whether or not the scheduler is locked.
It is locked when the value is non-zero.
OS_CRITICAL_EXIT() decrements OSSchedLockNestingCtr and when the value reaches
zero, invokes the scheduler.
OS_CRITICAL_EXIT_NO_SCHED() also decrements OSSchedLockNestingCtr , but does not
invoke the scheduler when the value reaches zero.
The code for the macros is shown in Listing 4-2.
71
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