IO.pdf

The I/O Subsystem

Chapter Seven

7.1

Chapter Overview

typical program does three basic activities: input, computation, and output. In this section we will discuss

the other two activities beyond computation: input and output or I/O.

This chapter concentrates on low-level

CPU I/O rather than high level file or character I/O.

This chapter discusses how the CPU transfers bytes of data

to and from the outside world.

This chapter discusses the mechanisms and performance issues behind the I/O.

7.2

Connecting a CPU to the Outside World

Most I/O devices interface to the CPU in a fashion quite similar to memory

. Indeed, many devices appear to

the CPU as though they were memory devices.

o output data to the outside world the CPU simply stores data

into a "memory" location and the data magically appears on some connectors external to the computer

. Simi

larly

, to input data from some external device, the CPU simply transfers data from a "memory" location into the

CPU; this "memory" location holds the value found on the pins of some external connector

An output port

is a device that looks like a memory cell to the computer but contains connections to the out

side world.

An I/O port typically uses a latch rather than a flip-flop to implement the memory cell.

When the

CPU writes to the address associated with the latch, the latch device captures the data and makes it available on a

set of wires external to the CPU and memory system (see

Figure 7.1

). Note that output ports can be write-only

or read/write.

The port in

Figure 7.1

, for example, is a write-only port. Since the outputs on the latch do not loop

back to the CPU

s data bus, the CPU cannot read the data the latch contains. Both the address decode and write

control lines must be active for the latch to operate; when reading from the latch

s address the decode line is

active, but the write control line is not.

CPU write control line

Address decode line

Data Bus from CPU

Data

Data to outside world

Figure 7.1

A Typical Output Port

Figure 7.2

shows how to create a

read/write input/output port.

The data written to the output port loops back

to a transparent latch.

Whenever the CPU reads the decoded address the read and decode lines are active and this

activates the lower latch.

This places the data previously written to the output port on the CPU

s data bus, allow

ing the CPU to read that data.

read-only (input) port is simply the lower half of

Figure 7.2

; the system ignores

any data written to an input port.

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CPU write control line

Address decode line

Data Bus from CPU

Data

CPU read control line

Data to outside world

Address decode line

Data Bus to CPU

Data

Figure 7.2

An Output Port that Supports Read/Write Access

Note that the port in

Figure 7.2

is not an input port.

Although the CPU can read this data, this port or

ganiza

tion simply lets the CPU read the data it previously wrote to the port.

The data appearing on an external connec

tor is an output port (only). One could create a (read-only) input port by using the lower half of the circuit in

Figure 7.2

The input to the latch would appear on the CPU

s data bus whenever the CPU reads the latch data.

perfect example of an output port is a parallel printer port.

The CPU typically writes an

ASCII character to

a byte-wide output port that connects to the DB-25F connector on the back of the computer

s case.

cable trans

mits this data to the printer where an input port (to the printer) receives the data.

processor inside the printer

typically converts this

ASCII character to a sequence of dots it prints on the paper

Generally

, a given peripheral device will use more than a single I/O port.

typical PC parallel printer inter

face, for example, uses three ports: a read/write port, an input port, and an output port.

The read/write port is the

data port (it is read/write to allow the CPU to read the last

ASCII character it wrote to the printer port).

The input

port returns control signals from the printer; these signals indicate whether the printer is ready to accept another

character

, is of

f-line, is out of paper

, etc.

The output port transmits control information to the printer such as

whether data is available to print.

The first thing to learn about the input/output subsystem is that I/O in a typical computer system is radically

dif

ferent than I/O in a typical high level programming language. In a real computer system you will rarely find

machine instructions that behave like

writeln

cout

printf

, or even the HLA

stdin

and

stdout

statements. In fact,

most input/output instructions behave exactly like the 80x86

s MOV

instruction.

o send data to an output

device, the CPU simply moves that data to a special memory location.

o read data from an input device, the

CPU simply moves data from the address of that device into the CPU. Other than there are usually more wait

states associated with a typical peripheral device than actual memory

, the input or output operation looks very

similar to a memory read or write operation.

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328

7.3

Read-Only, Write-Only, Read/Write, and Dual I/O Ports

e can classify input/output ports into four categories based on the CPU

s ability to read and write data at a

given port address.

These four categories are read-only ports, write-only ports, read/write ports, and dual I/O

ports.

read-only port is (obviously) an input port. If the CPU can only read the data from the port, then that port

is providing data appearing on lines external to the CPU.

The system typically ignores any attempt to write data

to a read-only port

good example of a read-only port is the status port on a PC

s parallel printer interface.

Reading data from this port lets you test the current condition of the printer

The system ignores any data written

to this port.

A write-only port is always an output port. Writing data to such a port presents the data for use by an external

device. Attempting to read data from a write-only port generally returns garbage (i.e., whatever values that just

happen to be on the data bus at that time). You generally cannot depend on the meaning of any value read from a

write-only port.

