Editing Program optimization (section)

===Assembly level===
At the lowest level, writing code using an [[assembly language]], designed for a particular hardware platform can produce the most efficient and compact code if the programmer takes advantage of the full repertoire of [[machine instruction]]s. Many [[operating system]]s used on [[embedded system]]s have been traditionally written in assembler code for this reason. Programs (other than very small programs) are seldom written from start to finish in assembly due to the time and cost involved. Most are compiled down from a high level language to assembly and hand optimized from there. When efficiency and size are less important large parts may be written in a high-level language.

With more modern [[optimizing compiler]]s and the greater complexity of recent [[CPU]]s, it is harder to write more efficient code than what the compiler generates, and few projects need this "ultimate" optimization step.

Much of the code written today is intended to run on as many machines as possible. As a consequence, programmers and compilers don't always take advantage of the more efficient instructions provided by newer CPUs or quirks of older models. Additionally, assembly code tuned for a particular processor without using such instructions might still be suboptimal on a different processor, expecting a different tuning of the code.

Typically today rather than writing in assembly language, programmers will use a [[disassembler]] to analyze the output of a compiler and change the high-level source code so that it can be compiled more efficiently, or understand why it is inefficient.