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King Fahd University of Petroleum and MineralsKing Fahd University of Petroleum and Minerals

Computer Engineering Department


College of ComputerScience And Engineering


Lectures 2 & 3 – Computer Organization

1

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Lectures 2 & 3:

Computer Organization

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Memory• Storage Device• Stores Programs and

Data coded in binary format.

• Technically “similar” to a two-dimensional array of “switches”

• A “switch” called a bit (abbr. for binary digit)

• n Address lines means 2n words of m bits each

0

1

m

n

2n

Data

Address

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Memory• 2 Operations:

– Read: Copy Data stored in word of Address (on Address lines) to Data Bus

– Write: Store Data on Data Bus into word of Address (on Address lines)

m

n

ReadWrite

Addr

Data

Memory

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Concept of Address• It is an index in the memory

• It represents a “geographic” location of a word in the memory

• Number of Address lines and Word size determine Memory Capacity (Size)

• Most of the time:– Memory size = 2n words = 2n * m bits

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Random Access• Access to Memory locations is equal delay.• Means access to any word which address

picked “randomly” will take the SAME amount of access time.

• Think about it as structurally different than a tape structure.

• In a tape: unequal access time to information stored in different locations (beginning of tape, middle or end of tape)

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RAM• RAM: Random Access Memory• Although the name is about the way memory is

accessed. Historically, volatile memory has been called RAM.

• Volatile (do not retain information on power off)• Used mainly as Central Memory for CPUs• Two types of RAM

– Static: Continuous Retention of Information– Dynamic (DRAM): needs refresh cycle to maintain

information

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ROM• Non Volatile• Used to store data (programs) that do not

change often (fixed)• Many types

– Mask ROM: Values set at fabrication stage. Values cannot be changed

– Fuse PROM: Values set at burning phase. Values cannot be changed

– EPROM: Can be erased (UV)– EEPROM: Electrically erased– Flash EEPROM: Easily reprogrammable.– New: NVRAM (Non Volatile RAM): Fast access time.

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Disk Drives• Hard Drive: suite of

magnetic disks. Mechanically read and write data by moving a set of magnetic head over the disks

• CD-ROM, DVD-ROM: Suite of optical disks read by measuring the time of laser reflexion between “1” and “0”

“1” “0”

N S

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Program Execution• A Program is a suite of

instructions

• Program Execution is Sequential

• Program is stored in Memory

• Program is executed by CPU

Instruction 1

Instruction 2

Instruction 3

Instruction n

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CPU

• Executes Programs Stored in Memory

• Executes Instructions ONE by ONE

• Only “knows” instructions: Instruction Set

• DO NOT know any notion of Program as a single entity.

• Everything is a suite of instructions

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CPU Structure (1)Is Mainly a Data Processing Unit Controlledby a Control Unit.• Data Processing Unit: Datapath

– Registers (Scratch pad working space or temporary data storage)

– ALU: Arithmetic and Logic Unit– Internal Buses

• Control Unit: Generates Commands to “drive” Datapath operations

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CPU Structure (2)

Control Unit

Data Path is Similar to a Pipe Structure where valves are controlled by the Control Unit

Dat

apat

h ALU

Reg

iste

r

Reg

iste

r

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Master Clock• Instructions Executed step by step• Need a “Rhythm” Generator to move forward in the

steps:

TimeClock Cycle

Every CPU needs a Clock to control the transition from one execution step to the next

Clock Frequency = 1/Clock Cycle Period : MHz

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Instruction Set• Instruction Set is the Catalog of the CPU• Defines what are ALL the possible

operations that the CPU can execute• Only Instructions are recognized by CPU.• CPU does NOT “understand” High Level

Language (text).• CPU understands instructions coded in

numbers called machine code.

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Instruction parameters• Each Instruction specifies an action or a suite of

actions:– Action(s) “identifier” or Operation Code or Opcode– Action arguments or operands– Methods specifying how to access the operands,

called addressing modes

Instruction specified as:<Opcode> <Operand 1, addr_mode1> <Operand 2, addr_mode2> ….

