diff --git a/02_runtime_init/kernel b/02_runtime_init/kernel
index 64add528..e7aea5b7 100644
Binary files a/02_runtime_init/kernel and b/02_runtime_init/kernel differ
diff --git a/03_hacky_hello_world/README.md b/03_hacky_hello_world/README.md
index e8515cc8..6ffa8232 100644
--- a/03_hacky_hello_world/README.md
+++ b/03_hacky_hello_world/README.md
@@ -92,7 +92,7 @@ diff -uNr 02_runtime_init/src/bsp/rpi.rs 03_hacky_hello_world/src/bsp/rpi.rs
 diff -uNr 02_runtime_init/src/interface.rs 03_hacky_hello_world/src/interface.rs
 --- 02_runtime_init/src/interface.rs
 +++ 03_hacky_hello_world/src/interface.rs
-@@ -0,0 +1,36 @@
+@@ -0,0 +1,37 @@
 +// SPDX-License-Identifier: MIT OR Apache-2.0
 +//
 +// Copyright (c) 2018-2019 Andre Richter <andre.o.richter@gmail.com>
@@ -124,7 +124,8 @@ diff -uNr 02_runtime_init/src/interface.rs 03_hacky_hello_world/src/interface.rs
 +
 +    /// Console read functions.
 +    pub trait Read {
-+        fn read_char(&mut self) -> char {
++        /// Read a single character.
++        fn read_char(&self) -> char {
 +            ' '
 +        }
 +    }
@@ -203,7 +204,7 @@ diff -uNr 02_runtime_init/src/panic_wait.rs 03_hacky_hello_world/src/panic_wait.
 diff -uNr 02_runtime_init/src/print.rs 03_hacky_hello_world/src/print.rs
 --- 02_runtime_init/src/print.rs
 +++ 03_hacky_hello_world/src/print.rs
-@@ -0,0 +1,33 @@
+@@ -0,0 +1,34 @@
 +// SPDX-License-Identifier: MIT OR Apache-2.0
 +//
 +// Copyright (c) 2018-2019 Andre Richter <andre.o.richter@gmail.com>
@@ -213,6 +214,7 @@ diff -uNr 02_runtime_init/src/print.rs 03_hacky_hello_world/src/print.rs
 +use crate::{bsp, interface};
 +use core::fmt;
 +
++#[doc(hidden)]
 +pub fn _print(args: fmt::Arguments) {
 +    use interface::console::Write;
 +
diff --git a/03_hacky_hello_world/kernel b/03_hacky_hello_world/kernel
index 3bed3f75..d21f01dd 100755
Binary files a/03_hacky_hello_world/kernel and b/03_hacky_hello_world/kernel differ
diff --git a/03_hacky_hello_world/src/interface.rs b/03_hacky_hello_world/src/interface.rs
index 28b17379..b7c96aad 100644
--- a/03_hacky_hello_world/src/interface.rs
+++ b/03_hacky_hello_world/src/interface.rs
@@ -29,7 +29,8 @@ pub mod console {
 
     /// Console read functions.
     pub trait Read {
-        fn read_char(&mut self) -> char {
+        /// Read a single character.
+        fn read_char(&self) -> char {
             ' '
         }
     }
diff --git a/03_hacky_hello_world/src/print.rs b/03_hacky_hello_world/src/print.rs
index 86e5da83..3a7bed6c 100644
--- a/03_hacky_hello_world/src/print.rs
+++ b/03_hacky_hello_world/src/print.rs
@@ -7,6 +7,7 @@
 use crate::{bsp, interface};
 use core::fmt;
 
+#[doc(hidden)]
 pub fn _print(args: fmt::Arguments) {
     use interface::console::Write;
 
diff --git a/04_zero_overhead_abstraction/Cargo.lock b/04_zero_overhead_abstraction/Cargo.lock
index 10ccb746..b90ca892 100644
--- a/04_zero_overhead_abstraction/Cargo.lock
+++ b/04_zero_overhead_abstraction/Cargo.lock
@@ -5,7 +5,7 @@ name = "cortex-a"
 version = "2.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -23,7 +23,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "register"
-version = "0.4.1"
+version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -37,5 +37,5 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [metadata]
 "checksum cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)" = "4aab2f5271d9bf17a52b34dd99993648132df3dacb79312a33332f2b6ae1d0fd"
 "checksum r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "e2a38df5b15c8d5c7e8654189744d8e396bddc18ad48041a500ce52d6948941f"
-"checksum register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "7c92f7cc61c67d0f283777ce3c678789788d1b48e8f5e822a257513c16194955"
+"checksum register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)" = "a51f149d257caa7d9aed1f870d573ba5e2429576e6fed0693341f23078c98e55"
 "checksum tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "50af9c49c55cfb4437dd78c1fada3be5d088cbe1bea641db8171283503606a70"
diff --git a/04_zero_overhead_abstraction/README.md b/04_zero_overhead_abstraction/README.md
index 10c7d27f..446d8806 100644
--- a/04_zero_overhead_abstraction/README.md
+++ b/04_zero_overhead_abstraction/README.md
@@ -103,11 +103,14 @@ diff -uNr 03_hacky_hello_world/src/arch/aarch64.rs 04_zero_overhead_abstraction/
 diff -uNr 03_hacky_hello_world/src/bsp/rpi.rs 04_zero_overhead_abstraction/src/bsp/rpi.rs
 --- 03_hacky_hello_world/src/bsp/rpi.rs
 +++ 04_zero_overhead_abstraction/src/bsp/rpi.rs
-@@ -7,6 +7,9 @@
+@@ -7,6 +7,12 @@
  use crate::interface;
  use core::fmt;
 
++/// Used by `arch` code to find the early boot core.
 +pub const BOOT_CORE_ID: u64 = 0;
++
++/// The early boot core's stack address.
 +pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
 +
  /// A mystical, magical device for generating QEMU output out of the void.
diff --git a/04_zero_overhead_abstraction/kernel b/04_zero_overhead_abstraction/kernel
index 06269ad0..5f204deb 100755
Binary files a/04_zero_overhead_abstraction/kernel and b/04_zero_overhead_abstraction/kernel differ
diff --git a/04_zero_overhead_abstraction/kernel8.img b/04_zero_overhead_abstraction/kernel8.img
index 4b009eea..65fbc00f 100755
Binary files a/04_zero_overhead_abstraction/kernel8.img and b/04_zero_overhead_abstraction/kernel8.img differ
diff --git a/04_zero_overhead_abstraction/src/bsp/rpi.rs b/04_zero_overhead_abstraction/src/bsp/rpi.rs
index 7c9a6e36..e42a4779 100644
--- a/04_zero_overhead_abstraction/src/bsp/rpi.rs
+++ b/04_zero_overhead_abstraction/src/bsp/rpi.rs
@@ -7,7 +7,10 @@
 use crate::interface;
 use core::fmt;
 
+/// Used by `arch` code to find the early boot core.
 pub const BOOT_CORE_ID: u64 = 0;
+
+/// The early boot core's stack address.
 pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
 
 /// A mystical, magical device for generating QEMU output out of the void.
diff --git a/04_zero_overhead_abstraction/src/interface.rs b/04_zero_overhead_abstraction/src/interface.rs
index 28b17379..b7c96aad 100644
--- a/04_zero_overhead_abstraction/src/interface.rs
+++ b/04_zero_overhead_abstraction/src/interface.rs
@@ -29,7 +29,8 @@ pub mod console {
 
     /// Console read functions.
     pub trait Read {
-        fn read_char(&mut self) -> char {
+        /// Read a single character.
+        fn read_char(&self) -> char {
             ' '
         }
     }
diff --git a/04_zero_overhead_abstraction/src/print.rs b/04_zero_overhead_abstraction/src/print.rs
index 86e5da83..3a7bed6c 100644
--- a/04_zero_overhead_abstraction/src/print.rs
+++ b/04_zero_overhead_abstraction/src/print.rs
@@ -7,6 +7,7 @@
 use crate::{bsp, interface};
 use core::fmt;
 
