I/O Port Addressing - Intel 80C188EC User Manual

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OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE

Destination EA

Figure 2-21. String Operand

I/O Port Addressing

Any memory operand addressing modes can be used to access an I/O port if the port is memory-

mapped. String instructions can also be used to transfer data to memory-mapped ports with an

appropriate hardware interface.

Two addressing modes can be used to access ports located in the I/O space (see Figure 2-22). For

direct I/O port addressing, the port number is an 8-bit immediate operand. This allows fixed ac-

cess to ports numbered 0 to 255. Indirect I/O port addressing is similar to register indirect address-

ing of memory operands. The DX register contains the port number, which can range from 0 to

65,535. Adjusting the contents of the DX register allows one instruction to access any port in the

I/O space. A group of adjacent ports can be accessed using a simple software loop that adjusts the

value of the DX register.

Port Address

Indirect Port

Figure 2-22. I/O Port Addressing