Why address bus is 20 bit?

A 20-bit address bus can address 2^20 distinct memory locations, which equals 1,048,576 (or 1 megabyte) of memory. This allows the system to access a sufficient amount of RAM and memory-mapped I/O devices. The choice of a 20-bit address bus is often a design decision to balance performance and cost, enabling support for a wide range of applications without requiring larger, more expensive memory configurations.