Microprocessor, Bus interface unit, Data & instruction cache memory, ALU

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Introduction to Computing CS101

LESSON 7

Goals for Today

Today we want to learn about the microprocessor, the key component, the brain, of a computer

We'll learn about the function of a microprocessor

And its various sub-systems

Bus interface unit

Data & instruction cache memory

Instruction decoder

Arithmetic-Logic unit

Floating-point unit

Control unit

7.1 Microprocessor

A microprocessor (abbreviated as µP or uP) is a computer processor on a microchip. It's sometimes

called a logic chip. A microprocessor is designed to perform arithmetic and logic operations that make

use of small number-holding areas called registers. Typical microprocessor operations include adding,

subtracting, comparing two numbers, and fetching numbers from one area to another. These operations

are the result of a set of instructions that are part of the microprocessor design. When the computer is

turned on, the microprocessor is designed to get the first instruction from the basic input/output system

(BIOS) that comes with the computer as part of its memory. After that, either the BIOS, or the operating

system that BIOS loads into computer memory, or an application program is "driving" the

microprocessor, giving it instructions to perform.

The number of transistors available has a huge effect on the performance of a processor. As seen earlier,

a typical instruction in a processor like an 8088 took 15 clock cycles to execute. Because of the design

of the multiplier, it took approximately 80 cycles just to do one 16-bit multiplication on the 8088. With

more transistors, much more powerful multipliers capable of single-cycle speeds become possible.

A microprocessor is made from miniaturized transistors and other circuit elements on a single

semiconductor integrated circuit (IC) . These are made up oof semiconductor and silicon.

7.2 Integrated Circuits

A chip is also called an (integrated circuit (IC) (aka microchip or just chip). It is a microelectronic

semiconductor device consisting of many interconnected transistors and other components.Generally it

is a small, thin piece of silicon onto which the transistors making up the microprocessor have been

etched.

A chip might be as large as an inch on a side and can contain tens of millions of transistors. Simpler

processors might consist of a few thousand transistors etched onto a chip just a few millimeters square.

Integrated circuits can be classified into analog, digital and mixed signal (both analog and digital on the

same chip). Digital integrated circuits can contain anything from one to millions of logic gates, flip-

flops, multiplexers, etc. in a few square millimeters. The small size of these circuits allows high speed,

low power dissipation, and reduced manufacturing cost compared with board-level integration.

The growth of complexity of integrated circuits follows a trend called "Moore's Law", it states that the

number of transistors in an integrated circuit doubles every two years. Integrated circuits can be

classified into analog, digital and mixed signal (both analog and digital on the same chip). Digital

integrated circuits can contain anything from one to millions of logic gates, flip-flops, multiplexers, etc.

in a few square millimeters. The small size of these circuits allows high speed, low power dissipation,

and reduced manufacturing cost compared with board-level integration.

Introduction to Computing CS101

7.3 Devices

7.3.1 Transistors

The transistor is a solid state semiconductor device used for amplification and switching, and has three

terminals. A small current or voltage applied to one terminal controls the current through the other two,

hence the term transistor; a voltage- or current-controlled resistor. It is the key component in all modern

electronics. In digital circuits, transistors are used as very fast electrical switches, and arrangements of

transistors can function as logic gates, RAM-type memory and other devices. In analog circuits,

transistors are essentially used as amplifiers.

7.3.2 Diodes

A diode functions as the electronic version of a one-way valve. By restricting the direction of

movement of charge carriers, it allows an electric current to flow in one direction, but blocks it in the

opposite direction.

A diode's current-voltage, or I-V, characteristic can be approximated by two regions of operation.

Below a certain difference in potential between the two leads, the diode can be thought of as an open

(non-conductive) circuit. As the potential difference is increased, at some stage the diode will become

conductive and allow current to flow, at which point it can be thought of as a connection with zero (or at

least very low) resistance. In a typical semiconductor p-n diode, conventional current can flow from the

p-doped side to the n-doped side, but not in the opposite direction. When the diode is reverse-biased, the

charge carriers are pulled away from the center of the device, creating a depletion region. More

specifically, the transfer function is logarithmic, but so sharp that it looks like a corner.

7.3.3 Resistors

A resistor is an electrical component designed to have an electrical resistance that is independent of the

current flowing through it. The common type of resistor is also designed to be independent of

temperature and other factors. Resistors may be fixed or variable. Variable resistors are also called

potentiometers or rheostats

A few resistor types

Some resistors are long and thin, with the actual resisting material in the centre, and a conducting metal

leg on each end. This is called an axial package.

Resistors used in computers and other devices are typically much smaller, often in surface-mount

(Surface-mount technology) packages without leads.

Larger power resistors come in more sturdy packages designed to dissipate heat efficiently, but they are

all basically the same structure. Resistors are used as part of electrical networks and incorporated into

microelectronic semiconductor devices. The critical measurement of a resistor is its resistance, which

serves as a ratio of voltage to current and is measured in ohms, an SI unit. Any physical object is a kind

of resistor. Most metals are conductors, and have low resistance to the flow of electricity. The human

body, a piece of plastic, or even a vacuum has a resistance that can be measured. Materials that have

very high resistance are called insulators.

7.3.4 Capacitors

A capacitor (historically known as a "condenser") is a device that stores energy in an electric field, by

accumulating an internal imbalance of electric charge. An ideal capacitor can store electronic energy

when disconnected from its charging circuit, so it can be used like a fast battery. In AC or signal circuits

it induces a phase difference of 90 degrees, current leading potential.

They are connected in parallel with the power circuits of most electronic devices and larger systems

(such as factories) to shunt away and conceal current fluctuations from the primary power source to

Introduction to Computing CS101

provide a "clean" power supply for signal or control circuits. The effect of such capacitors can be

thought of in two different ways. One way of thinking about it is that the capacitors act as a local

reserve for the DC power source, to smooth out fluctuations by charging and discharging each cycle.

The other way to think about it is that the capacitor and resistance of the power supply circuitry acts as a

filter and removes high frequencies, leaving only DC.

Wires

And are made of the following materials

Silicon - semiconductor

Copper - conductor

Silicon Dioxide insulator

7.4 Microprocessor system

Microprocessors are powerful pieces of hardware, but not much useful on their own. They do not have

the sense of their own. Like the human sample it needs some instructions inputs and outputs to process

some task. As per instruction given to the microprocessor.

A microprocessor system is microprocessor plus all the components it requires to do a certain task.

Shortly, a microprocessor needs help of some components to make up the task to fulfill. These

components are input, output, storage, and memory. All these components and microprocessor make up

a microprocessor system.

Personal Computer is an example of microprocessor System. Another example is the microcontroller.

7.5 Micro-controllers

A microcontroller is a microprocessor optimised to be used to control electronic equipment.

Microcontrollers represent the vast majority of all computer chips sold, over 50% are "simple"

controllers, and another 20% are more specialized decipline processors. While you may have one or two

general-purpose microprocessors in your house (you're using one to read this), you likely have

somewhere between one and two dozen microcontrollers. They can be found in almost any electrical

device, washing machines, microwave ovens, telephones etc.

A microcontroller includes CPU, memory for the program (ROM), memory for data (RAM), I/O lines to

communicate with peripherals and complementary resources, all this in a closed chip. A microcontroller

differs from a standalone CPU, because the first one generally is quite easy to make into a working

computer, with a minimum of external support chips. The idea is that the microcontroller will be placed

in the device to control, hooked up to power and any information it needs, and that's that.

7.5 The Main Memory Bottleneck

Modern super-fast microprocessors can process a huge amount of data in a short duration. They need

data to be processed at the same speed. Other wise they have to sit idle and wait for the input/data,

because speed of input is rather small then processing of data. They require quick access to data to

maximize their performance. If they don't receive the data that they require, they literally stop and wait,

this results in reduced performance and wasted power.

Current microprocessors can process an instruction in about ns (nanosecond). Time required for

fetching data from main memory (RAM) is of the order of 100 ns

Introduction to Computing CS101

Solution to the Bottleneck Problem

In order to eliminate the solution it was suggested to make the main memory faster. But that evolved a

problem that the 1-ns memory is extremely expensive as compared the currently popular 100-ns

memory.

Finally it was decided that in addition to the relatively slow main memory, put a small amount of ultra-

fast RAM right next to the microprocessor on the same chip and make sure that frequently used data and

instructions resides in that ultra-fast memory

It increases the performance. It supports better over performance due to fast access to frequently used

data and instructions.

7.7 Cache

A cache is a collection of duplicate data, where the original data is expensive to fetch or compute

(usually in terms of access time) relative to the cache. Future accesses to the data can be made by

accessing the cached copy rather than refetching or recomputing the original data, so that the perceived

average access time is lower. Caches may mark the cached data as 'stale' when the original data is

changed, but this is not always the case.

On-Chip Cache Memory (1)

That small amount of memory located on the same chip as the microprocessor is called On-Chip Cache

Memory.

The microprocessor stores a copy of frequently used data and instructions in its cache memory. When

the microprocessor desires to look at a piece of data, it checks in the cache first. If it is not there, only

then the microprocessor asks for the same from the main memory

On-Chip Cache Memory (2)

L2, cache memory, which is on a separate chip from the microprocessor but faster to access than regular

RAM.

It is the small size and proximity to the microprocessor makes access times short, resulting in a boost in

performance. Microprocessors predict what data will be required for future calculations and it pre-

fetches that data and places it in the cache so that it is available immediately when the need arises.

7.8 Microprocessors Building Blocks

Microprocessor

Data

Cache

Memory

Bus

Arithmetic

Control

RAM

& Logic

Bus

Unit

Unit

Interface

Unit

I/O

Instruction

Registers

Decoder

System

Floating

Bus

Point

Unit

Instruction

Cache

Registers

Introduction to Computing CS101

Bus Interface Unit

The bus interface unit is the part of the processor that interfaces with the rest of the PC. Its name comes

from the fact that it deals with moving information over the processor data bus, the primary conduit for

the transfer of information to and from the CPU. The bus interface unit is responsible for responding to

all signals that go to the processor, and generating all signals that go from the processor to other parts of

the system.

It receives instructions & data from main memory to be processed and operations. After the operations

are processed it then sends back the information (processed data) to the cache. It also receives the

processed data to send it to the main memory.

Instruction Decoder

The instruction decoder of a processor is a combinatorial circuit sometimes in the form of a read-only

memory, sometimes in the form of an ordinary combinatorial circuit. Its purpose is to translate an

instruction code into the address in the micro memory where the micro code for the instruction starts.

A decoder is a device which is the reverse, undoing the encoding so that the original information can be

retrieved. The same method used to encode is usually just reversed in order to decode.This unit receives

the programming instructions and decodes them into a form that is understandable by the processing

units, i.e. The ALU or FPU Then, it passes on the decoded instruction to the ALU or FPUs as desired.

Arithmetic & Logic Unit (ALU)

An arithmetic and logical unit (ALU) also known as "Integer Unit" is one of the core components of

all central processing units. It is capable of calculating the results of a wide variety of common

computations. The most common available operations are the integer arithmetic operations of addition,

subtraction, and multiplication, the bitwise logic operations of AND, NOT, OR, and XOR, and various

shift operations.

The ALU takes as inputs the data to be operated on and a code from the control unit indicating which

operation to perform, and for output provides the result of the computation. In some designs it may also

take as input and output a set of condition codes, which can be used to indicate cases such as carry-in or

carry-out, overflow, or other statuses.

The new breed of popular microprocessors have not one but two almost identical ALU's that can do

calculations simultaneously, doubling the capability

Floating-Point Unit (FPU)

A floating point unit (FPU) is a part of a CPU specially designed to carry out operations on floating

point numbers. Typical operations are floating point arithmetic (such as addition and multiplication), but

some systems may be capable of performing exponential or trigonometric calculations as well (such as

square roots or cosines).

Not all CPUs have a dedicated FPU. In the absence of an FPU, the CPU may use a microcode program

to emulate an FPUs function using an arithmetic and logical unit (ALU), which saves the added

hardware cost of an FPU but is significantly slower.

In some computer architectures, floating point operations are handled completely separate from integer

operations, with dedicated floating point registers and independent clocking schemes. Floating point

addition and multiplication operations are typically pipelined, but more complicated operations, like

division, may not be, and some systems may even have a dedicated floating point divider circuit.

Introduction to Computing CS101

Registers

A register is a device for storing data. It is a small amount of very fast computer memory used to speed

the execution of computer programs by providing quick access to commonly used values. These

registers are the top of the memory hierarchy, and are the fastest way for the system to manipulate data.

It is common to measure registers by the number of bits it can hold, for example, an "8-bit register" or

"32-bit register". Registers are now usually implemented as an array of SRAMs, but they have also been

implemented using individual flip flops, high speed core memory, thin film memory, and other ways in

various machines.

There are several other classes of registers:

Data registers are used to store integer numbers.

Address registers hold memory addresses and are used to access memory.

General Purpose registers can store both data and addresses.

Floating Point registers are used to store floating point numbers.

Constant registers hold read-only values (e.g zero or one).

Vector registers hold data for Single Instruction Multiple Data (SIMD) instructions.

Special Purpose registers which store internal CPU data like the stack pointer or processor status

words.

The ALU & FPU store intermediate and final results from their calculations in these registers. Then the

processed data goes back to the data cache and then to main memory from these registers.

Control Unit

A control unit is the part of a CPU or other device that directs its operation. The outputs of the unit

control the activity of the rest of the device. A control unit can be thought of as a finite state machine. It

is called the brain of computer microprcessor. It manages whole process of the microprocessor. For it

identifes which data is sent to the ALU or memory etc.

At one time control units for CPUs were ad-hoc logic, and they were difficult to design. Now they are

often implemented as a microprogram that is stored in a control store.

Microprocessor

Data

Cache

Memory

Bus

Arithmetic

RAM

Control

& Logic

Bus

Unit

Unit

Interface

Unit

Instruction

I/O

Registers

Decoder

System

Floating

Bus

Point

Unit

Instruction

Registers

Cache

That was the structure, now let's talk about the language of a microprocessor

Instruction Set

The set of machine instructions that a microprocessor recognizes and can execute the only language

microprocessor knows

Introduction to Computing CS101

An instruction set includes low-level, a single step-at-a-time instructions, such as add, subtract,

multiply, and divide

Each microprocessor family has its unique instruction set

Bigger instruction-sets mean more complex chips (higher costs, reduced efficiency), but shorter

programs

An instruction set, or instruction set architecture (ISA), is a specification detailing the commands that a

computer's CPU should be able to understand and execute, or the set of all commands implemented by a

particular CPU design. The term describes the aspects of a computer or microprocessor typically visible

to a programmer, including the native datatypes, instructions, registers, memory architecture, interrupt

and fault system, and external I/O (if any). "Instruction set architecture" is sometimes used to

distinguish this set of characteristics from the Micro-Architecture, which are the elements and

techniques used to implement the ISA, e.g. microcode, pipelining, cache systems, etc. Bigger

instruction-sets mean more complex chips (higher costs, reduced efficiency), but shorter programs. Each

microprocessor family has its unique instruction set. Following are the few ISA;

MIPS

Motorola 6800

ARM

PowerPC

x86 (Pentium)

ALGOL Object Code

SPARC

7.9The 1st microprocessor : Intel 4004

The first microprocessor was the Intel 4004, introduced in 1971. The 4004 was not very powerful all it

could do was add and subtract, and it could only do that 4 bits at a time. But it was amazing that

everything was on one chip. Prior to the 4004, engineers built computers either from collections of chips

or from discrete components (transistors wired one at a time). The 4004 powered one of the first

portable electronic calculators. It was as powerful as ENIAC which had 18000 tubes and occupied a

large room. It cost less then $100. Its targeted use was of calculation. It consisted of 2250 transistors and

16pins. Speed was 108 kHz, 60,000 ops/sec.

Why Intel came up with the idea?

A Japanese calculator manufacturer, Busicom wanted Intel to develop 16 separate IC's for a line of new

calculators. Intel, at that point in time known only as a memory manufacturer, was quite small and did

not have the resources to do all 16 chips. Then Ted Hoff came up with the idea of doing all 16 on a

single chip. Later, Intel realized that the 4004 could have other uses as well.

Currently Intel came with Intel Pentium 4 (2.2GHz).

It was introduced in December 2001. It got 55 million transistors. 32-bit word size. Within the processor

it has 2 ALU's each working at 4.4GHz. It costs around $600.

Moore's Law

Moore's law(1965) is an empirical observation stating in effect that at our rate of technological

development and advances in the semiconductor industry the complexity of integrated circuits doubles

every 18 months. His original empirical observation was that the number of components on

semiconductor chips with lowest per-component cost doubles roughly every 12 months, and he

conjectured that the trend will stay for at least 10 years. In 1975, Moore revised his estimate for the

expected doubling time, arguing that it was slowing down to about two years

Introduction to Computing CS101

Evolution of Intel Microprocessors

4-, 8-, 16-, 32-, 64-bit (Word Length)

The 4004 dealt with data in chunks of 4-bits at a time

Pentium 4 deals with data in chunks (words) of 32-bit length

The new Itanium processor deals with 64-bit chunks (words) at a time

kHz, MHz, GHz (Clock Frequency)

4004 worked at a clock frequency of 108kHz

The latest processors have clock freqs. in GHz

Out of 2 microprocessors having similar designs, one with higher clock frequency will be more

powerful

Same is not true for 2 microprocessors of dissimilar designs. Example: Out of PowerPC & Pentium 4

microprocessors working at the same freq, the former performs better due to superior design. Same for

the Athlon microprocessor when compared with a Pentium

Enhancing the capability of a microprocessor ?

The computing capability of a microprocessor can be enhanced in many different ways:

By increasing the clock frequency

By increasing the word-width

By having a more effective caching algorithm and the right cache size

By adding more functional units (e.g. ALU's, FPU's, Vector/SIMD units, etc.)

Improving the architecture

What have we learnt today?

Today we learnt about the microprocessor, the key component, the brain, of a computer

We learnt about the function of a microprocessor

And its various sub-systems

Bus interface unit

Data & instruction cache memory

Instruction decoder

ALU

Floating-point unit

Control unit

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