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+## Guided Overview
+
+Let's say you have a couple LEDs that are controlled by memory-mapped
+registers, a common case for embedded microcontrollers. To begin, you'll need
+to define the register used to control the LEDs.
+
+For an 8-bit register at memory address `0x0021`, you would write:
+
+```cpp
+using PORTA_OUT = fr::MemRegister<uint8_t, 0x0021>;
+```
+
+`MemRegister` is an `Register` that uses `MemoryIO` access. Registers
+have static functions for interacting with their contents; for example, we
+could now do `PORTA_OUT::write(0x10)` or `auto state = PORTA::read()`.
+
+A lot more can be done with registers once we define some register masks. A
+`RegisterMask` lets us name one or more bits within a register.
+
+To name our two LEDs, which are only controlled by single bits, we write:
+
+```cpp
+using LED_1 = fr::RegisterMask<PORTA_OUT, (1 << 0)>; // bit zero
+using LED_2 = fr::RegisterMask<PORTA_OUT, (1 << 2)>; // bit two
+```
+
+We can also give a name to both LEDs:
+
+```cpp
+using LED_ALL = fr::RegisterMask<PORTA_OUT, (1 << 0) | (1 << 2)>;
+```
+
+Now, we can control the LEDs either directly or through the register. Direct
+calls would be like `LED_1::set()`; there is also `clear`, `toggle`, `read`,
+`write`, and `test`.
+
+These calls can also be made through the register using template parameters:
+
+```cpp
+PORTA_OUT::set<LED_2>();
+```
+
+Registers can take multiple masks at once too. The masks will be merged so that
+the register is only read and written once:
+
+```cpp
+PORTA_OUT::toggle<LED_1, LED_2>();
+```
+
+A `modify` function is also supported, which takes a list of mask operations as
+shown below. This allows the different operations to be carried out together,
+still keeping to a single register read and write:
+
+```cpp
+PORTA_OUT::modify<LED_1::set, LED_2::clear>(); // Only have LED_1 turned on
+```
+
+What if we need to add a third LED? And what if that LED is on a different
+register, PORTB?
+
+This is where `RegisterGroup` comes in handy:
+
+```cpp
+using PORTB_OUT = fr::MemRegister<uint8_t, 0x0041>;
+
+using LED_3 = fr::RegisterMask<PORTB_OUT, (1 << 5)>;
+
+// Group the output ports together:
+using LEDS = fr::RegisterGroup<PORTA_OUT, PORTB_OUT>;
+```
+
+By grouping the two registers, we can carry out our modification calls without
+worrying about which mask is for what register. The `RegisterGroup` will take
+of that, while still merging operations when possible to maintain minimal I/O:
+
+```cpp
+LEDS::clear<LED_1, LED_2, LED_3>();
+
+LEDS::modify<LED_1::set,
+             LED_2::clear,
+             LED_3::toggle>(); // You get the idea...
+```
+
+## Other features
+
+### Multi-bit masks
+
+Say bits two through five in a register select a clock's prescaler:
+
+```cpp
+using CLOCK_DIV = fr::RegisterMask<CLOCK_CONTROL, 0x3C>;
+```
+
+The mask's `write` function will let you write values to the field:
+
+```cpp
+CLOCK_DIV::write<0x03>();
+```
+
+This will read the register's current value, clear all bits selected by the
+mask, set the new value `0x03` in the mask's location, then update the
+register.
+
+`write` can also be included in `modify` chains:
+
+```cpp
+CLOCK_CONTROL::modify<CLOCK_DIV::write<0x03>, CLOCK_ENABLE::set>();
+```
+
+A `RegisterMaskValue` can also be defined to identify specific values:
+
+```cpp
+using CLOCK_DIV4 = fr::RegisterMaskValue<CLOCK_DIV, 0x03>;
+```
+
+`RegisterMaskValue` supports three functions: `set`, which would call
+`CLOCK_DIV::write<0x03>()`; `clear`, which clears the masked bits; and `test`,
+which would confirm that the register contains the value `0x03` in the masked
+bits' location.
+
+### External registers
+
+"External" registers are registers that are not memory-mapped. These are also
+supported in *funreg*, and can even be placed in `RegisterGroup`s with
+other register types.
+
+An "access type" must be defined to specify how the register is accessed. Here
+is the definition of `MemoryIO`, which is used for memory-mapped registers:
+
+```cpp
+template<typename T, uintptr_t Addr>
+struct MemoryIO {
+    using type = T;
+    constexpr static auto addr = Addr;
+
+    /**
+     * Reads the register's value.
+     */
+    constexpr static T read() {
+        return *reinterpret_cast<volatile T*>(Addr);
+    }
+
+    /**
+     * Overwrites the register's value.
+     */
+    constexpr static void write(const T& value) {
+        *reinterpret_cast<volatile T*>(Addr) = value;
+    }
+};
+```
+
+Custom access types should use `MemoryIO` as a template. For example, here is
+an access type for ports on x86 processors:
+
+```cpp
+template<typename T, uintptr_t Addr>
+struct PortIO {
+    using type = T;
+    constexpr static auto addr = Addr;
+
+    static T read() {
+        T ret;
+        asm volatile("in %0, %1" : "=r" (ret) : "r" (Addr));
+        return ret; 
+    }
+
+    static void write(const T& value) {
+        asm volatile("out %0, %1" :: "r" (value), "r" (Addr));
+    }
+};
+```
+
+Now, just define your register(s) using `ExtRegister`:
+
+```cpp
+using KEYBOARD = fr::ExtRegister<PortIO, uint8_t, 0x60>;
+```
+