Logic Gates Mastery for IGCSE OCR Computer Science | 逻辑门精通：IGCSE OCR 计算机科学考点精讲

📚 Logic Gates Mastery for IGCSE OCR Computer Science | 逻辑门精通：IGCSE OCR 计算机科学考点精讲

Logic gates are the fundamental building blocks of digital electronics and a core topic in IGCSE OCR Computer Science. These simple circuits take one or more binary inputs and produce a single binary output based on Boolean functions. Mastering their symbols, truth tables, expressions and combinations is essential for both the theory paper and for understanding how computers process data at the hardware level.

逻辑门是数字电子的基本构建块，也是IGCSE OCR计算机科学的核心主题。这些简单电路接收一个或多个二进制输入，并基于布尔函数产生单个二进制输出。掌握它们的符号、真值表、表达式以及组合方法，对于理论考试以及理解计算机如何在硬件层面处理数据至关重要。

1. What Are Logic Gates? | 什么是逻辑门？

Logic gates are the simplest form of digital circuit, operating on binary signals represented by 0 (FALSE, low voltage) and 1 (TRUE, high voltage). They obey the rules of Boolean algebra and are used to build everything from basic memory cells to complex arithmetic logic units. The IGCSE OCR specification requires you to know six core gates: AND, OR, NOT, NAND, NOR, and XOR, plus occasionally the XNOR gate.

逻辑门是最简单的数字电路形式，对由0（假，低电压）和1（真，高电压）表示的二进制信号进行运算。它们遵循布尔代数规则，用于构建从基本存储单元到复杂算术逻辑单元的一切。IGCSE OCR考纲要求你掌握六种核心门：与门、或门、非门、与非门、或非门和异或门，偶尔也会涉及同或门。

Every gate has a unique symbol, a truth table that lists all possible input combinations and the corresponding output, and a Boolean expression that describes its logic function algebraically. Being able to move confidently between these three representations is a key exam skill.

每个门都有一个独特的符号、一个列出所有可能输入组合及对应输出的真值表，以及一个用代数方式描述其逻辑功能的布尔表达式。能够自信地在三者之间转换是一项重要的考试技能。

2. Basic Gates: AND, OR, NOT | 基本门：与门、或门、非门

The AND gate gives a high output (1) only if all its inputs are high. For a 2-input AND, the Boolean expression is Y = A·B, read as ‘A AND B’. Its symbol is a D-shaped block with two inputs on the left and the output on the right.

与门仅在所有输入都为高电平时才给出高电平输出（1）。对于2输入与门，布尔表达式为Y = A·B，读作“A与B”。其符号是一个D形块，左侧有两个输入，右侧是输出。

The OR gate outputs 1 if at least one input is 1. Its expression is Y = A+B (A OR B). The symbol is a curved shape, often called a shield, with the inputs merging into the output. Notice that the plus sign here means logical OR, not arithmetic addition.

或门只要至少有一个输入为1就输出1。其表达式为Y = A+B（A或B）。符号是一个曲线形状，常称为盾牌，输入汇合到输出。注意这里的加号表示逻辑或，而不是算术加法。

The NOT gate, also called an inverter, has only one input. It flips the logic level: an input of 1 gives an output of 0, and vice versa. The expression is Y = A (NOT A). The symbol is a triangle pointing right with a small bubble at the output, indicating inversion.

非门，也称为反相器，只有一个输入。它翻转逻辑电平：输入1产生输出0，反之亦然。表达式为Y = A（非A）。符号是一个指向右的三角形，输出端有一个小气泡，表示反相。

The truth tables below summarise the behaviour of these three fundamental gates in one view.

下面的真值表将这三个基本门的行为总结在一个视图中。

Remember that the NOT output depends only on A; the B column is irrelevant for NOT but is shown for completeness.

请记住，非输出仅取决于A；B列对非门无关，但为了完整性而显示。

3. Universal Gates: NAND and NOR | 通用门：与非门和或非门

The NAND gate is the logical complement of AND: its output is 0 only when both inputs are 1; otherwise the output is 1. The Boolean expression is Y = A·B, which reads as ‘A NAND B’. The symbol is an AND gate with a small bubble on the output.

与非门是与门的逻辑补集：仅当两个输入均为1时输出为0；否则输出为1。布尔表达式为Y = A·B，读作“A与非B”。符号是一个与门，输出端带有一个小气泡。

The NOR gate is the complement of OR: it outputs 1 only when all inputs are 0. Its expression is Y = A+B (A NOR B). The symbol is an OR gate with a bubble on the output.

或非门是或门的补集：仅当所有输入都为0时才输出1。其表达式为Y = A+B（A或非B）。符号是一个或门，输出端带有气泡。

NAND and NOR are called universal gates because any Boolean function can be constructed entirely from NAND gates alone, or entirely from NOR gates alone. This property is extremely useful in chip design, where keeping a single gate type simplifies manufacturing.

与非门和或非门被称为通用门，因为任何布尔函数都可以仅用与非门或仅用或非门来完全构建。这一特性在芯片设计中极为有用，保持单一门类型可以简化制造。

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