Computer Organization

A Preliminary Study

Parts of a Computer

• Let us look a little closer into Computers

• The simplest and oldest organization

– Central Processing Unit (CPU)

– Memory (Storage Unit)

– Input/Output Devices (I/O)

– Disk, Monitor, Printer

– all connected by a single bus (collection of wires)

Block Diagram of a Computer

CPU

• Brain of the computer

• a number of storage units, called Registers

– Instruction Register (IR), Program Counter (PC)

• Control Unit

• Arithmetic and Logical Unit (ALU)

CPU Function

• Control unit fetches instruction from memory to IR

• PC indicates which instruction to be brought

• PC holds the `address' in memory

• Instructions specify operation and operands

• Control unit identifies the operation and operands

• Control unit brings the operands from memory to registers

• ALU computes the operation

• Control unit stores back or prints the result

• All the above steps repeated forever

Data Path of a Processor

Modern Processor

• Highly Parallel Architecture

• Multiple ALUs

• Control unit divided into various sub units that run in parallel

• Pipelined Computation

CPU Power

Computing power of a CPU measured in various terms

• The rate at with which basic steps are performed measured in terms of clock frequency (1GHz)

• number of instructions executed in a second (MFlops)

• Size of the data values that can be operated upon in a single instruction (16 bit, 32 bit, 64 bit)

How big is a CPU?

• CPU is a very tiny VLSI chip

• Very Large Scale Integrated Circuit

• size of 1/4th square inches!

• Made of semi-conducting material

• Millions of Transistors

• Transistors are switches (0 and 1)

• Size of a CPU keeps decreasing

• No. of transistors doubles every eighteen months

(Moore’s Law)

Binary Encoding

• All entities (operations, operands) encoded using BITS

• BITS - Binary Digits: 0 and 1

• Decimal Digits: 0,1,2,...,9

• Sequence of 0 and 1 enough to represent any information!

• Idea: any type of info. can be enumerated (Is this true?)

• Numbers can be represented by sequence of bits

Binary Representation

• Conventional representation – Decimal

458 = 4´10² + 5 ´10¹ + 8 ´10⁰

• 10 is the base - 10 distinct symbols

• Binary Representation - base 2

• 2 distinct symbol 0,1

• 0100, 10001,110 are examples

• value of 0100 = 0 ´ 2³ + 1 ´ 2² + 0 ´ 2¹ + 0 ´2⁰ = 4

• Compute the other values?

• How do you represent 458 in binary?

Hexadecimal Representation

• Any base can be used

• Another common representation

• Often used in System programming

• Base is 16

• Distinct symbols are: 0,...,9,A,B,C,D,E,F

• Examples: 9CAD0, FFF, AAA

• value of FFF = 15 ´16² + 15 ´ 16¹ + 15 ´ 16⁰

= 4095

Conversion of Representation

• Binary to Decimal: Expand the number

• Example: 1011 = 1 ´ 2³ + 0 ´ 2² + 1 ´ 2¹ + 1 ´ 2⁰

= 11

• Decimal to Binary:

– keep dividing the number by 2 till you get a quotient less than 2

– write down the reminders at each stage from left to right

Example

• Convert 28 to binary

2 28 0

2 14 0

2 7 1 Binary representation of

2 3 1 28 is 11100

1

How to convert decimal to Hexadecimal?

Memory

• Instructions and operands in memory during execution

• physically, memory is an array of cells

• each cell contain one BIT of information

• chunks of 8 adjacent bits is called a BYTE

• WORDS are chunks of adjacent bytes (usually 2 or 4 bytes)

• logical view of memory is - array of words

• words contain meaningful data

• each word has a numerical address (0, 127, 2067 etc.)

Main Memory

Memory Operations

• CPU performs two operations on memory

– READ and WRITE

– contents of any word can be read or written

– Random Access Memory (RAM)

• CPU refers to the contents of memory by their addresses

Memory capacity

• Measured in various terms

• Size of the memory: no. of bytes of data it can store

– Current common sizes are:

– 128 Kilo bytes, 256 Mega Bytes, 1 Giga Bytes

1 Kilo byte = 1024 bytes (2¹⁰)

1 Mega Byte = 1024 KB (2²⁰)

1 Giga Byte = 1024 MB (2³⁰)

Memory Capacity

• Size of the word:

– Transfer from/to CPU and memory take place in units of words

– word size determines the number of bytes that can be transferred in a single operation

– larger it is, faster the processing power

– Common word sizes are: 16 bits, 32 bits, 64 bits

Different Memory Types

• Main memory Speed

– 100s of Mbytes per second

• Types of Memory

– Random Access Memory (RAM)

– Read Only Memory (ROM)

– Programmable ROM (PROM)

– Erasable/Programmable Memory (EPROM)

Peripherals

• Secondary Memory - Hard Disk

• Can store large amount of data (10 GB, 256 GB, ...)

– Slower (tens of MB/sec)

– Date retained even when not powered

– Magnetic memory

– Program and Data files are stored here

• Video Display Unit (Monitor)

• Keyboard, Mouse, Printer, Floppy Disk, CDROM

• Network Devices (Ethernet card, Modem)

• All these are interfaced to the main bus via CONTROLLERS

• CPU deals with controllers and the latter with the devices

Machine Programs

Machine instructions

• involve primitive operations like

– reading and writing memory/registers

– arithmetic and logical operations on binary nos.

• involve references to registers, memory locations and binary strings

• only one kind of data values: binary numbers

• Too difficult to program, understand, debug, analyze and maintain

• Not portable - instructions are machine peculiarities

• Example: mov R1,R2
lda R1,#0AB
add R1,R2,R3
jmp end

High Level Language

• hides details of machine peculiarities

• provides high level of abstractions of memory and registers

• High level data types like integers,real,characters

• suitable operations on these data types

• easier to program, debug, analyze and maintain

• portable - you need a compiler for each (type of) machine

Running a HL program

• cannot be directly executed

• translate to low-level programs

• compilers do the job

• source and object programs

• compilers are machine dependent

Simple Picture