+#[doc(hidden)]
 pub fn _print(args: fmt::Arguments) {
     use interface::console::Write;
 
diff --git a/05_safe_globals/Cargo.lock b/05_safe_globals/Cargo.lock
index 10ccb746..b90ca892 100644
--- a/05_safe_globals/Cargo.lock
+++ b/05_safe_globals/Cargo.lock
@@ -5,7 +5,7 @@ name = "cortex-a"
 version = "2.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -23,7 +23,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "register"
-version = "0.4.1"
+version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -37,5 +37,5 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [metadata]
 "checksum cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)" = "4aab2f5271d9bf17a52b34dd99993648132df3dacb79312a33332f2b6ae1d0fd"
 "checksum r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "e2a38df5b15c8d5c7e8654189744d8e396bddc18ad48041a500ce52d6948941f"
-"checksum register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "7c92f7cc61c67d0f283777ce3c678789788d1b48e8f5e822a257513c16194955"
+"checksum register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)" = "a51f149d257caa7d9aed1f870d573ba5e2429576e6fed0693341f23078c98e55"
 "checksum tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "50af9c49c55cfb4437dd78c1fada3be5d088cbe1bea641db8171283503606a70"
diff --git a/05_safe_globals/README.md b/05_safe_globals/README.md
index 9b97f0f3..95d4a672 100644
--- a/05_safe_globals/README.md
+++ b/05_safe_globals/README.md
@@ -55,7 +55,7 @@ make qemu
 diff -uNr 04_zero_overhead_abstraction/src/arch/aarch64/sync.rs 05_safe_globals/src/arch/aarch64/sync.rs
 --- 04_zero_overhead_abstraction/src/arch/aarch64/sync.rs
 +++ 05_safe_globals/src/arch/aarch64/sync.rs
-@@ -0,0 +1,52 @@
+@@ -0,0 +1,53 @@
 +// SPDX-License-Identifier: MIT OR Apache-2.0
 +//
 +// Copyright (c) 2018-2019 Andre Richter <andre.o.richter@gmail.com>
@@ -88,6 +88,7 @@ diff -uNr 04_zero_overhead_abstraction/src/arch/aarch64/sync.rs 05_safe_globals/
 +unsafe impl<T: ?Sized + Send> Sync for NullLock<T> {}
 +
 +impl<T> NullLock<T> {
++    /// Wraps `data` into a new `NullLock`.
 +    pub const fn new(data: T) -> NullLock<T> {
 +        NullLock {
 +            data: UnsafeCell::new(data),
@@ -125,7 +126,7 @@ diff -uNr 04_zero_overhead_abstraction/src/arch/aarch64.rs 05_safe_globals/src/a
 diff -uNr 04_zero_overhead_abstraction/src/bsp/rpi.rs 05_safe_globals/src/bsp/rpi.rs
 --- 04_zero_overhead_abstraction/src/bsp/rpi.rs
 +++ 05_safe_globals/src/bsp/rpi.rs
-@@ -4,38 +4,114 @@
+@@ -4,7 +4,7 @@
 
  //! Board Support Package for the Raspberry Pi.
 
@@ -133,7 +134,8 @@ diff -uNr 04_zero_overhead_abstraction/src/bsp/rpi.rs 05_safe_globals/src/bsp/rp
 +use crate::{arch::sync::NullLock, interface};
  use core::fmt;
 
- pub const BOOT_CORE_ID: u64 = 0;
+ /// Used by `arch` code to find the early boot core.
+@@ -14,31 +14,107 @@
  pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
 
  /// A mystical, magical device for generating QEMU output out of the void.
@@ -254,7 +256,7 @@ diff -uNr 04_zero_overhead_abstraction/src/bsp/rpi.rs 05_safe_globals/src/bsp/rp
 diff -uNr 04_zero_overhead_abstraction/src/interface.rs 05_safe_globals/src/interface.rs
 --- 04_zero_overhead_abstraction/src/interface.rs
 +++ 05_safe_globals/src/interface.rs
-@@ -20,17 +20,66 @@
+@@ -20,12 +20,13 @@
 
  /// System console operations.
  pub mod console {
@@ -267,13 +269,13 @@ diff -uNr 04_zero_overhead_abstraction/src/interface.rs 05_safe_globals/src/inte
 -    /// intention.
 -    pub use core::fmt::Write;
 +    pub trait Write {
++        /// Write a Rust format string.
 +        fn write_fmt(&self, args: fmt::Arguments) -> fmt::Result;
 +    }
 
      /// Console read functions.
      pub trait Read {
--        fn read_char(&mut self) -> char {
-+        fn read_char(&self) -> char {
+@@ -34,4 +35,53 @@
              ' '
          }
      }
diff --git a/05_safe_globals/kernel b/05_safe_globals/kernel
index 96ed10e5..ca52d607 100755
Binary files a/05_safe_globals/kernel and b/05_safe_globals/kernel differ
diff --git a/05_safe_globals/kernel8.img b/05_safe_globals/kernel8.img
index e07c2390..94851bfe 100755
Binary files a/05_safe_globals/kernel8.img and b/05_safe_globals/kernel8.img differ
diff --git a/05_safe_globals/src/arch/aarch64/sync.rs b/05_safe_globals/src/arch/aarch64/sync.rs
index 96ceff03..7e8781e6 100644
--- a/05_safe_globals/src/arch/aarch64/sync.rs
+++ b/05_safe_globals/src/arch/aarch64/sync.rs
@@ -30,6 +30,7 @@ unsafe impl<T: ?Sized + Send> Send for NullLock<T> {}
 unsafe impl<T: ?Sized + Send> Sync for NullLock<T> {}
 
 impl<T> NullLock<T> {
+    /// Wraps `data` into a new `NullLock`.
     pub const fn new(data: T) -> NullLock<T> {
         NullLock {
             data: UnsafeCell::new(data),
diff --git a/05_safe_globals/src/bsp/rpi.rs b/05_safe_globals/src/bsp/rpi.rs
index ce467ee4..bf962b82 100644
--- a/05_safe_globals/src/bsp/rpi.rs
+++ b/05_safe_globals/src/bsp/rpi.rs
@@ -7,7 +7,10 @@
 use crate::{arch::sync::NullLock, interface};
 use core::fmt;
 
+/// Used by `arch` code to find the early boot core.
 pub const BOOT_CORE_ID: u64 = 0;
+
+/// The early boot core's stack address.
 pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
 
 /// A mystical, magical device for generating QEMU output out of the void.
diff --git a/05_safe_globals/src/interface.rs b/05_safe_globals/src/interface.rs
index 84c0b317..3c7ceeca 100644
--- a/05_safe_globals/src/interface.rs
+++ b/05_safe_globals/src/interface.rs
@@ -24,11 +24,13 @@ pub mod console {
 
     /// Console write functions.
     pub trait Write {
+        /// Write a Rust format string.
         fn write_fmt(&self, args: fmt::Arguments) -> fmt::Result;
     }
 
     /// Console read functions.
     pub trait Read {
+        /// Read a single character.
         fn read_char(&self) -> char {
             ' '
         }
diff --git a/05_safe_globals/src/print.rs b/05_safe_globals/src/print.rs
index 86e5da83..3a7bed6c 100644
--- a/05_safe_globals/src/print.rs
+++ b/05_safe_globals/src/print.rs
@@ -7,6 +7,7 @@
 use crate::{bsp, interface};
 use core::fmt;
 
