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docs/lectures/compilers/01_structures.md

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+# Compilers - COMP 3012
+
+A compiler is a tool that maps one language into another language. It takes a program written in a source programming language and maps it to program written in a target programming language. A compiler is written in an **implementation language**.
+
+![img](img/a.png)
+
+Notation of semantics of program $A$: $[\![A]\!]$
+
+An **interpreter** is a program that takes a source program and executes the program.
+
+![img](img/b.png)
+
+> NOTE: Java uses both. A java source program is compiled into byte code (by a compiler) which is then executed by an interpreter (called java virtual machine - JVM). JVM will also compile fragments of code so that if there is a call back, it can execute the compiled code. This is called compilation on the fly.
+
+Compilers will often use an **intermediate representation (IR)** to bridge the gap between the source language and the executable language. Converting source language to IR is called **front end**, where as converting IR to executable code is called **back end**.
+
+* The front end focuses on understand the source-language program. 
+* The back end focuses on mapping programs to the target machine
+
+![img](img/c.png)
+
+* The front end, intermediate representation and the back end are all part of the compiler.
+
+IR is stored as an Abstract Syntax Tree **AST**.
+
+The syntactic details needed for parsing the source program are represented in the structure of the tree.
+
+The **IR** could be broken down into many sub-steps i.e. a IR1 could be created which is then ran through an optimiser to create IR2 which is fed into the back end instead of IR1. This is called a *three-phase compiler*.
+
+![img](img/d.png)