# Target Architecture

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][src_export].

[src_export]: https://github.com/rtic-rs/cortex-m-rtic/blob/master/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*

| Processor  | Architecture | Priority Ceiling | Source Masking |
| :--------- | :----------: | :--------------: | :------------: |
| Cortex-M0  | ARMv6-M      |                  |     &#2713     |
| Cortex-M0+ | ARMv6-M      |                  |     &#2713     |
| Cortex-M3  | ARMv7-M      |      &#2713      |                |
| Cortex-M4  | ARMv7-M      |      &#2713      |                |
| Cortex-M7  | ARMv7-M      |      &#2713      |                |
| Cortex-M23 | ARMv8-M-base |                  |     &#2713     |
| Cortex-M33 | ARMv8-M-main |      &#2713      |                |

## Priority Ceiling

This implementation is covered in depth by the [Critical Sections][critical_sections] 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*

```text
  ┌────────────────────────────────────────────────────────────────┐
  │                                                                │
  │                                                                │
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][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.

[critical_sections]: https://github.com/rtic-rs/cortex-m-rtic/blob/master/book/en/src/internals/critical-sections.md