The address bus is the set of wire traces that is used to identify which address in memory the CPU is accessing.

The number of wire traces in the address bus limits the maximum amount of RAM which the CPU can address.

CPU Address Bus Sizes

CPU Address Bus Size
8086 20 bit
8088 20 bit
80286 24 bit
80386SX 24 bit
80386DX 32 bit
80486SX 32 bit
80486DX 32 bit
Pentium I 32 bit
K6 32 bit
Duron 32 bit
Athlon 32 bit
Athlon XP 32 bit
Celeron 36 bit
Pentium Pro 36 bit
Pentium II 36 bit
Pentium III 36 bit
Pentium 4 36 bit
Athlon 40 bit
Athlon-64 40 bit
Athlon-64 FX 40 bit
Opteron 40 bit
Itanium 44 bit
Itanium 2 44 bit

Address Bus Memory Capacity Chart

Address Bus Size Maximum RAM
20 bits 1MB
24 bits 16MB
32 bits 4GB
36 bits 64GB
40 bits 1TB
44 bits 16TB