Instruction

Enhanced Definition

An instruction in the mainframe context is the most fundamental, atomic operation that a Central Processing Unit (CPU) can execute. It is a command encoded in a specific binary format that tells the CPU to perform a single, predefined task, such as adding two numbers, moving data, or branching to another part of a program. Programs are essentially sequences of these machine-level instructions.

Key Characteristics

- Fixed Format: Instructions on IBM z/Architecture typically have a fixed length (2, 4, or 6 bytes) and a defined structure, including an operation code (opcode) and operands.
- Opcode and Operands: The opcode specifies the operation to be performed (e.g., ADD, LOAD), while operands specify the data or memory locations involved in the operation (e.g., registers, memory addresses, immediate values).
- Atomic Execution: Each instruction is executed as a single, indivisible unit by the CPU's instruction execution unit, often involving fetching, decoding, executing, and storing results.
- Part of Instruction Set Architecture (ISA): The complete set of instructions a CPU can understand and execute is defined by its ISA, such as the z/Architecture for IBM mainframes, which ensures backward compatibility across generations.
- Privileged vs. Non-Privileged: Some instructions are privileged (e.g., I/O operations, system control), meaning they can only be executed by the operating system (z/OS) in supervisor state, while non-privileged instructions are available to user programs.
- Sequential Execution: Instructions are typically executed sequentially from memory, with the Program Counter (or Instruction Address Register) pointing to the next instruction, unless a branch instruction alters the flow.

Use Cases

- Arithmetic and Logical Operations: Performing calculations (AP - Add Packed, SP - Subtract Packed) or bitwise manipulations (NR - AND Register, OR - OR Register) on data within registers or memory.
- Data Movement: Loading data from memory into a register (L - Load), storing data from a register to memory (ST - Store), or moving blocks of data between memory locations (MVC - Move Character).
- Control Flow: