Chapter 4

I/O Subsystem Organization and Interfacing

Cs 147

Peter Nguyen

General Information

Computer generally consists of three subsystemsCommunication between subsystems by busesThe processors sends out the address to be accessed in memory or the address of the I/O device via the address busData bus carries data between the subsystems Information sent on the control bus coordinates all data transfers

The processor goes through an instruction cycle

Fetch an instruction from memory, and decode the instruction, and execute the instruction

All instructions are fetched and decoded in the same way, but the execute cycle is different for every instruction

Three Sections of the CPU

The register section, used for data storage

The arithmetic/logic unit, performs computations on data with the CPU

The control unit outputs signals to control the rest of the processor

Types of Memory Chips

ROM, PROM, EPROM, and EEPROM are all read only memory chips (data does not change)

SRAM and DRAM, these are random access memory (data change or lost when the computer is turned off)

Two types of I/O access

Memory mapped I/O

Treats I/O device as if it were a memory location

The same instructions are used to access both memory and I/O devices, but memory and the I/O device cannot both use the same address

Isolated I/O

Different instructions are used to access I/O devices and memoryRequires an additional control line to distinguish between the two, but this allows memory and I/O devices to both use the same address

Presentation Outline : Part I

Input Module

Output Module

Bi-directional Module

An overall view within the organization of a simple computer

-Definition -

_________________________

Memory is defined as homogeneous

From the CPU’s perspective, each location is read from and written to in

exactly the same way

What’s the correlation ?

They are quite different but the comparisons is needed in order to

understand the structure and organization of the computer

Memory vs. I/O

Every memory has a unique address

Each memory location is read from and written to in exactly the same way, performs the same function – that is, its stores a data value or an instruction for use by the CPU

Every I/O device has a unique address

Perform vastly different functions; even though, they are part of the same subsystem

The major components of the Input Module

Input device (keyboard, mouse, …)

Tri-state buffers

Data bus

Enable logic (Address bus and Control Bus)

An input device: (a) with its interface and (b) the enable logic for the tri-state buffers

The tri-state buffers are used in input device interfaces to make sure that no more than one device writes data to the bus at any time.

What is Enable Logic ?

Enable logic is the key to this design.

Because of the unique address, enable logic must not enable the buffers unless it receives the correct address from the address bus. It must also get the

correct control signals from the control bus.

Components of the Output Module

Output device (monitor)

Register

Data bus

Enable Logic (Address bus and Control bus)

Output Device

___

The output devices read data from the bus, rather that write data to it, they do not need the buffers.

The data can be made available to all output devices; only the device with the correct address

will read it in.

Load Logic

In the Output Module – the load logic plays the role of the enable logic in the input device interface. When this logic

receives the correct address and control signals, it asserts the LD signal of the register, causing it to read data from the

system’s data bus.

__

By doing so, the output device can then read the data from the register at its leisure while the CPU

performs other tasks.

Components of the bi-directional module

I/O device (hard disk drive)

Two interfaces (one input, tri-state and one output, register)

Logic elements (gates to check address on the address bus)

Bi-directional module

I/O devices are much slower than CPUs and memory

Timing problems exist when interacting with the CPU

How does the CPU work around this processes ?

Ready

Normal inputs are high … when the CPU outputs the address of the I/O device and the correct control signals,

enabling the tri-state buffers of the I/O device interface, the I/O device sets READY low.

The CPU reads this signal and continues to output the same address and control signals

… which cause the buffers to remain enabled.

By having the extra clock cycles generated when READY is set low

This is called wait states

Ready is good for small processes

Many systems actually use the interrupts

For larger computers

Interrupts

Direct memory access, DMA

Interrupts

Performs useful work while waiting for the much slower I/O devices

Direct memory access (DMA)

… method used to bypass the CPU in the transferring process

Building a Simple Computer

CPU

8K of ROM, starting with address 0

8K of RAM

Memory mapped

Bidirectional I/O with a port address of 80000H

Read and Write control lines

Central Processing Unit (CPU)

Relatively Simple

CPU

16-bit addresses pin

Systems data bus accesses

Read control lines

Write control lines

The memory subsystem

Memory Subsystem

0000 0000 0000 00000001 1111 1111 1111

To access the memory chip, the processor must supply an address used by the chip, as well as the

proper control signals

Memory

ROM

RAM

Address 000

Address 001

Combinatorial logic is used to realize these functions and set the chip enable signals of the memory chips

ROM chip will output data only when its output enable and chip enable signals are asserted

RAM has two control inputs , RD and WR. Both the read and write signals from the control bus can

drive these two signals

I/O subsystem

Register Tri-state buffers

I/O Device

Freehand drawing of a computer organization

Historical Perspective

Please see attached

Questions ?