How a compiler turns source into a running program

Source code

the text you write

characters in a file, meant for humans to read

becomes tokens,

a lexer scans the characters

splitting them into keywords, names, numbers, and symbols

discarding spaces and comments along the way

then a syntax tree,

a parser groups the tokens

following the language grammar

nesting expressions inside statements inside blocks

gets checked for meaning,

a semantic pass verifies names and types

catching mistakes long before the program ever runs

turns into an intermediate form,

a simpler internal language

easier to reason about than the original text

is optimized,

the compiler rewrites the intermediate form

to run faster and take less space

without changing what the program actually does

and finally lowered

a backend picks real machine instructions

matching the target processor and its rules

to machine code,

numbers the CPU can decode directly

each one a tiny, concrete operation

that runs.

the processor executes the instructions

one after another, billions per second