A20总线，是x86体系的扩充电子线路之一。A20总线是专门用来转换地址总线的第二十一位。

当IBM设计IBM PC AT机器时，他们决定采用性能更好但是在实模式下并不完全兼容以前的Intel 8088与Intel 8086系列的Intel 80286微处理器。以前的x86体系微处理器并没有从A20到A23的总线。80286微处理器能够寻址到16MB的系统内存。

许多实模式进程仍然利用这一点，因此使不改变微处理器的段寄存器而去访问最开始的64KB内存成为一个通用的技巧。为了和这些进程保持兼容性，IBM自己在主板上去修复这个问题。在微处理器与系统总线间插入一个逻辑门完成了这个修复。这个逻辑门也因此被命名为A20总线。A20总线能被软件关闭或打开，以此来阻止或允许地址总线收到A20传来的信号。在引导系统时，BIOS先打开A20总线来统计和测试所有的系统内存。而当BIOS准备将计算机的控制权交给操作系统时会先将A20总线关闭。一开始，这个逻辑门连接到Intel 8042的键盘控制器。控制它是相对较慢。

The A20 or addressing line 20 is one of the electrical lines that make up the system bus of an x86-based computer system. The A20 line in particular is used to transmit the 21st bit on the address bus.

A microprocessor will typically have a number of addressing lines equal to the base-two logarithm of its physical addressing space. For example, a processor with 4 gigabytes of physical addressing space requires 32 lines, which are named A0 through A31. The lines are named after the zero-based number of the bit in the address they are transmitting, where the least significant bit is first, therefore numbered bit 0, and signaled on line A0. A20 transmits bit 20 (the 21st bit) and will become active once addresses reach 1 megabyte or 2.