+#[doc(hidden)]
 pub fn _print(args: fmt::Arguments) {
     use interface::console::Write;
 
diff --git a/06_drivers_gpio_uart/Cargo.lock b/06_drivers_gpio_uart/Cargo.lock
index 1b2c9ac6..4f7a0366 100644
--- a/06_drivers_gpio_uart/Cargo.lock
+++ b/06_drivers_gpio_uart/Cargo.lock
@@ -5,7 +5,7 @@ name = "cortex-a"
 version = "2.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -14,7 +14,7 @@ version = "0.1.0"
 dependencies = [
  "cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -24,7 +24,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "register"
-version = "0.4.1"
+version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -38,5 +38,5 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [metadata]
 "checksum cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)" = "4aab2f5271d9bf17a52b34dd99993648132df3dacb79312a33332f2b6ae1d0fd"
 "checksum r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "e2a38df5b15c8d5c7e8654189744d8e396bddc18ad48041a500ce52d6948941f"
-"checksum register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "7c92f7cc61c67d0f283777ce3c678789788d1b48e8f5e822a257513c16194955"
+"checksum register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)" = "a51f149d257caa7d9aed1f870d573ba5e2429576e6fed0693341f23078c98e55"
 "checksum tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "50af9c49c55cfb4437dd78c1fada3be5d088cbe1bea641db8171283503606a70"
diff --git a/06_drivers_gpio_uart/README.md b/06_drivers_gpio_uart/README.md
index e85d5f15..1e392920 100644
--- a/06_drivers_gpio_uart/README.md
+++ b/06_drivers_gpio_uart/README.md
@@ -642,7 +642,7 @@ diff -uNr 05_safe_globals/src/bsp/rpi/memory_map.rs 06_drivers_gpio_uart/src/bsp
 diff -uNr 05_safe_globals/src/bsp/rpi.rs 06_drivers_gpio_uart/src/bsp/rpi.rs
 --- 05_safe_globals/src/bsp/rpi.rs
 +++ 06_drivers_gpio_uart/src/bsp/rpi.rs
-@@ -4,114 +4,68 @@
+@@ -4,7 +4,10 @@
 
  //! Board Support Package for the Raspberry Pi.
 
@@ -653,7 +653,9 @@ diff -uNr 05_safe_globals/src/bsp/rpi.rs 06_drivers_gpio_uart/src/bsp/rpi.rs
 +use crate::interface;
  use core::fmt;
 
- pub const BOOT_CORE_ID: u64 = 0;
+ /// Used by `arch` code to find the early boot core.
+@@ -13,108 +16,59 @@
+ /// The early boot core's stack address.
  pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
 
 -/// A mystical, magical device for generating QEMU output out of the void.
@@ -818,15 +820,17 @@ diff -uNr 05_safe_globals/src/bsp.rs 06_drivers_gpio_uart/src/bsp.rs
 diff -uNr 05_safe_globals/src/interface.rs 06_drivers_gpio_uart/src/interface.rs
 --- 05_safe_globals/src/interface.rs
 +++ 06_drivers_gpio_uart/src/interface.rs
-@@ -24,6 +24,7 @@
+@@ -24,6 +24,9 @@
 
      /// Console write functions.
      pub trait Write {
++        /// Write a single character.
 +        fn write_char(&self, c: char);
++
+         /// Write a Rust format string.
          fn write_fmt(&self, args: fmt::Arguments) -> fmt::Result;
      }
-
-@@ -83,3 +84,20 @@
+@@ -85,3 +88,20 @@
          fn lock<R>(&mut self, f: impl FnOnce(&mut Self::Data) -> R) -> R;
      }
  }
diff --git a/06_drivers_gpio_uart/kernel b/06_drivers_gpio_uart/kernel
index dd911cf3..c909017d 100755
Binary files a/06_drivers_gpio_uart/kernel and b/06_drivers_gpio_uart/kernel differ
diff --git a/06_drivers_gpio_uart/kernel8.img b/06_drivers_gpio_uart/kernel8.img
index 860a4f06..41d1e853 100755
Binary files a/06_drivers_gpio_uart/kernel8.img and b/06_drivers_gpio_uart/kernel8.img differ
diff --git a/06_drivers_gpio_uart/src/arch/aarch64/sync.rs b/06_drivers_gpio_uart/src/arch/aarch64/sync.rs
index 96ceff03..7e8781e6 100644
--- a/06_drivers_gpio_uart/src/arch/aarch64/sync.rs
+++ b/06_drivers_gpio_uart/src/arch/aarch64/sync.rs
@@ -30,6 +30,7 @@ unsafe impl<T: ?Sized + Send> Send for NullLock<T> {}
 unsafe impl<T: ?Sized + Send> Sync for NullLock<T> {}
 
 impl<T> NullLock<T> {
+    /// Wraps `data` into a new `NullLock`.
     pub const fn new(data: T) -> NullLock<T> {
         NullLock {
             data: UnsafeCell::new(data),
diff --git a/06_drivers_gpio_uart/src/bsp/rpi.rs b/06_drivers_gpio_uart/src/bsp/rpi.rs
index 197258c1..0acb693c 100644
--- a/06_drivers_gpio_uart/src/bsp/rpi.rs
+++ b/06_drivers_gpio_uart/src/bsp/rpi.rs
@@ -10,7 +10,10 @@ use super::driver;
 use crate::interface;
 use core::fmt;
 
+/// Used by `arch` code to find the early boot core.
 pub const BOOT_CORE_ID: u64 = 0;
+
+/// The early boot core's stack address.
 pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
 
 //--------------------------------------------------------------------------------------------------
diff --git a/06_drivers_gpio_uart/src/interface.rs b/06_drivers_gpio_uart/src/interface.rs
index 9178eb60..66c22f1a 100644
--- a/06_drivers_gpio_uart/src/interface.rs
+++ b/06_drivers_gpio_uart/src/interface.rs
@@ -24,12 +24,16 @@ pub mod console {
 
     /// Console write functions.
     pub trait Write {
+        /// Write a single character.
         fn write_char(&self, c: char);
+
+        /// Write a Rust format string.
         fn write_fmt(&self, args: fmt::Arguments) -> fmt::Result;
     }
 
     /// Console read functions.
     pub trait Read {
+        /// Read a single character.
         fn read_char(&self) -> char {
             ' '
         }
diff --git a/06_drivers_gpio_uart/src/print.rs b/06_drivers_gpio_uart/src/print.rs
index 86e5da83..3a7bed6c 100644
--- a/06_drivers_gpio_uart/src/print.rs
+++ b/06_drivers_gpio_uart/src/print.rs
@@ -7,6 +7,7 @@
 use crate::{bsp, interface};
 use core::fmt;
 
+#[doc(hidden)]
 pub fn _print(args: fmt::Arguments) {
     use interface::console::Write;
 
diff --git a/07_uart_chainloader/Cargo.lock b/07_uart_chainloader/Cargo.lock
index 1b2c9ac6..4f7a0366 100644
--- a/07_uart_chainloader/Cargo.lock
+++ b/07_uart_chainloader/Cargo.lock
@@ -5,7 +5,7 @@ name = "cortex-a"
 version = "2.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -14,7 +14,7 @@ version = "0.1.0"
 dependencies = [
  "cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -24,7 +24,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "register"
-version = "0.4.1"
+version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -38,5 +38,5 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [metadata]
 "checksum cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)" = "4aab2f5271d9bf17a52b34dd99993648132df3dacb79312a33332f2b6ae1d0fd"
 "checksum r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "e2a38df5b15c8d5c7e8654189744d8e396bddc18ad48041a500ce52d6948941f"
-"checksum register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "7c92f7cc61c67d0f283777ce3c678789788d1b48e8f5e822a257513c16194955"
+"checksum register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)" = "a51f149d257caa7d9aed1f870d573ba5e2429576e6fed0693341f23078c98e55"
 "checksum tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "50af9c49c55cfb4437dd78c1fada3be5d088cbe1bea641db8171283503606a70"
diff --git a/07_uart_chainloader/README.md b/07_uart_chainloader/README.md
index a0e56715..b90022e4 100644
--- a/07_uart_chainloader/README.md
+++ b/07_uart_chainloader/README.md
@@ -239,8 +239,8 @@ diff -uNr 06_drivers_gpio_uart/src/bsp/rpi/link.ld 07_uart_chainloader/src/bsp/r
 diff -uNr 06_drivers_gpio_uart/src/bsp/rpi.rs 07_uart_chainloader/src/bsp/rpi.rs
 --- 06_drivers_gpio_uart/src/bsp/rpi.rs
 +++ 07_uart_chainloader/src/bsp/rpi.rs
-@@ -13,6 +13,9 @@
- pub const BOOT_CORE_ID: u64 = 0;
+@@ -16,6 +16,9 @@
+ /// The early boot core's stack address.
  pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
 
