Many of the compile-time decisions for run-time storage involve procedure calls. For each procedure or function, including the main program, the compiler constructs a program unit to be used at execution time. A program unit is composed of a code segment, which is the code to be executed, and an activation record, which is the information necessary to execute the code segment.
A code segment is fixed since it consists of (machine code) instructions, while the activation record information is changeable since it references the variables which are to receive values.