X86-64 Instruction Encoding
Encoding of x86 and x86-64 instructions is well documented in Intel or AMD’s manuals. Here is a list of these manuals/tutorials for x86-64 instruction encoding.
x86-64 (and x86) ISA Reference from Intel and AMD: [urlf="./3024/x86-64-isa-assembly-references/"]x86-64 (and x86) ISA Reference[/urlf].
A very good page from OSDev: [url="http://wiki.osdev.org/X86-64InstructionEncoding"]X86-64 Instruction Encoding[/url].