This book describes the specification and proof assembly language compiler for complex realistic level. Piton is the assembly-level programming language simple microprocessor called the FM9001 to be explained at the level of machine code. How Piton implemented on the FM9001? Also discussed is the character evolution of the software, the implementation of Piton in particular, and how the evidence played a continuing role in the design and improvement. Piton is a simple programming language but non-trivial. It provides execute-only programs, recursive subroutine call and return, stack based parameter passing, local variables, global variables and arrays, user-visible stack for intermediate results, and seven abstract data types including integers, data addresses, program addresses and subroutine names.