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+For cached decodings, this emulator uses the layout:
+
+* 8 bits: operation;
+* 8 bits:
+  * b7: address size — 16- or 32-bit;
+  * b6: set if this instruction has a displacement or offset attached;
+  * b5: set if this instruction has an immediate operand attached;
+  * [b4, b0]: the source operand's `Source`;
+* 16 bits:
+  * [b15, b14]: this instruction's data size;
+  * [b13, b10]: the length of this instruction in bytes (or 0 to indicate a length extension word is present);
+  * [b9, b5]: the top five bits of this instruction's SIB;
+  * [b4, b0]: the destination operand's `Source`.
+
+The low three bits of the SIB are stored in the low three bits of its operand's `Source` if necessary; the `Source` enum treats all values from `11000b` upwards as having the equivalent meaning of `Indirect` for this reason.
+
+Extension words are 16 bits in length for 16-bit decodings and 32 bits in length for 32-bit decodings. Up to three may be present, in the order:
+1. an immediate operand;
+2. an offset or displacement; and
+3. a length extension.
+
+If a length extension is present then it is laid out as:
+* [b15/b31, b6]: instruction length in bytes;
+* [b5, b4]: repetition attached to this instruction — repe/repne;
+* [b3, b1]: segment override attached to this instruction;
+* b0: whether the lock prefix was found.
+
+Therefore each decoded instruction is:
+* between 4 and 10 bytes for 16-bit decodings; and
+* between 4 and 16 bytes for 32-bit decodings.
+
+`sizeof(Instruction)` is therefore either `10` or `16`; it provides `packing_size` to give the size in bytes that are actually in use. `Instruction` is plain-old-data with a trivial destructor so it is safe to place them into memory such that instruction n+1 is placed at the address of instruction n + its `packing_size()`. Extension words therefore need be paid for only when required.
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