Add documentation for different targets

book/en/src/by-example/starting_a_project.md

@@ -1,6 +1,9 @@
 # Starting a new project
 
-A recommendation when starting a RTIC project from scratch is to follow RTIC's [`defmt-app-template`].
+A recommendation when starting a RTIC project from scratch on an ARMv7-M or ARMv8-M-main MCU is to 
+follow RTIC's [`defmt-app-template`]. For ARMv6-M or ARMv8-M-base, check out Section 4.? of
+this book for more information on hardware and implementation differences to be aware of before
+starting with RTIC.
 
 [`defmt-app-template`]: https://github.com/rtic-rs/defmt-app-template
 

book/en/src/internals/targets.md

@@ -0,0 +1,59 @@
+# Target Architecture
+
+While RTIC can currently target all Cortex-m devices there are some key architecure differences that 
+users should be aware. Namely the absence of hardware priority ceiling (BASEPRI) support in the 
+ARMv6-M and ARMv8-M-base architectures requires a few tweaks from RTIC to deliver the same 
+features. These differences result in two flavors of critical sections: priority ceiling, and source 
+masking. 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-M23 | ARMv8-M-base |                  |     &#2713     |
+| Cortex-M33 | ARMv8-M-main |      &#2713      |                |
+| Cortex-M7  | ARMv7-M      |      &#2713      |                |
+
+## Priority Ceiling
+
+This implementation is covered in depth by Chapter 4.5 of this book.
+
+## Source Masking
+
+Since there is no hardware support for a priority ceiling, RTIC must instead rely on the Nested 
+Vectored Interrupt Controller (NVIC) present in the core architecture. 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 all other tasks which share 
+the resource using the NVIC. In effect this raisis the virtual priority ceiling. 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 it's priority is 
+greater. This is because the NVIC is preventing it from starting due to task A's source mask 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.