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Addressing Modes• Direct

• Indirect

• Indexed

• Immediate

• Register

• Inherent

• Combination of the above (some)

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Number of Operands• Many types of Instruction Sets

• Instruction Set with One Operand: Implicit Register Called Accumulator. Everything goes to and from the accumulator:

• Instruction Set with Two Operands: Many registers can be used as accumulators

• Instruction Set with Three Operands: Mainly Register Based.

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Instruction Format• Specifies the map of the instruction at the code machine

level.• Formatted scheme for the instruction word(s)• Considers a word (or multiple words) in memory as a

single instruction composed of different contiguous bit-fields containing the opcode and the different operands and their addressing modes

• Many formats can co-exist in the same instruction set• Opcode value determines remaining format

Opcode Reg Immediate

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CPU Execution• Executes Instruction by Instruction• Instructions stored in Memory• Capable of doing a certain number of operations

(Defined by Instruction Set)

• Get Instruction from Memory• Recognize What instruction it is• Activates the associated set of commands (step-by-step)

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Fetch – Decode - Execute

Fetch

Decode

Execute

CpuMemory

Address of next Instruction

Instruction

Read Command

Opcode Reg Immediate

InstructionDecoder

Fetch

Decode

Execute

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Address of Programs• Where the Address of next instruction is Stored ?

Need for an Instruction PointerCalled: “Program Counter”

PC

• Critical Component of CPU• Conveniently useful for changing program

sequence (Branch instructions)

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Instruction Register• Where is the current instruction going to be stored during

its execution ?Need for a RegisterCalled: “Instruction Register”

• Critical Component of CPU• Internal Register. Cannot be used

(accessed) by instructions• Holds the current instruction until its

execution is completed• Tightly Coupled to the decoding

portion of the control unit• Connected to the datapath (to transfer

operand fields)

Opcode Op1 Op2

Instruction Decoder

Data Bus

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Program in Memory• Binary code (machine code).

B8

00

05

MOV AX,5

03

C3ADD AX,BX

EB

E7JMP Next

Memory (8-bit)

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High Level Languages• Machine independent. • Cannot be run directly on the target machine• Need to be translated to machine language • Compiler: program that translates a HLL program to a machine

language program of a specific platform • The Machine language program produced by the compiler is the

executable program.• Translating HLL programs to machine language programs is not a

one-to-one mapping– A HLL statement translated to one or more machine language

instructions– Usually, machine language programs produced by compilers are not

efficient• Deals with Data types (integer, real, complex, user-defined) vs.

machine language: no data types only binary words.

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Assembly Language• Text version of machine language• Human friendly representation of machine

language• Based on mnemonics (easy to memorize

abbreviations of actions) instead of dealing with opcode numbers.

• Complicated format simplified with some conventions

• Text file translated into machine code by the Assembler

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Assembler• Program that assemble the

programs written in assembly language into machine language

• Because there is a ONE to ONE mapping between instructions written in assembly language and machine language instructions, the process is called: assembly rather than translation.

• Disassembly (reverse process) is also easy because of the ONE to ONE relation between the assembly language instructions and the machine language instructions

MOV DX, 1

MOV AX, BX

MOV AX, CX

MOV AX, DX

ADD AX, 1

ADD AX, 2

ADD AX, BX

ADD AX, CX

ADD AX, i

SUB AX, 1

SUB AX, BX

ADD AX, 1234h

BA 0001

8B C3

8B C1

8B C2

83 C0 01

83 C0 02

03 C3

03 C1

03 06 0000

83 E8 01

2B C3

05 1234

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Linker

• Program used to link together separately assembled/compiled programs into a single executable code

• Allows the programmers to develop different parts of a large program separately, test them separately and ‘freeze’ them for future use.

• Allows the programmer to develop store portions of programs that have been intensively tested and used into a “program library” for anyone to re-use them.

• Produces modular programs and greatly enables the management of large programming projects

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Debugger/Monitor

These are tools that allow the assembly programmers to:• Display and alter the contents of memory and registers

while running their code, • Perform disassembly of their machine code (show the

assembly language equivalent),• Permit them to run their programs, stop (or halt) them,

run them step-by-step or insert break points.– Break points: Positions in the program that if are encountered

during run time, the program will be halted so the programmer can examine the memory and registers contents and determine what went wrong.

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