ASM

Assembler

Enhanced Definition

A low-level programming language that provides a one-to-one correspondence between mnemonic operation codes and machine language instructions. On mainframes, assembler is used for system programming, performance-critical applications, and low-level system interfaces.

Key Characteristics

- Direct control over CPU instructions and registers
- Access to system control blocks and hardware
- Maximum performance and minimal overhead
- Required for system exits and specialized functions
- Steep learning curve but powerful capabilities
Assembler Components:
- Instructions: Machine operations (MVC, LA, ST, L, BAL)
- Registers: General (R0-R15), floating-point, access
- Macros: Reusable code sequences
- Data definitions: DC (Define Constant), DS (Define Storage)
- Control sections: CSECT, DSECT for program organization
- System Programming: Operating system components, device drivers
- Performance-Critical Code: Inner loops, high-frequency routines
- System Exits: User exits, security exits, subsystem interfaces
- Hardware Interface: Direct I/O, privileged instructions
- Utility Programs: System tools and diagnostic utilities
Assembler Types:
- High Level Assembler (HLASM): IBM's current assembler
- Macro Assembler: Legacy assembler version
- Cross-Assemblers: Development on other platforms

Use Cases

- Developing system management facilities (SMF) exits
- Creating high-performance data transformation routines
- Implementing security exits for RACF or ACF2
- Writing CICS or IMS exits and customizations
- Building specialized utility programs