While RTIC can currently target all Cortex-m devices there are some key architecure differences that
users should be aware of. Namely the absence of Base Priority Mask Register (
BASEPRI) which lends
itself exceptionally well to the hardware priority ceiling support used in RTIC, in the ARMv6-M and
ARMv8-M-base architectures, which forces RTIC to use source masking instead. For each implementation
of lock and a detailed commentary of pros and cons, see the implementation of
lock in src/export.rs.
These differences influence how critical sections are realized, but functionality should be the same except that ARMv6-M/ARMv8-M-base cannot have tasks with shared resources bound to exception handlers, as these cannot be masked in hardware.
Table 1 below shows a list of Cortex-m processors and which type of critical section they employ.
|Processor||Architecture||Priority Ceiling||Source Masking|
This implementation is covered in depth by the Critical Sections page of this book.
BASEPRI register which allows for directly setting a priority ceiling in the Nested
Vectored Interrupt Controller (NVIC), RTIC must instead rely on disabling (masking) interrupts.
Consider Figure 1 below, showing two tasks A and B where A has higher priority but shares a resource
┌────────────────────────────────────────────────────────────────┐ │ │ │ │ 3 │ Pending Preempts │ 2 │ ↑- - -A- - - - -↓A─────────► │ 1 │ B───────────────────► - - - - B────────► │ 0 │Idle┌─────► Resumes ┌────────► │ ├────┴────────────────────────────────────────────┴──────────────┤ │ │ └────────────────────────────────────────────────────────────────┴──► Time t1 t2 t3 t4
At time t1, task B locks the shared resource by selectively disabling (using the NVIC) all other
tasks which have a priority equal to or less than any task which shares resouces with B. In effect
this creates a virtual priority ceiling, miroring the
BASEPRI approach described in the
Critical Sections page. Task A is one such task that shares resources with
task B. At time t2, task A is either spawned by task B or becomes pending through an interrupt
condition, but does not yet preempt task B even though its priority is greater. This is because the
NVIC is preventing it from starting due to task A being being disabled. At time t3, task B
releases the lock by re-enabling the tasks in the NVIC. Because task A was pending and has a higher
priority than task B, it immediately preempts task B and is free to use the shared resource without
risk of data race conditions. At time t4, task A completes and returns the execution context to B.
Since source masking relies on use of the NVIC, core exception sources such as HardFault, SVCall, PendSV, and SysTick cannot share data with other tasks.