Computer Systems Architecture

Computer

A computer is an electronic device that can accept input, process it according to instructions, store the instructions and the result and output information.

Data

Data refers to information/facts that is processed or stored by a computer to provide information.

Hardware

The physical components of a computer system, allowing input and output

Software

Programs that tell the computer what to do

5 Basic Hardware Components

Input devices, CPU, Primary Storage, Secondary Storage, Output devices

John Vonn Neuman Model

Input unit, Memory unit, ALU, CU, Output unit.

Harvard Architecture

Stores data and instructions in separate memory units

Basic Data Formats

Numeric, Alphanumeric, Graphic, Audio, Video

Bit

The smallest unit of data in a computer, representing a binary value of either 0 or 1

Byte

A unit of data that is a group of 8 bits, used to represent a single character of data (letter , number , symbol etc.)

Kilobyte to byte?

1 Kilobyte = 2^10 bytes = 1,024 bytes

3 Performance Measures

MIPS, MFLOPS, Gigahertz.

MIPS

A general measure of “the amount of work a computer can do”. It stands for "Million Instructions Per Second" and indicates the speed of a computer's instruction processing.

MFLOPS

A measure of a computer's performance, specifically the number of million floating-point operations it can perform per second.

Gigahertz

Unit of measure of the “clock speed” of the processor.

Computer Classification

Supercomputers, Mainframes, Minicomputers, Microcomputers

Supercomputer

Have most processing power available, used for scientific / military work or large simulation models

Mainframes

Used by corporations with large databases ( banks etc.), has high storage etc.

Minicomputers

Midrange computers, small, cheap and compact, somewhat obsolete ( replaced by servers).

Microcomputers

Built around 1 chip processor, relatively small, designed for one individual, “Personal Computers”.

Binary Numbers

Term comes from a Latin word and means ‘two’ in

English (0 and 1).

This means that while counting in binary you cannot

exceed 1.

Unsigned Binary

Direct binary equivalent of any decimal number. Range of 0-255

Binary Coded Decimal

Digit-by-digit binary. Numbers stored as 4 bits per digit.

Signed Magnitude ( Binary )

Using leftmost digit as representation of sign ( + or - ). Range of [-127, -0 , +0 , 127]

ONE’s Complement Process

Magnitude of negative numbers is represented by taking 1’s compliment of each bit. If there is a “carry 1”, we add it to the result.

TWO’s Compliment Process

Turning numbers into negative is: 1. Taking one’s compliment, 2. Adding 1 to that result to ignore carry bits.

Octal Numbers Range

Octal numbers are built from 8 octal digits: 0, 1, 2, 3, 4, 5, 6, 7.

Hexadecimal Numbers Range

Hex numbers are built from 16 digits: 0-9 and A-F

Boolean Algebra

Implies something that can be either true or false ( 0 or 1 ).

4 Main Boolean Operators

NOT, AND, OR, XOR

NOT Boolean Operator

Given a Boolean A, is the opposite of A.

AND Boolean Operator

Given 2 Booleans (A and B), is true if both A and B are true.

OR Boolean Operator

Given 2 Booleans (A and B), is true if either or both A or B are true.

XOR Boolean Operator

Given 2 Booleans (A and B), is true if either but not both A or B are true.

Truth Table

Mathematical table used to represent outputs of a Boolean function of all possible input combinations. n variables = 2^n rows.

Logic Gates

Boolean logic in computers is done through gates, made of transistor switches and others.

De Morgan’s Theorem

( A + B )’ = A’ * B’ or ( A * B )’ = A’ + B’

Boolean Functions

A Boolean function is a mathematical expression consisting of Boolean variables and operators ( OR, AND, NOT).

Literal: Boolean Functions

The variables used (A, B, C’)

Product term: Boolean Functions

Expression of literals combined by AND operator

Sum Term: Boolean Functions

Expression of literals combined by OR operator

Sum of Products: Boolean Functions

The sum of product terms

Product of Sums: Boolean Functions

The product of sum terms

Normal Term: Boolean Functions

A term without repeated variables

Shannon’s Expansion Theorem Steps

1. Identify pos or neg cofactors (e.g A = 1 and A = 0),

2. Apply theorem: (e.g. A · f (1, B, C) + A′ · f (0, B, C))

3. Substitute cofactors into equation

Combinatorial Logic

Digital logic where the result depends only on present inputs of the operation

Sequential Logic

Digital logic where the result depends not only on current inputs but also on past ones

Switches

A switch simply lets current flow completely or not at all. There are:

1. Relay (mechanical)

2. Vacuum tubes (electronic)

3. Transistors (electronic)

4. IC’s (tiny transistors)

Transistor

A 3 terminal electronic device made of semiconductor material. When input voltage at Base is below a certain value, transistor turns off and has HIGH output, and vise versa.

Universal gates

NOR and NAND. Easier to make many of same gates than many different gates.

Karnaugh Maps

Convenient way of representing a Boolean Function, with combinations in order: 00, 01, 11, 10. n variables = 2^n squares

Bus

A physical connect that allows transfer of data from one system location to another

Bus Width

Number of bits that can be sent through the bus (measured: bits)

Bus Speed

How quickly data can transfer across the bus. (measured: MHz)

3 Types of Bus Lines

1. Address Bus

2. Data Bus

3. Control Bus

Address Bus

Defines the location of communication

Data Bus

Defines data being communicated

Control Bus

Controls operations (e.g. start / end of transmissions etc.)

