What is a source program?

An interpreter is a software that reads one statement from the source code, translates it to the machine code or virtual machine code, and then executes it right away

A compiler is a software that translates a program in high-level language into machine language code.

How does a compiler work?

Compilers vary in the methods they use for analyzing and converting source code to output code. Despite their differences, they typically carry out the following steps:

• Lexical analysis. The compiler splits the source code into lexemes, which are individual code fragments that represent specific patterns in the code. The lexemes are then tokenized in preparation for syntax and semantic analyses.
• Syntax analysis. The compiler verifies that the code's syntax is correct, based on the rules for the source language. This process is also referred to as parsing. During this step, the compiler typically creates abstract syntax trees that represent the logical structures of specific code elements.
• Semantic analysis. The compiler verifies the validity of the code's logic. This step goes beyond syntax analysis by validating the code's accuracy. For example, the semantic analysis might check whether variables have been assigned the right types or have been properly declared.
• IR code generation. After the code passes through all three analysis phases, the compiler generates an intermediate representation (IR) of the source code. The IR code makes it easier to translate the source code into a different format. However, it must accurately represent the source code in every respect, without omitting any functionality.
• Optimization. The compiler optimizes the IR code in preparation for the final code generation. The type and extent of optimization depends on the compiler. Some compilers let users configure the degree of optimization.
• Output code generation. The compiler generates the final output code, using the optimized IR code.

An interpreter reads one statement from the source code, and translates it to the machine code or virtual machine code, and then executes it right away. A compiler translates the entire source code into a machine code file, and the machine code file is then executed.