Target Architecture

While RTIC can currently target all Cortex-m devices there are some key architecture 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.

Table 1: Critical Section Implementation by Processor Architecture

Priority Ceiling

This is covered by the Resources page of this book.

Source Masking

Without a 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 with B.

Figure 1: Shared Resources and Source Masking

  ┌────────────────────────────────────────────────────────────────┐
  │                                                                │
  │                                                                │
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 resources with B. In effect this creates a virtual priority ceiling, mirroring the BASEPRI approach. 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 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.