A read/write port is an output port as far as the outside world is concerned. However, the CPU can read as

well as write data to such a port. Whenever the CPU reads data from a read/write port, it reads the data that was

last written to the port. Reading the port does not affect the data the external peripheral device sees, reading the

port is a simple convenience for the programmer so that s/he doesn t have to save the value last written to the

port should they want to retrieve the value.

A dual I/O port is also a read/write port, but reading the port reads data from some external device while writ-

ing data to the port transmits data to a different external device. Figure 7.3 shows how you could interface such

a device to the system. Note that the input and output ports are actually a read-only and a write-only port that

share the same address. Reading the address accesses one port while writing to the address accesses the other

port. Essentially, this port arrangement uses the R/W control line(s) as an extra address bit when selecting these

ports.

1. Note, however, that some devices may fail if you attempt to write to their corresponding input ports, so it s never a good

idea to write data to a read-only port.

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CPU write control line

Address decode line

Data to the

outside world

Data Bus from CPU

Data

Data Bus

CPU read control line

Address decode line

Data from the

outside world

Data Bus to CPU

Data

Figure 7.3

An Input and an Output Device That Share the Same Address (a Dual I/O Port)

These examples may leave you with the impression that the CPU always reads and writes data to peripheral

devices using data on the data bus (that is, whatever data the CPU places on the data bus when it writes to an out-

put port is the data actually written to that output port). While this is generally true for input ports (that is, the

CPU transfers input data across the data bus when reading data from the input port), this isn t necessarily true for

output ports. In fact, a very common output mechanism is simply accessing a port. Figure 7.4 provides a very

simple example. In this circuit, an address decoder decodes two separate addresses. Any access (read or write)

to the first address sets the output line high; any read or write of the second address clears the output line. Note

that this circuit ignores the data on the CPU s data lines. It is not important whether the CPU reads or writes data

to these addresses, nor is the data written of any consequence. The only thing that matters is that the CPU access

one of these two addresses.

Address decode line #1

Single bit output

to the outside

world.

Address decode line #2

S/R

Flip-Flop

Figure 7.4

Outputting Data to a Port by Simply Accessing That Port

Another possible way to connect an output port to the CPU is to use a D flip-flop and connect the read/write

status lines to the D input on the flip-flop. Figure 7.5 shows how you could design such a device. In this dia-

gram any read of the selected port sets the output bit to zero while a write to this output port sets the output bit to

one.

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Address decode line #1

Clk

Single bit output

to the outside

world.

Read control line

(active low)

Flip-Flop

Figure 7.5

Outputting Data Using the Read/Write Control as the Data to Output

There are a wide variety of ways you can connect external devices to the CPU. This section only provides a

few examples as a sampling of what is possible. In the real world, there are an amazing number of different ways

that engineers connect external devices to the CPU. Unless otherwise noted, the rest of this chapter will assume

that the CPU reads and writes data to an external device using the data bus. This is not to imply that this is the

only type of I/O that one could use in a given example.

7.4 I/O (Input/Output) Mechanisms

There are three basic forms of input and output that a typical computer system will use: I/O-mapped I/O,

memory-mapped I/O, and direct memory access (DMA). I/O-mapped input/output uses special instructions to

transfer data between the computer system and the outside world; memory-mapped I/O uses special memory

locations in the normal address space of the CPU to communicate with real-world devices; DMA is a special

form of memory-mapped I/O where the peripheral device reads and writes data in memory without going

through the CPU. Each I/O mechanism has its own set of advantages and disadvantages, we will discuss these in

this section.

7.4.1 Memory Mapped Input/Output

A memory mapped peripheral device is connected to the CPU s address and data lines exactly like memory,

so whenever the CPU reads or writes the address associated with the peripheral device, the CPU transfers data to

or from the device. This mechanism has several benefits and only a few disadvantages.

The principle advantage of a memory-mapped I/O subsystem is that the CPU can use any instruction that

accesses memory to transfer data between the CPU and a memory-mapped I/O device. The MOV instruction is

the one most commonly used to send and receive data from a memory-mapped I/O device, but any instruction

that reads or writes data in memory is also legal. For example, if you have an I/O port that is read/write, you can

use the ADD instruction to read the port, add data to the value read, and then write data back to the port.

Of course, this feature is only usable if the port is a read/write port (or the port is readable and you ve speci-

fied the port address as the source operand of your ADD instruction). If the port is read-only or write-only, an

instruction that reads memory, modifies the value, and then writes the modified value back to memory will be of

little use. You should use such read/modify/write instructions only with read/write ports (or dual I/O ports if

such an operation makes sense).

Nevertheless, the fact that you can use any instruction that accesses memory to manipulate port data is often

a big advantage since you can operate on the data with a single instruction rather than first moving the data into

the CPU, manipulating the data, and then writing the data back to the I/O port.

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