Subject: system programming and compiler construction

Topic: Intermediate Code Generation

Difficulty: Medium

• A program as a source code is merely a collection of text (code, statements etc.) and to make it alive, it requires actions to be performed on the target machine.
• A program needs memory resources to execute instructions.
• A program contains names for procedures, identifiers etc., that require mapping with the actual memory location at runtime.
• By runtime, we mean a program in execution.
• Runtime environment is a state of the target machine, which may include software libraries, environment variables, etc., to provide services to the processes running in the system.
• Runtime support system is a package, mostly generated with the executable program itself and facilitates the process communication between the process and the runtime environment.
• It takes care of memory allocation and de-allocation while the program is being executed.

Activation Records

• Recall that an activation is an execution of a subprogram. In most languages, local variables are allocated when this execution begins (activation time).
• The storage (for formals, local variables, function results etc.) needed for an activation is organized as an activation record (or frame).

• In a language with recursion, each simultaneous activation of a recursive subprogram can have different parameters, different values for local variables, return a different result.

  • Each activation needs its own activation record
  • The number of activation records needed isn’t known at compile time
  • Allocation of activation records is dynamic, and local variables are stack dynamic (unless declared static).

• A program is a sequence of instructions combined into a number of procedures.

• Instructions in a procedure are executed sequentially.

• A procedure has a start and an end delimiter and everything inside it is called the body of the procedure.

• The procedure identifier and the sequence of finite instructions inside it make up the body of the procedure.

• The execution of a procedure is called its activation.
• An activation record contains all the necessary information required to call a procedure.
• An activation record may contain the following units (depending upon the source language used).

Activate Record

• It is used to store the current record and the record is been stored in the stack.
• It contains return value .After the execution the value is been return.
• It can be called as return value. Parameter
• It specifies the number of parameters used in functions.

Local Data

• The data that is been used inside the function is called as local address Temporary Data
• It is used to store the data in temporary variables.

Links

• It specifies the additional links that are required by the program.

Status

• It specifies the status of program that is the flag used.

• Whenever a procedure is executed, its activation record is stored on the stack, also known as control stack.
• When a procedure calls another procedure, the execution of the caller is suspended until the called procedure finishes execution.
• At this time, the activation record of the called procedure is stored on the stack.
• We assume that the program control flows in a sequential manner and when a procedure is called, its control is transferred to the called procedure.
• When a called procedure is executed, it returns the control back to the caller. This type of control flow makes it easier to represent a series of activations in the form of a tree, known as the activation tree.

To understand this concept, we take a piece of code as an example:

            . . .
            Printf (“Enter Your Name: “);
            Scanf (“%s”, username);
            Show_data (username);
            printf (“Press any key to continue…”);
            . . .
             int show_data (char *user)
            {
            Printf (“Your name is %s”, username);
             return 0;
             }
             . . .

• Below is the activation tree of the code given.

• Now we understand that procedures are executed in depth-first manner, thus stack allocation is the best suitable form of storage for procedure activations.