Start writing about MMIO.

src/SUMMARY.md

@@ -256,7 +256,7 @@
 # Bare Metal Rust: Afternoon
 
 - [Application processors]()
-  - [MMIO]()
+  - [MMIO](bare-metal/aps/mmio.md)
   - [Let's write a UART driver]()
   - [Logging]()
 - [Useful crates]()

src/bare-metal/aps/mmio.md

@@ -0,0 +1,17 @@
+# Volatile memory access for MMIO
+
+ * Use `pointer::read_volatile` and `pointer::write_volatile`.
+ * Never hold a reference.
+ * `addr_of!` lets you get fields of structs without creating an intermediate reference.
+
+<details>
+
+ * Volatile access: read or write operations may have side-effects, so prevent the compiler or
+   hardware from reordering, duplicating or eliding them.
+   * Usually if you write and then read, e.g. via a mutable reference, the compiler may assume that
+     the value read is the same as the value just written, and not bother actually reading memory.
+ * Some existing crates for volatile access to hardware do hold references, but this is unsound.
+   Whenever a reference exist, the compiler may choose to dereference it.
+ * Use the `addr_of!` macro to get struct field pointers from a pointer to the struct.
+
+</details>