 +/// The address on which the RPi3 firmware loads every binary by default.
@@ -253,9 +253,9 @@ diff -uNr 06_drivers_gpio_uart/src/bsp/rpi.rs 07_uart_chainloader/src/bsp/rpi.rs
 diff -uNr 06_drivers_gpio_uart/src/interface.rs 07_uart_chainloader/src/interface.rs
 --- 06_drivers_gpio_uart/src/interface.rs
 +++ 07_uart_chainloader/src/interface.rs
-@@ -26,6 +26,10 @@
-     pub trait Write {
-         fn write_char(&self, c: char);
+@@ -29,6 +29,10 @@
+
+         /// Write a Rust format string.
          fn write_fmt(&self, args: fmt::Arguments) -> fmt::Result;
 +
 +        /// Block execution until the last character has been physically put on the TX wire
@@ -264,7 +264,7 @@ diff -uNr 06_drivers_gpio_uart/src/interface.rs 07_uart_chainloader/src/interfac
      }
 
      /// Console read functions.
-@@ -33,6 +37,9 @@
+@@ -37,6 +41,9 @@
          fn read_char(&self) -> char {
              ' '
          }
diff --git a/07_uart_chainloader/kernel b/07_uart_chainloader/kernel
index f136ea06..04602d52 100755
Binary files a/07_uart_chainloader/kernel and b/07_uart_chainloader/kernel differ
diff --git a/07_uart_chainloader/kernel8.img b/07_uart_chainloader/kernel8.img
index 7f74a22a..654ac5b8 100755
Binary files a/07_uart_chainloader/kernel8.img and b/07_uart_chainloader/kernel8.img differ
diff --git a/07_uart_chainloader/src/arch/aarch64/sync.rs b/07_uart_chainloader/src/arch/aarch64/sync.rs
index 96ceff03..7e8781e6 100644
--- a/07_uart_chainloader/src/arch/aarch64/sync.rs
+++ b/07_uart_chainloader/src/arch/aarch64/sync.rs
@@ -30,6 +30,7 @@ unsafe impl<T: ?Sized + Send> Send for NullLock<T> {}
 unsafe impl<T: ?Sized + Send> Sync for NullLock<T> {}
 
 impl<T> NullLock<T> {
+    /// Wraps `data` into a new `NullLock`.
     pub const fn new(data: T) -> NullLock<T> {
         NullLock {
             data: UnsafeCell::new(data),
diff --git a/07_uart_chainloader/src/bsp/rpi.rs b/07_uart_chainloader/src/bsp/rpi.rs
index 77fef475..79fbeca0 100644
--- a/07_uart_chainloader/src/bsp/rpi.rs
+++ b/07_uart_chainloader/src/bsp/rpi.rs
@@ -10,7 +10,10 @@ use super::driver;
 use crate::interface;
 use core::fmt;
 
+/// Used by `arch` code to find the early boot core.
 pub const BOOT_CORE_ID: u64 = 0;
+
+/// The early boot core's stack address.
 pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
 
 /// The address on which the RPi3 firmware loads every binary by default.
diff --git a/07_uart_chainloader/src/interface.rs b/07_uart_chainloader/src/interface.rs
index 420aeaf3..866f2d4d 100644
--- a/07_uart_chainloader/src/interface.rs
+++ b/07_uart_chainloader/src/interface.rs
@@ -24,7 +24,10 @@ pub mod console {
 
     /// Console write functions.
     pub trait Write {
+        /// Write a single character.
         fn write_char(&self, c: char);
+
+        /// Write a Rust format string.
         fn write_fmt(&self, args: fmt::Arguments) -> fmt::Result;
 
         /// Block execution until the last character has been physically put on the TX wire
@@ -34,6 +37,7 @@ pub mod console {
 
     /// Console read functions.
     pub trait Read {
+        /// Read a single character.
         fn read_char(&self) -> char {
             ' '
         }
diff --git a/07_uart_chainloader/src/print.rs b/07_uart_chainloader/src/print.rs
index 86e5da83..3a7bed6c 100644
--- a/07_uart_chainloader/src/print.rs
+++ b/07_uart_chainloader/src/print.rs
@@ -7,6 +7,7 @@
 use crate::{bsp, interface};
 use core::fmt;
 
+#[doc(hidden)]
 pub fn _print(args: fmt::Arguments) {
     use interface::console::Write;
 
diff --git a/08_timestamps/Cargo.lock b/08_timestamps/Cargo.lock
index 1b2c9ac6..4f7a0366 100644
--- a/08_timestamps/Cargo.lock
+++ b/08_timestamps/Cargo.lock
@@ -5,7 +5,7 @@ name = "cortex-a"
 version = "2.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -14,7 +14,7 @@ version = "0.1.0"
 dependencies = [
  "cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -24,7 +24,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "register"
-version = "0.4.1"
+version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -38,5 +38,5 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [metadata]
 "checksum cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)" = "4aab2f5271d9bf17a52b34dd99993648132df3dacb79312a33332f2b6ae1d0fd"
 "checksum r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "e2a38df5b15c8d5c7e8654189744d8e396bddc18ad48041a500ce52d6948941f"
-"checksum register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "7c92f7cc61c67d0f283777ce3c678789788d1b48e8f5e822a257513c16194955"
+"checksum register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)" = "a51f149d257caa7d9aed1f870d573ba5e2429576e6fed0693341f23078c98e55"
 "checksum tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "50af9c49c55cfb4437dd78c1fada3be5d088cbe1bea641db8171283503606a70"
diff --git a/08_timestamps/README.md b/08_timestamps/README.md
index a9903732..9f83e5d0 100644
--- a/08_timestamps/README.md
+++ b/08_timestamps/README.md
@@ -287,8 +287,8 @@ diff -uNr 07_uart_chainloader/src/bsp/rpi/link.ld 08_timestamps/src/bsp/rpi/link
 diff -uNr 07_uart_chainloader/src/bsp/rpi.rs 08_timestamps/src/bsp/rpi.rs
 --- 07_uart_chainloader/src/bsp/rpi.rs
 +++ 08_timestamps/src/bsp/rpi.rs
-@@ -13,9 +13,6 @@
- pub const BOOT_CORE_ID: u64 = 0;
+@@ -16,9 +16,6 @@
+ /// The early boot core's stack address.
  pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
 
 -/// The address on which the RPi3 firmware loads every binary by default.
@@ -301,7 +301,7 @@ diff -uNr 07_uart_chainloader/src/bsp/rpi.rs 08_timestamps/src/bsp/rpi.rs
 diff -uNr 07_uart_chainloader/src/interface.rs 08_timestamps/src/interface.rs
 --- 07_uart_chainloader/src/interface.rs
 +++ 08_timestamps/src/interface.rs
-@@ -108,3 +108,22 @@
+@@ -112,3 +112,22 @@
          }
      }
  }
@@ -413,12 +413,12 @@ diff -uNr 07_uart_chainloader/src/main.rs 08_timestamps/src/main.rs
 diff -uNr 07_uart_chainloader/src/print.rs 08_timestamps/src/print.rs
 --- 07_uart_chainloader/src/print.rs
 +++ 08_timestamps/src/print.rs
-@@ -31,3 +31,71 @@
+@@ -32,3 +32,71 @@
          $crate::print::_print(format_args_nl!($($arg)*));
      })
  }
 +
-+/// Prints am info, with newline.
++/// Prints an info, with newline.
 +#[macro_export]
 +macro_rules! info {
 +    ($string:expr) => ({
diff --git a/08_timestamps/kernel b/08_timestamps/kernel
index 1929dc8c..396f84df 100755
Binary files a/08_timestamps/kernel and b/08_timestamps/kernel differ
diff --git a/08_timestamps/kernel8.img b/08_timestamps/kernel8.img
index f553ef75..a30cd440 100755
Binary files a/08_timestamps/kernel8.img and b/08_timestamps/kernel8.img differ
diff --git a/08_timestamps/src/arch/aarch64/sync.rs b/08_timestamps/src/arch/aarch64/sync.rs
index 96ceff03..7e8781e6 100644
--- a/08_timestamps/src/arch/aarch64/sync.rs
+++ b/08_timestamps/src/arch/aarch64/sync.rs
@@ -30,6 +30,7 @@ unsafe impl<T: ?Sized + Send> Send for NullLock<T> {}
 unsafe impl<T: ?Sized + Send> Sync for NullLock<T> {}
 
 impl<T> NullLock<T> {
+    /// Wraps `data` into a new `NullLock`.
     pub const fn new(data: T) -> NullLock<T> {
         NullLock {
             data: UnsafeCell::new(data),
diff --git a/08_timestamps/src/bsp/rpi.rs b/08_timestamps/src/bsp/rpi.rs
index 197258c1..0acb693c 100644
--- a/08_timestamps/src/bsp/rpi.rs
+++ b/08_timestamps/src/bsp/rpi.rs
@@ -10,7 +10,10 @@ use super::driver;
 use crate::interface;
 use core::fmt;
 
