ALLPCB
Introduction
After writing a lot of code, it is natural to ask how a computer actually understands and executes that code. This topic leads into the computer's internal workings, from binary representation to logic gates, and on to the coordinated operation of the arithmetic logic unit and the controller.
1. Exploring the binary world
Before examining how a computer executes instructions, review the basic unit: binary. Computers use 0 and 1 to represent information, but these states are implemented as different voltage levels (low and high). In practice, 0 denotes a low voltage level and 1 denotes a high voltage level. Using 0 and 1 is an abstraction that makes the internal operations easier to reason about, similar to how blueprints represent a building.
2. Logic gates and circuits
Binary is the foundation, but it is only a starting point. Computers must convert those 0 and 1 states into concrete operations and results. This introduces logic gates and circuits. Logic gates are circuits built from electronic components that perform logical operations such as AND, OR, NOT, XOR. By combining logic gates, a computer can implement complex logical functions.
For example, an AND gate outputs 1 only when both inputs are 1. An OR gate outputs 1 when at least one input is 1. The outputs of these logic gates form the basis of internal operations.
3. Arithmetic Logic Unit (ALU)
Arithmetic and logical operations are performed by the arithmetic logic unit, or ALU. The ALU is a core component of the processor and executes arithmetic operations (addition, subtraction, etc.) and logical operations (AND, OR, NOT, etc.). The ALU receives inputs from registers, performs the operation specified by the opcode and operands, and writes the result back to registers.
For example, a simple ALU can add two binary numbers while preserving carry bits for chained operations. The ALU's internal circuits implement addition, subtraction, and logical operations through combinations of logic gates.
4. Control unit
Computers must not only perform arithmetic and logical operations, but also execute instructions in a specific sequence. The control unit coordinates the computer's activities. Based on an instruction's opcode, the control unit decides which action to perform during a clock cycle, such as reading data from memory, invoking the ALU, or writing results back to memory. The control unit generates specific control signals to orchestrate the behavior of other components.
5. Instruction set architecture
To enable complex tasks, a computer uses an instruction set architecture, or ISA, which defines the operations and instructions the machine supports. The ISA determines basic operation types, the number and purpose of registers, instruction formats, and more. Different ISAs support different operations and are suited to different application domains.
6. Compilation and execution
When you write code, you typically use a high-level programming language such as C or Python. Computers cannot directly understand high-level languages. High-level code must be translated into machine code through compilation (for compiled languages) or interpretation (for interpreted languages). A compiler converts high-level source code into corresponding machine instructions that the processor can execute.
At runtime, the processor fetches instructions from memory according to the ISA. The control unit decodes the opcode and drives the ALU and other components to carry out the specified operation. Data are fetched from registers or memory, processed by the ALU, and the results are stored back to registers or memory. This sequence of steps enables the computer to execute the logic expressed in your code.
7. Summary
The execution process inside a computer is a highly coordinated, staged sequence. From binary representation to logic gates, and through the joint operation of the ALU and the control unit, electronic signals carry information and drive operations. The instruction set architecture defines what the machine can do, and compilers translate high-level code into machine code. The control unit then orchestrates the components so the machine performs the intended tasks. Although the process is mechanical, it is the mechanism that enables computers to perform the functions used in everyday life.
