Intel 64 and IA-32 Architectures Software Developers Manual Volume 1, Basic Architecture

Vol. 1 3-33
BASIC EXECUTION ENVIRONMENT
combination of these components based on the language construct a programmer
defines.
3.7.7 I/O Port Addressing
The processor supports an I/O address space that contains up to 65,536 8-bit I/O
ports. Ports that are 16-bit and 32-bit may also be defined in the I/O address space.
An I/O port can be addressed with either an immediate operand or a value in the DX
register. See Chapter 13, “Input/Output,” for more information about I/O port
addressing.