Branch Table

Enhanced Definition

In mainframe computing, particularly in assembly language programming and compiler optimization, a **branch table** is a data structure, typically an array of memory addresses or offsets, used to implement efficient multi-way branching. It allows a program to transfer control to one of several predefined locations based on an index value, significantly optimizing decision logic compared to a series of sequential conditional checks.

Key Characteristics

- Data Structure: Consists of an ordered list of memory addresses or relative offsets, each pointing to a specific execution path, routine, or program section.
- Indexed Access: A computed or provided index value is used to select the appropriate entry (address/offset) from the table, which then becomes the target of a branch instruction.
- Efficient Multi-way Branching: Provides a direct jump mechanism, eliminating the need for multiple COMPARE and BRANCH instructions (BC, BCT, BNE, BE) often found in long IF-THEN-ELSE IF constructs.
- Compiler Generation: Often generated by compilers for high-level language constructs like COBOL's GOTO DEPENDING ON or C/PL/I's SWITCH statements, optimizing the compiled code.
- Assembly Implementation: Can be explicitly coded in mainframe assembly language using DC (Define Constant) directives to create the table of addresses, followed by an indexed BR (Branch Register) or B (Branch) instruction.

Use Cases

- COBOL GOTO DEPENDING ON: Compilers translate this construct into a branch table to efficiently dispatch control to one of several paragraphs or sections based on the value of a data item.
- Transaction Dispatching: In online systems like CICS, a branch table might be used to dispatch control to different processing routines based on a transaction code or message type.
- Command Processors: Implementing command parsers where different commands map to different execution routines, using an index derived from the command.
- Menu Systems: Directing program flow to specific menu options based on user input, providing a fast lookup mechanism.
- Optimizing Decision Logic: Replacing a long series of IF-THEN-ELSE IF statements with a single indexed lookup and branch, particularly when there are many discrete conditions.

Related Concepts

Branch tables are a fundamental technique for program control flow, closely related to assembly language programming where they are directly implemented using instructions like BR or B with an index register. They are a common compiler optimization strategy, allowing high -level language constructs (like COBOL's GOTO DEPENDING ON or C's SWITCH) to be translated into highly efficient machine code. This technique directly impacts program performance by reducing instruction cycles compared to sequential conditional branching, especially for a large number of conditions.