10-1
CHAPTER 10
INPUT/OUTPUT
In addition to transferring data to and from external memory, Intel Architecture (IA) processors
can also transfer data to and from input/output ports (I/O ports). I/O ports are created in system
hardware by circuity that decodes the control, data, and address pins on the processor. These I/O
ports are then configured to communicate with peripheral devices. An I/O port can be an input
port, an output port, or a bidirectional port. Some I/O ports are used for transmitting data, such
as to and from the transmit and receive registers, respectively, of a serial interface device. Other
I/O ports are used to control peripheral devices, such as the control registers of a disk controller.
This chapter describes the processors I/O architecture. The topics discussed include:
I/O port addressing.
I/O instructions.
I/O protection mechanism.
10.1.I/O PORT ADDRESSING
The processor allows I/O ports to be accessed in either of two ways:
Through a separate I/O address space.
Through memory-mapped I/O.
Accessing I/O ports through the I/O address space is handled through a set of I/O instructions
and a special I/O protection mechanism. Accessing I/O ports through memory-mapped I/O is
handled with the processors general-purpose move and string instructions, with protection
provided through segmentation or paging. I/O ports can be mapped so that they appear in the
I/O address space or the physical-memory address space (memory mapped I/O) or both.
One benefit of using the I/O address space is that writes to I/O ports are guaranteed to be
completed before the next instruction in the instruction stream is executed. Thus, I/O writes to
control system hardware cause the hardware to be set to its new state before any other instruc-
tions are executed. Refer to Section 10.6. for more information on serializing of I/O operations.
10.2.I/O PORT HARDWARE
From a hardware point of view, I/O addressing is handled through the processors address lines.
For Pentium
®
Pro, Pentium II, and Pentium
®
III processors, a special memory-I/O transaction
on the system bus indicates whether the address lines are being driven with a memory address
or an I/O address; for Pentium and earlier IA processors, the M/IO pin indicates a memory
address (1) or an I/O address (0). When the separate I/O address space is selected, it is the
responsibility of the hardware to decode the memory-I/O bus transaction to select I/O ports
rather than memory.
Data is transmitted between the processor and an I/O device through the data lines.
