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
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
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, flipflops,
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
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.
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.
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
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.
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.
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
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.
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.
A microcontroller is a microprocessor optimised to be used to control
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
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
Finally it was decided that in addition to the relatively slow main
memory, put a small amount of ultrafast
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.
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
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
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 prefetches
that data and places it in the cache so that it is available immediately
when the need arises.
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
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.
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
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.
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
There are several other classes of registers:
are used to store integer numbers.
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.
hold read-only values (e.g zero or one).
hold data for Single Instruction Multiple Data (SIMD) instructions.
Special Purpose registers
which store internal CPU data like the stack pointer or processor status
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
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.
That was the structure, now let’s talk about the language of a
The set of machine instructions that a microprocessor recognizes and can
execute – the only language
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
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
ALGOL Object Code
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
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(1965) is an empirical observation stating in effect that at our rate of
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 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
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
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,
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