+/// Used by `arch` code to find the early boot core.
 pub const BOOT_CORE_ID: u64 = 0;
+
+/// The early boot core's stack address.
 pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
 
 //--------------------------------------------------------------------------------------------------
diff --git a/08_timestamps/src/interface.rs b/08_timestamps/src/interface.rs
index 134dbfd5..8a547ab3 100644
--- a/08_timestamps/src/interface.rs
+++ b/08_timestamps/src/interface.rs
@@ -24,7 +24,10 @@ pub mod console {
 
     /// Console write functions.
     pub trait Write {
+        /// Write a single character.
         fn write_char(&self, c: char);
+
+        /// Write a Rust format string.
         fn write_fmt(&self, args: fmt::Arguments) -> fmt::Result;
 
         /// Block execution until the last character has been physically put on the TX wire
@@ -34,6 +37,7 @@ pub mod console {
 
     /// Console read functions.
     pub trait Read {
+        /// Read a single character.
         fn read_char(&self) -> char {
             ' '
         }
diff --git a/08_timestamps/src/print.rs b/08_timestamps/src/print.rs
index 7bd8d6ef..16f1fb21 100644
--- a/08_timestamps/src/print.rs
+++ b/08_timestamps/src/print.rs
@@ -7,6 +7,7 @@
 use crate::{bsp, interface};
 use core::fmt;
 
+#[doc(hidden)]
 pub fn _print(args: fmt::Arguments) {
     use interface::console::Write;
 
@@ -32,7 +33,7 @@ macro_rules! println {
     })
 }
 
-/// Prints am info, with newline.
+/// Prints an info, with newline.
 #[macro_export]
 macro_rules! info {
     ($string:expr) => ({
diff --git a/09_hw_debug_JTAG/Cargo.lock b/09_hw_debug_JTAG/Cargo.lock
index 1b2c9ac6..4f7a0366 100644
--- a/09_hw_debug_JTAG/Cargo.lock
+++ b/09_hw_debug_JTAG/Cargo.lock
@@ -5,7 +5,7 @@ name = "cortex-a"
 version = "2.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -14,7 +14,7 @@ version = "0.1.0"
 dependencies = [
  "cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -24,7 +24,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "register"
-version = "0.4.1"
+version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -38,5 +38,5 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [metadata]
 "checksum cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)" = "4aab2f5271d9bf17a52b34dd99993648132df3dacb79312a33332f2b6ae1d0fd"
 "checksum r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "e2a38df5b15c8d5c7e8654189744d8e396bddc18ad48041a500ce52d6948941f"
-"checksum register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "7c92f7cc61c67d0f283777ce3c678789788d1b48e8f5e822a257513c16194955"
+"checksum register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)" = "a51f149d257caa7d9aed1f870d573ba5e2429576e6fed0693341f23078c98e55"
 "checksum tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "50af9c49c55cfb4437dd78c1fada3be5d088cbe1bea641db8171283503606a70"
diff --git a/09_hw_debug_JTAG/kernel b/09_hw_debug_JTAG/kernel
index 1929dc8c..396f84df 100755
Binary files a/09_hw_debug_JTAG/kernel and b/09_hw_debug_JTAG/kernel differ
diff --git a/09_hw_debug_JTAG/kernel8.img b/09_hw_debug_JTAG/kernel8.img
index f553ef75..a30cd440 100755
Binary files a/09_hw_debug_JTAG/kernel8.img and b/09_hw_debug_JTAG/kernel8.img differ
diff --git a/09_hw_debug_JTAG/src/arch/aarch64/sync.rs b/09_hw_debug_JTAG/src/arch/aarch64/sync.rs
index 96ceff03..7e8781e6 100644
--- a/09_hw_debug_JTAG/src/arch/aarch64/sync.rs
+++ b/09_hw_debug_JTAG/src/arch/aarch64/sync.rs
@@ -30,6 +30,7 @@ unsafe impl<T: ?Sized + Send> Send for NullLock<T> {}
 unsafe impl<T: ?Sized + Send> Sync for NullLock<T> {}
 
 impl<T> NullLock<T> {
+    /// Wraps `data` into a new `NullLock`.
     pub const fn new(data: T) -> NullLock<T> {
         NullLock {
             data: UnsafeCell::new(data),
diff --git a/09_hw_debug_JTAG/src/bsp/rpi.rs b/09_hw_debug_JTAG/src/bsp/rpi.rs
index 197258c1..0acb693c 100644
--- a/09_hw_debug_JTAG/src/bsp/rpi.rs
+++ b/09_hw_debug_JTAG/src/bsp/rpi.rs
@@ -10,7 +10,10 @@ use super::driver;
 use crate::interface;
 use core::fmt;
 
+/// Used by `arch` code to find the early boot core.
 pub const BOOT_CORE_ID: u64 = 0;
+
+/// The early boot core's stack address.
 pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
 
 //--------------------------------------------------------------------------------------------------
diff --git a/09_hw_debug_JTAG/src/interface.rs b/09_hw_debug_JTAG/src/interface.rs
index 134dbfd5..8a547ab3 100644
--- a/09_hw_debug_JTAG/src/interface.rs
+++ b/09_hw_debug_JTAG/src/interface.rs
@@ -24,7 +24,10 @@ pub mod console {
 
     /// Console write functions.
     pub trait Write {
+        /// Write a single character.
         fn write_char(&self, c: char);
+
+        /// Write a Rust format string.
         fn write_fmt(&self, args: fmt::Arguments) -> fmt::Result;
 
         /// Block execution until the last character has been physically put on the TX wire
@@ -34,6 +37,7 @@ pub mod console {
 
     /// Console read functions.
     pub trait Read {
+        /// Read a single character.
         fn read_char(&self) -> char {
             ' '
         }
diff --git a/09_hw_debug_JTAG/src/print.rs b/09_hw_debug_JTAG/src/print.rs
index 7bd8d6ef..16f1fb21 100644
--- a/09_hw_debug_JTAG/src/print.rs
+++ b/09_hw_debug_JTAG/src/print.rs
@@ -7,6 +7,7 @@
 use crate::{bsp, interface};
 use core::fmt;
 
+#[doc(hidden)]
 pub fn _print(args: fmt::Arguments) {
     use interface::console::Write;
 