System Bus

Bus used to connect CPU, Memory, and a set of I/O modules

Point-to-Point Bus

Bus that carries signals from one specific location to a specific destination (e.g. ports)

Multipoint Bus

Bus that connects many points together

Memory

Electronic circuitry that holds data and program instructions. Often referred to as “primary Storage”

4 Types of Memory

RAM, ROM, Cache, and Virtual

2 Memory Categories (power related)

1. Volatile (needs power : RAM)

2. Nonvolatile (doesn’t need power: ROM)

2 Memory Categories (RAM related)

1. DRAM (Dynamic RAM): cheaper, needs less power, needs refresh, e.g. RAM)

2. SRAM (Static RAM): expensive\, complex, no need of refresh, e.g. Cache)

RAM

“Main Memory” , temporarily stores data during processing.

Properties of RAM

1. Grid of digital circuits representing 1 bit

2. Transistors to change bit value

3. 1-bit circuits group into bytes

4. Each byte has a specific address

5. Volatile

Memory Address

Each memory cell has a unique address, to read or write from the cells via the address

Representation of 8 bits on a memory cell (RAM)

1. Part of an integer / real number

2. One alphanumeric character

3. small sound sample

4. One pixel value (8-bit color)

5. 1/3 pixel value (24-bit color)

Clock Speed of CPU

Measured in GHz, determines processing speed. E.g. 2 GHz = 2 billion times/second

Cache

Smaller, faster memory between CPU and Main Memory for frequently used information.

Cache Hit

CPU looks in cache for data, if data is found: cache hit!

Cache Miss

CPU looks in cache for data, if data is not found: cache miss

Locality of Reference Principle (Cache)

What was accessed previously is likely to be accessed again, what is near previously accessed is more likely to be accessed.

L1 Cache

Even smaller, faster memory put directly onto CPU chip

L2 Cache

Sits between CPU and Main Memory, is checked after L1.

Virtual Memory

Virtual memory is used to “pretend” we have enough memory, e.g. a Hard Disk which creates a large virtual memory space. Ram is divided into pages, inactive pages go into Hard Disk and active ones into RAM. When a page is needed, they are swapped out.

Hard Drive’s Disk Cache

Hard Drives have their own cache, because of the dramatic difference between Hard Drive and Memory. The cache is put in the Hard Drives "Disk Controller”

ROM

Integrated circuit programmed with specific data when it’s manufactured. It has instructions for:

1. Starting computer

2. Runnings system diagnostics

3. Controlling BIOS.

You cannot reprogram or rewrite a ROM chip.

4 Types of ROM

1. PROM: Programmable ( Only once)

2. EPROM: Erasable Programmable ( Can be erased and rewritten with tools, after being removed and erased )

3. EEPROM: Electrically Erasable Programmable ( Can be erased with no special tools, does not have to be removed or FULLY erased, is slow )

4. FLASH Memory: Type of EEPROM, writes in chunks (is faster)

BIOS

“Basic Input/Output System”

Most common use of Flash Memory.

1. Boots OS

2. POST (power-on-self-test): ensure HW functions correctly

3. Activates other BIOS chips (e.g. Graphics card)

4. Provides instructions to OS for different HW devices

Structure of a Processor

1. Internal Interconnections (value transfer among components)

2. Controller (oversees operations, coordinates, manages timing)

3. Computational Engine (directed by controller, one operation at a time e.g. arithmetic, logical, bitwise)

4. Local Storage (temporarily store data values for operations)

5. External Interface (provides interaction with external: memory or I/O devices)

5 Processor Categories

1. Coprocessor

2. Microcontrollers

3. Embedded System Processors

4. Micro-sequencers

5. General Purpose Processors (CPU)

Coprocessor : Processor Categories

Works in conjunction and under control of other processors

Can perform a single task at high speed.

E.g. FPU, GPU

Microcontrollers : Processor Categories

Programmable

Control physical systems ( external HW)

Disadv : Tiny memory, slow processing capabilities

Adv: Require Limited power

E.g. Automatic doors, airplane landing gear, remote controls, toys etc.

Embedded Systems Processors : Processor Categories

Run sophisticated electronic devices

E.g. DVD Player (commands from remote or front panel), tablets, smartphones etc.

Micro-sequencers : Processor Categories

Controls coprocessor and others within a LARGER processor.

Does not perform operations, but generates addresses used by microprograms in CU.

CPU

Heart and Brain of a computer.

Carries out programs instructions, operates on the data stored in memory.

All circuits are stored on a single circuit

4 Main Components of the CPU

1. Control Unit

2. Arithmetic/Logic Unit

3. Registers

4. Clock

CPU - Memory Interaction

CPU:

1. Fetches data/instructions

2. Decodes and Executes

3. Stores data back to memory

4. next …

Registers

Single, permanent location in CPU for a specific purpose

Holds a binary value temporarily for:

• storage

• manipulation

• simple calculations

Registers ( what they can hold )

• Data being processed

• Instruction being executed

• Address to be accessed

• Binary codes for other purposes

Registers ( operations performed on them )

1. Loading values (previous one destroyed)

2. Arithmetic operations on the value

3. Bit shifting (bits shifted left/right by 1 or more)

General Purpose Registers (GPR’s)

Can hold both data and memory addresses

Control Unit Registers

• Program Counter ( PC )

• Instruction Register ( IR )

• Memory Address Register ( MAR )

• Memory Data Register ( MDR / MBR )

ALU Registers

Accumulator: Temporarily holds data used for arithmetic or logical operations

Program Counter ( PC )

Holds address of next instruction to be executed

Instruction Register ( IR )

Holds instruction being executed

Memory Address Register ( MAR )

Holds memory address being accessed

Memory Data Register ( MDR / MBR )

Holds data to or from the memory address