Instructions and Operation in Machine Code
Machine code is the lowest-level programming language and is the direct set of instructions that a computer's central processing unit (CPU) can execute. Each instruction in machine code performs a very specific operation, and the set of all possible instructions for a particular CPU is known as its instruction set architecture (ISA).
Structure of Instructions
In a typical instruction set, each machine instruction is composed of an opcode (operation code) and zero or more operands. The opcode specifies the operation to be performed, such as addition or data movement, whereas the operands are the data or memory locations that the operation acts upon.
Opcodes
Opcodes are unique binary or hexadecimal representations that are recognized by the CPU as commands. Each opcode corresponds to a specific operation within the CPU's capabilities, defined by the CPU's microarchitecture.
Operands and Addressing Modes
Operands provide the data required for the operation. They can be immediate values, registers, or memory addresses. The method of specifying operands is known as the addressing mode. Common addressing modes include:
- Immediate addressing, where the operand is directly specified in the instruction.
- Register addressing, which uses data from CPU registers.
- Direct and indirect addressing, where memory addresses are used to fetch operands.
Instruction Set Architectures (ISA)
ISAs define the supported instructions and the format for machine code. Some prominent examples include:
- x86 architecture: Predominantly used in personal computers, this CISC (Complex Instruction Set Computing) architecture supports a wide array of instructions.
- ARM architecture: A RISC (Reduced Instruction Set Computing) architecture, emphasizing efficiency and used in many mobile and embedded devices.
- MIPS architecture: Another RISC architecture known for its simplicity and wide use in educational contexts.
RISC vs. CISC
RISC architectures streamline operations by using a smaller set of simple instructions, while CISC architectures have more complex instructions capable of performing multiple operations. This difference affects both the instruction set and the microarchitecture of the CPU.
Execution of Instructions
Machine instructions are executed by the CPU in a series of steps known as the fetch-decode-execute cycle. During this cycle, the CPU fetches an instruction from memory, decodes it to determine the operation required and the operands involved, and then executes it.
Pipelining and Parallel Execution
Modern CPU designs often use techniques such as pipelining and multithreading to enhance performance by executing multiple instructions simultaneously or overlapping different stages of the fetch-decode-execute cycle.
Control Flow
Instructions in machine code can also include control flow operations that affect the sequence of execution, such as jumps and branches. These instructions are essential for implementing loops and conditional operations in higher-level programming languages.
Assembly Language
Assembly language, closely tied to machine code, provides a more human-readable representation of machine instructions. It uses symbolic names for opcodes and operands, making it easier to understand and write than raw machine code. An assembler is used to convert assembly language into machine code.