@@ -32,7 +33,7 @@ macro_rules! println {
     })
 }
 
-/// Prints am info, with newline.
+/// Prints an info, with newline.
 #[macro_export]
 macro_rules! info {
     ($string:expr) => ({
diff --git a/10_privilege_level/Cargo.lock b/10_privilege_level/Cargo.lock
index 1b2c9ac6..4f7a0366 100644
--- a/10_privilege_level/Cargo.lock
+++ b/10_privilege_level/Cargo.lock
@@ -5,7 +5,7 @@ name = "cortex-a"
 version = "2.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -14,7 +14,7 @@ version = "0.1.0"
 dependencies = [
  "cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -24,7 +24,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "register"
-version = "0.4.1"
+version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -38,5 +38,5 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [metadata]
 "checksum cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)" = "4aab2f5271d9bf17a52b34dd99993648132df3dacb79312a33332f2b6ae1d0fd"
 "checksum r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "e2a38df5b15c8d5c7e8654189744d8e396bddc18ad48041a500ce52d6948941f"
-"checksum register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "7c92f7cc61c67d0f283777ce3c678789788d1b48e8f5e822a257513c16194955"
+"checksum register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)" = "a51f149d257caa7d9aed1f870d573ba5e2429576e6fed0693341f23078c98e55"
 "checksum tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "50af9c49c55cfb4437dd78c1fada3be5d088cbe1bea641db8171283503606a70"
diff --git a/10_privilege_level/README.md b/10_privilege_level/README.md
index e94300a9..8b656a19 100644
--- a/10_privilege_level/README.md
+++ b/10_privilege_level/README.md
@@ -334,7 +334,7 @@ diff -uNr 09_hw_debug_JTAG/src/arch/aarch64.rs 10_privilege_level/src/arch/aarch
  //--------------------------------------------------------------------------------------------------
  // Global instances
  //--------------------------------------------------------------------------------------------------
-@@ -61,3 +103,36 @@
+@@ -61,3 +103,37 @@
          asm::wfe()
      }
  }
@@ -353,6 +353,7 @@ diff -uNr 09_hw_debug_JTAG/src/arch/aarch64.rs 10_privilege_level/src/arch/aarch
 +        }
 +    }
 +
++    /// Print the AArch64 exceptions status.
 +    #[rustfmt::skip]
 +    pub fn print_exception_state() {
 +        use super::{
@@ -361,7 +362,7 @@ diff -uNr 09_hw_debug_JTAG/src/arch/aarch64.rs 10_privilege_level/src/arch/aarch
 +        };
 +        use crate::info;
 +
-+        let to_mask_str = |x: bool| -> &'static str {
++        let to_mask_str = |x| -> _ {
 +            if x { "Masked" } else { "Unmasked" }
 +        };
 +
diff --git a/10_privilege_level/kernel b/10_privilege_level/kernel
index 7ceb1c98..0178ddf0 100755
Binary files a/10_privilege_level/kernel and b/10_privilege_level/kernel differ
diff --git a/10_privilege_level/kernel8.img b/10_privilege_level/kernel8.img
index 3c08349f..6e226f70 100755
Binary files a/10_privilege_level/kernel8.img and b/10_privilege_level/kernel8.img differ
diff --git a/10_privilege_level/src/arch/aarch64.rs b/10_privilege_level/src/arch/aarch64.rs
index 3ba64ea6..91c80d44 100644
--- a/10_privilege_level/src/arch/aarch64.rs
+++ b/10_privilege_level/src/arch/aarch64.rs
@@ -118,6 +118,7 @@ pub mod state {
         }
     }
 
+    /// Print the AArch64 exceptions status.
     #[rustfmt::skip]
     pub fn print_exception_state() {
         use super::{
@@ -126,7 +127,7 @@ pub mod state {
         };
         use crate::info;
 
-        let to_mask_str = |x: bool| -> &'static str {
+        let to_mask_str = |x| -> _ {
             if x { "Masked" } else { "Unmasked" }
         };
 
diff --git a/10_privilege_level/src/arch/aarch64/sync.rs b/10_privilege_level/src/arch/aarch64/sync.rs
index 96ceff03..7e8781e6 100644
--- a/10_privilege_level/src/arch/aarch64/sync.rs
+++ b/10_privilege_level/src/arch/aarch64/sync.rs
@@ -30,6 +30,7 @@ unsafe impl<T: ?Sized + Send> Send for NullLock<T> {}
 unsafe impl<T: ?Sized + Send> Sync for NullLock<T> {}
 
 impl<T> NullLock<T> {
+    /// Wraps `data` into a new `NullLock`.
     pub const fn new(data: T) -> NullLock<T> {
         NullLock {
             data: UnsafeCell::new(data),
diff --git a/10_privilege_level/src/bsp/rpi.rs b/10_privilege_level/src/bsp/rpi.rs
index 197258c1..0acb693c 100644
--- a/10_privilege_level/src/bsp/rpi.rs
+++ b/10_privilege_level/src/bsp/rpi.rs
@@ -10,7 +10,10 @@ use super::driver;
 use crate::interface;
 use core::fmt;
 
+/// Used by `arch` code to find the early boot core.
 pub const BOOT_CORE_ID: u64 = 0;
+
+/// The early boot core's stack address.
 pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
 
 //--------------------------------------------------------------------------------------------------
diff --git a/10_privilege_level/src/interface.rs b/10_privilege_level/src/interface.rs
index 134dbfd5..8a547ab3 100644
--- a/10_privilege_level/src/interface.rs
+++ b/10_privilege_level/src/interface.rs
@@ -24,7 +24,10 @@ pub mod console {
 
     /// Console write functions.
     pub trait Write {
+        /// Write a single character.
         fn write_char(&self, c: char);
+
+        /// Write a Rust format string.
         fn write_fmt(&self, args: fmt::Arguments) -> fmt::Result;
 
         /// Block execution until the last character has been physically put on the TX wire
@@ -34,6 +37,7 @@ pub mod console {
 
     /// Console read functions.
     pub trait Read {
+        /// Read a single character.
         fn read_char(&self) -> char {
             ' '
         }
diff --git a/10_privilege_level/src/print.rs b/10_privilege_level/src/print.rs
index 7bd8d6ef..16f1fb21 100644
--- a/10_privilege_level/src/print.rs
+++ b/10_privilege_level/src/print.rs
@@ -7,6 +7,7 @@
 use crate::{bsp, interface};
 use core::fmt;
 
+#[doc(hidden)]
 pub fn _print(args: fmt::Arguments) {
     use interface::console::Write;
 
@@ -32,7 +33,7 @@ macro_rules! println {
     })
 }
 
-/// Prints am info, with newline.
+/// Prints an info, with newline.
 #[macro_export]
 macro_rules! info {
     ($string:expr) => ({
diff --git a/11_virtual_memory/Cargo.lock b/11_virtual_memory/Cargo.lock
index 1b2c9ac6..4f7a0366 100644
--- a/11_virtual_memory/Cargo.lock
+++ b/11_virtual_memory/Cargo.lock
@@ -5,7 +5,7 @@ name = "cortex-a"
 version = "2.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -14,7 +14,7 @@ version = "0.1.0"
 dependencies = [
  "cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -24,7 +24,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "register"
-version = "0.4.1"
+version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -38,5 +38,5 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [metadata]
 "checksum cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)" = "4aab2f5271d9bf17a52b34dd99993648132df3dacb79312a33332f2b6ae1d0fd"
 "checksum r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "e2a38df5b15c8d5c7e8654189744d8e396bddc18ad48041a500ce52d6948941f"
-"checksum register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "7c92f7cc61c67d0f283777ce3c678789788d1b48e8f5e822a257513c16194955"
+"checksum register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)" = "a51f149d257caa7d9aed1f870d573ba5e2429576e6fed0693341f23078c98e55"
 "checksum tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "50af9c49c55cfb4437dd78c1fada3be5d088cbe1bea641db8171283503606a70"
diff --git a/11_virtual_memory/README.md b/11_virtual_memory/README.md
index 00bf0497..304d4387 100644
--- a/11_virtual_memory/README.md
+++ b/11_virtual_memory/README.md
@@ -602,15 +602,6 @@ diff -uNr 10_privilege_level/src/arch/aarch64.rs 11_virtual_memory/src/arch/aarc
  /// Information about the HW state.
  pub mod state {
      use cortex_a::regs::*;
-@@ -126,7 +133,7 @@
-         };
-         use crate::info;
-
--        let to_mask_str = |x: bool| -> &'static str {
-+        let to_mask_str = |x| -> _ {
-             if x { "Masked" } else { "Unmasked" }
-         };
-
 
 diff -uNr 10_privilege_level/src/bsp/rpi/link.ld 11_virtual_memory/src/bsp/rpi/link.ld
 --- 10_privilege_level/src/bsp/rpi/link.ld
@@ -759,8 +750,8 @@ diff -uNr 10_privilege_level/src/bsp/rpi.rs 11_virtual_memory/src/bsp/rpi.rs
 +use crate::{interface, memory::KernelVirtualLayout};
  use core::fmt;
 
- pub const BOOT_CORE_ID: u64 = 0;
-@@ -69,3 +70,13 @@
+ /// Used by `arch` code to find the early boot core.
+@@ -72,3 +73,13 @@
      // Configure PL011Uart's output pins.
      GPIO.map_pl011_uart();
  }
@@ -791,13 +782,14 @@ diff -uNr 10_privilege_level/src/bsp.rs 11_virtual_memory/src/bsp.rs
 diff -uNr 10_privilege_level/src/interface.rs 11_virtual_memory/src/interface.rs
 --- 10_privilege_level/src/interface.rs
 +++ 11_virtual_memory/src/interface.rs
-@@ -127,3 +127,11 @@
+@@ -131,3 +131,12 @@
          fn spin_for(&self, duration: Duration);
      }
  }
 +
 +/// Memory Management interfaces.
 +pub mod mm {
++    /// MMU functions.
 +    pub trait MMU {
 +        /// Called by the kernel early during init.
 +        unsafe fn init(&self) -> Result<(), &'static str>;
diff --git a/11_virtual_memory/kernel b/11_virtual_memory/kernel
index 5d939853..0927bfc3 100755
Binary files a/11_virtual_memory/kernel and b/11_virtual_memory/kernel differ
diff --git a/11_virtual_memory/kernel8.img b/11_virtual_memory/kernel8.img
index ae26afd1..4e6eede4 100755
Binary files a/11_virtual_memory/kernel8.img and b/11_virtual_memory/kernel8.img differ
diff --git a/11_virtual_memory/src/arch/aarch64.rs b/11_virtual_memory/src/arch/aarch64.rs
index a77782c8..32187b04 100644
--- a/11_virtual_memory/src/arch/aarch64.rs
+++ b/11_virtual_memory/src/arch/aarch64.rs
@@ -125,6 +125,7 @@ pub mod state {
         }
     }
 
+    /// Print the AArch64 exceptions status.
     #[rustfmt::skip]
     pub fn print_exception_state() {
         use super::{
diff --git a/11_virtual_memory/src/arch/aarch64/sync.rs b/11_virtual_memory/src/arch/aarch64/sync.rs
index 96ceff03..7e8781e6 100644
--- a/11_virtual_memory/src/arch/aarch64/sync.rs
+++ b/11_virtual_memory/src/arch/aarch64/sync.rs
@@ -30,6 +30,7 @@ unsafe impl<T: ?Sized + Send> Send for NullLock<T> {}
 unsafe impl<T: ?Sized + Send> Sync for NullLock<T> {}
 
 impl<T> NullLock<T> {
+    /// Wraps `data` into a new `NullLock`.
     pub const fn new(data: T) -> NullLock<T> {
         NullLock {
             data: UnsafeCell::new(data),
diff --git a/11_virtual_memory/src/bsp/rpi.rs b/11_virtual_memory/src/bsp/rpi.rs
index 27c6bc12..93120316 100644
--- a/11_virtual_memory/src/bsp/rpi.rs
+++ b/11_virtual_memory/src/bsp/rpi.rs
@@ -11,7 +11,10 @@ use super::driver;
 use crate::{interface, memory::KernelVirtualLayout};
 use core::fmt;
 
+/// Used by `arch` code to find the early boot core.
 pub const BOOT_CORE_ID: u64 = 0;
+
+/// The early boot core's stack address.
 pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
 
 //--------------------------------------------------------------------------------------------------
diff --git a/11_virtual_memory/src/interface.rs b/11_virtual_memory/src/interface.rs
index 0d4a3a23..19dad63c 100644
--- a/11_virtual_memory/src/interface.rs
+++ b/11_virtual_memory/src/interface.rs
@@ -24,7 +24,10 @@ pub mod console {
 
     /// Console write functions.
     pub trait Write {
+        /// Write a single character.
         fn write_char(&self, c: char);
+
+        /// Write a Rust format string.
         fn write_fmt(&self, args: fmt::Arguments) -> fmt::Result;
 
         /// Block execution until the last character has been physically put on the TX wire
@@ -34,6 +37,7 @@ pub mod console {
 
     /// Console read functions.
     pub trait Read {
+        /// Read a single character.
         fn read_char(&self) -> char {
             ' '
         }
@@ -130,6 +134,7 @@ pub mod time {
 
 /// Memory Management interfaces.
 pub mod mm {
+    /// MMU functions.
     pub trait MMU {
         /// Called by the kernel early during init.
         unsafe fn init(&self) -> Result<(), &'static str>;
diff --git a/11_virtual_memory/src/print.rs b/11_virtual_memory/src/print.rs
index 7bd8d6ef..16f1fb21 100644
--- a/11_virtual_memory/src/print.rs
+++ b/11_virtual_memory/src/print.rs
@@ -7,6 +7,7 @@
 use crate::{bsp, interface};
 use core::fmt;
 
+#[doc(hidden)]
 pub fn _print(args: fmt::Arguments) {
     use interface::console::Write;
 
@@ -32,7 +33,7 @@ macro_rules! println {
     })
 }
 
-/// Prints am info, with newline.
+/// Prints an info, with newline.
 #[macro_export]
 macro_rules! info {
     ($string:expr) => ({
diff --git a/12_cpu_exceptions_part1/Cargo.lock b/12_cpu_exceptions_part1/Cargo.lock
index 1b2c9ac6..4f7a0366 100644
--- a/12_cpu_exceptions_part1/Cargo.lock
+++ b/12_cpu_exceptions_part1/Cargo.lock
@@ -5,7 +5,7 @@ name = "cortex-a"
 version = "2.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -14,7 +14,7 @@ version = "0.1.0"
 dependencies = [
  "cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -24,7 +24,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "register"
-version = "0.4.1"
+version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -38,5 +38,5 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [metadata]
 "checksum cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)" = "4aab2f5271d9bf17a52b34dd99993648132df3dacb79312a33332f2b6ae1d0fd"
 "checksum r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "e2a38df5b15c8d5c7e8654189744d8e396bddc18ad48041a500ce52d6948941f"
-"checksum register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "7c92f7cc61c67d0f283777ce3c678789788d1b48e8f5e822a257513c16194955"
+"checksum register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)" = "a51f149d257caa7d9aed1f870d573ba5e2429576e6fed0693341f23078c98e55"
 "checksum tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "50af9c49c55cfb4437dd78c1fada3be5d088cbe1bea641db8171283503606a70"
diff --git a/12_cpu_exceptions_part1/kernel b/12_cpu_exceptions_part1/kernel
index e452eae8..f4dc62bd 100755
Binary files a/12_cpu_exceptions_part1/kernel and b/12_cpu_exceptions_part1/kernel differ
diff --git a/12_cpu_exceptions_part1/kernel8.img b/12_cpu_exceptions_part1/kernel8.img
index 4b51eea0..f393029b 100755
Binary files a/12_cpu_exceptions_part1/kernel8.img and b/12_cpu_exceptions_part1/kernel8.img differ
diff --git a/12_cpu_exceptions_part1/src/arch/aarch64.rs b/12_cpu_exceptions_part1/src/arch/aarch64.rs
index 588d87e5..0b13069c 100644
--- a/12_cpu_exceptions_part1/src/arch/aarch64.rs
+++ b/12_cpu_exceptions_part1/src/arch/aarch64.rs
@@ -134,6 +134,7 @@ pub mod state {
         }
     }
 
+    /// Print the AArch64 exceptions status.
     #[rustfmt::skip]
     pub fn print_exception_state() {
         use super::{
diff --git a/12_cpu_exceptions_part1/src/arch/aarch64/sync.rs b/12_cpu_exceptions_part1/src/arch/aarch64/sync.rs
index 96ceff03..7e8781e6 100644
--- a/12_cpu_exceptions_part1/src/arch/aarch64/sync.rs
+++ b/12_cpu_exceptions_part1/src/arch/aarch64/sync.rs
@@ -30,6 +30,7 @@ unsafe impl<T: ?Sized + Send> Send for NullLock<T> {}
 unsafe impl<T: ?Sized + Send> Sync for NullLock<T> {}
 
 impl<T> NullLock<T> {
+    /// Wraps `data` into a new `NullLock`.
     pub const fn new(data: T) -> NullLock<T> {
         NullLock {
             data: UnsafeCell::new(data),
diff --git a/12_cpu_exceptions_part1/src/bsp/rpi.rs b/12_cpu_exceptions_part1/src/bsp/rpi.rs
index 27c6bc12..93120316 100644
--- a/12_cpu_exceptions_part1/src/bsp/rpi.rs
+++ b/12_cpu_exceptions_part1/src/bsp/rpi.rs
@@ -11,7 +11,10 @@ use super::driver;
 use crate::{interface, memory::KernelVirtualLayout};
 use core::fmt;
 
+/// Used by `arch` code to find the early boot core.
 pub const BOOT_CORE_ID: u64 = 0;
+
+/// The early boot core's stack address.
 pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
 
 //--------------------------------------------------------------------------------------------------
diff --git a/12_cpu_exceptions_part1/src/interface.rs b/12_cpu_exceptions_part1/src/interface.rs
index 0d4a3a23..19dad63c 100644
--- a/12_cpu_exceptions_part1/src/interface.rs
+++ b/12_cpu_exceptions_part1/src/interface.rs
@@ -24,7 +24,10 @@ pub mod console {
 
     /// Console write functions.
     pub trait Write {
+        /// Write a single character.
         fn write_char(&self, c: char);
+
+        /// Write a Rust format string.
         fn write_fmt(&self, args: fmt::Arguments) -> fmt::Result;
 
         /// Block execution until the last character has been physically put on the TX wire
@@ -34,6 +37,7 @@ pub mod console {
 
     /// Console read functions.
     pub trait Read {
+        /// Read a single character.
         fn read_char(&self) -> char {
             ' '
         }
@@ -130,6 +134,7 @@ pub mod time {
 
 /// Memory Management interfaces.
 pub mod mm {
+    /// MMU functions.
     pub trait MMU {
         /// Called by the kernel early during init.
         unsafe fn init(&self) -> Result<(), &'static str>;
diff --git a/12_cpu_exceptions_part1/src/print.rs b/12_cpu_exceptions_part1/src/print.rs
index 7bd8d6ef..16f1fb21 100644
--- a/12_cpu_exceptions_part1/src/print.rs
+++ b/12_cpu_exceptions_part1/src/print.rs
@@ -7,6 +7,7 @@
 use crate::{bsp, interface};
 use core::fmt;
 
+#[doc(hidden)]
 pub fn _print(args: fmt::Arguments) {
     use interface::console::Write;
 
@@ -32,7 +33,7 @@ macro_rules! println {
     })
 }
 
-/// Prints am info, with newline.
+/// Prints an info, with newline.
 #[macro_export]
 macro_rules! info {
     ($string:expr) => ({
diff --git a/X1_JTAG_boot/Cargo.lock b/X1_JTAG_boot/Cargo.lock
index 1b2c9ac6..4f7a0366 100644
--- a/X1_JTAG_boot/Cargo.lock
+++ b/X1_JTAG_boot/Cargo.lock
@@ -5,7 +5,7 @@ name = "cortex-a"
 version = "2.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -14,7 +14,7 @@ version = "0.1.0"
 dependencies = [
  "cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
- "register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -24,7 +24,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "register"
-version = "0.4.1"
+version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -38,5 +38,5 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [metadata]
 "checksum cortex-a 2.8.2 (registry+https://github.com/rust-lang/crates.io-index)" = "4aab2f5271d9bf17a52b34dd99993648132df3dacb79312a33332f2b6ae1d0fd"
 "checksum r0 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "e2a38df5b15c8d5c7e8654189744d8e396bddc18ad48041a500ce52d6948941f"
-"checksum register 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "7c92f7cc61c67d0f283777ce3c678789788d1b48e8f5e822a257513c16194955"
+"checksum register 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)" = "a51f149d257caa7d9aed1f870d573ba5e2429576e6fed0693341f23078c98e55"
 "checksum tock-registers 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "50af9c49c55cfb4437dd78c1fada3be5d088cbe1bea641db8171283503606a70"
diff --git a/X1_JTAG_boot/jtag_boot_rpi3.img b/X1_JTAG_boot/jtag_boot_rpi3.img
index c9a8d970..68143d6f 100755
Binary files a/X1_JTAG_boot/jtag_boot_rpi3.img and b/X1_JTAG_boot/jtag_boot_rpi3.img differ
diff --git a/X1_JTAG_boot/jtag_boot_rpi4.img b/X1_JTAG_boot/jtag_boot_rpi4.img
index 142e1e38..ccb1ed02 100755
Binary files a/X1_JTAG_boot/jtag_boot_rpi4.img and b/X1_JTAG_boot/jtag_boot_rpi4.img differ
diff --git a/X1_JTAG_boot/src/arch/aarch64/sync.rs b/X1_JTAG_boot/src/arch/aarch64/sync.rs
index 96ceff03..7e8781e6 100644
--- a/X1_JTAG_boot/src/arch/aarch64/sync.rs
+++ b/X1_JTAG_boot/src/arch/aarch64/sync.rs
@@ -30,6 +30,7 @@ unsafe impl<T: ?Sized + Send> Send for NullLock<T> {}
 unsafe impl<T: ?Sized + Send> Sync for NullLock<T> {}
 
 impl<T> NullLock<T> {
+    /// Wraps `data` into a new `NullLock`.
     pub const fn new(data: T) -> NullLock<T> {
         NullLock {
             data: UnsafeCell::new(data),
diff --git a/X1_JTAG_boot/src/bsp/rpi.rs b/X1_JTAG_boot/src/bsp/rpi.rs
index 871c4b8b..d76314ba 100644
--- a/X1_JTAG_boot/src/bsp/rpi.rs
+++ b/X1_JTAG_boot/src/bsp/rpi.rs
@@ -10,7 +10,10 @@ use super::driver;
 use crate::interface;
 use core::fmt;
 
+/// Used by `arch` code to find the early boot core.
 pub const BOOT_CORE_ID: u64 = 0;
+
+/// The early boot core's stack address.
 pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
 
 //--------------------------------------------------------------------------------------------------
diff --git a/X1_JTAG_boot/src/interface.rs b/X1_JTAG_boot/src/interface.rs
index 376be32c..84633d71 100644
--- a/X1_JTAG_boot/src/interface.rs
+++ b/X1_JTAG_boot/src/interface.rs
@@ -24,7 +24,10 @@ pub mod console {
 
     /// Console write functions.
     pub trait Write {
+        /// Write a single character.
         fn write_char(&self, c: char);
+
+        /// Write a Rust format string.
         fn write_fmt(&self, args: fmt::Arguments) -> fmt::Result;
 
         /// Block execution until the last character has been physically put on the TX wire
@@ -34,6 +37,7 @@ pub mod console {
 
     /// Console read functions.
     pub trait Read {
+        /// Read a single character.
         fn read_char(&self) -> char {
             ' '
         }
diff --git a/X1_JTAG_boot/src/print.rs b/X1_JTAG_boot/src/print.rs
index 7bd8d6ef..16f1fb21 100644
--- a/X1_JTAG_boot/src/print.rs
+++ b/X1_JTAG_boot/src/print.rs
@@ -7,6 +7,7 @@
 use crate::{bsp, interface};
 use core::fmt;
 
+#[doc(hidden)]
 pub fn _print(args: fmt::Arguments) {
     use interface::console::Write;
 
@@ -32,7 +33,7 @@ macro_rules! println {
     })
 }
 
-/// Prints am info, with newline.
+/// Prints an info, with newline.
 #[macro_export]
 macro_rules! info {
     ($string:expr) => ({