1. Computer Systems
An Introducton

2. Computer Systems: Introduction
Programmable machines
Hardware + Software (program)
Hardware
Program
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Computer Systems: Introduction

3. Computer Systems: Introduction
What does a program do?
Take input data from users or world
Process data
Output the result
Take input from keyboard and mouse
Determine if the input is a click at button “save”, or a click at button “slide show”, or a press on keyboard “K”.
Then, do it!
Display the message “saved”, or show slide, or letter “K”.
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Computer Systems: Introduction

4. Representation of Data and Programs
Bits: 0 or 1
Bytes: string/sequence of bits e.g
Normally, a byte is composed of 8 bits.
Words: sequence of bytes
A word can be composed of 4 or 8 bytes.
There are standard formats of data stored in a computer.
ASCII code, Unicode
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Computer Systems: Introduction

5. Languages and Programs
A program is a sequence of instructions for computer.
A format of instructions = a language
Computer languages
High-level languages
Java, Python, C, ASP, HTML, …
Low-level languages
Assembly languages
Machine languages
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Computer Systems: Introduction

6. Computer Systems: Introduction
Examples of Languages
High-level languages C
int main(void) {
int n, m, sum=0;
for (n=0; n<7; n++)
{ scanf(“%d”, &m);
sum+=m; }
return 0;
Assembly languages
6800 assembly
ldx i
ldaa ar,x
staa max ; max = ar[0]
lp:ldaa i+1
cmpa #2
bgt ex ; while i <= N
cmpa max ; if max<ar[i]
ble sp
staa max ; then max=ar[i]
sp:inx ; i=i+1
stx i
jmp lp
ex:
Easier for human to understand
Easier for machines to understand
(Also, for machine languages)
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Computer Systems: Introduction

7. Computer Systems: Introduction
Language Translation
A compiler is a program which translates a program in one language into another language.
Source
program
compiler
Target
program
Source language
Target language
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Computer Systems: Introduction

8. Computer Systems: Introduction
Compiler
Usually, the source language is a high-level language, and the target language is a low-level language.
Assembler is a compiler which translates from an assembly language to a machine language.
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Computer Systems: Introduction

9. Where are programs/data stored?
In memory
Components of computers
Input Unit
Processor
Output Unit
Memory
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Computer Systems: Introduction

10. Computer Systems: Introduction
Memory
Linear storage
Addressable in words
“Random access”
Data/program can be read from/ write to memory, one word at a time.
To write data d to memory addressed a ,
data d and address a must be specified.
To read data from memory addressed a ,
address a must be specified.
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Computer Systems: Introduction

11. Components of a computer: More detail
processor
memory
registers
ALU
data
program
Control units
Instruction register
data
address
Program counter
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Computer Systems: Introduction

12. Components of Processors
Arithmetic-logic unit (ALU)
Performs arithmetic and logic functions: add, substract, multiply, divide, and, or, not, etc.
Registers
Fast-access memory used by a processor to store intermediate results.
Program counter (PC)
Keeps track where the current instruction is.
Normally, incremented to point to the next instruction.
Changed by instructions that alter the flow of control of a program (if-then-else, loop, and function call).
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Computer Systems: Introduction

13. Components of Processors
Instruction register (IR)
Store the current instruction fetched from memory.
Its content (the instruction) signals the control unit to initiate the execution of that instruction.
Control unit
Most complex part of a processor.
Send control signals to all parts in the processor to co-ordinate their activities.
A large finite state machine.
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Computer Systems: Introduction

14. How a computer system works
Fetch-execute cycle
Fetch: get an instruction (addressed by PC) from memory and store in IR, and update PC.
Execute: work according to the instruction in IR
If the instruction causes PC changed, PC is updated again.
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Computer Systems: Introduction

15. Instruction Execution Cycle
processor
memory
registers
ALU
data
program
Control units
Instruction register
data
address
Program counter
increment
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Computer Systems: Introduction

16. Computer Systems: Introduction
Processor
Control unit
Data path
Logic
Multiplexor
Registers
Bus
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Computer Systems: Introduction

17. Computer Systems: Introduction
Data Path
Logic is a combinational function, out = f(in1 ... inn) where f is a Boolean function {not, and, or}.
out = x1 + x2 · x3 + x1 ·x2 f x1 x2 x3
out
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Computer Systems: Introduction

18. Computer Systems: Introduction
Data Path
Registers are storage elements
out(t+1) = in(t)
control signal “load”
width of a register defines the number of bits that can be stored
register n n in
out
load
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Computer Systems: Introduction

19. Computer Systems: Introduction
Data Path
Muliplexors have n inputs (of width m) and select one input to be the output, called n:1 multiplexor.
The control signal to determine the output is called the select signal. x0 x1 x2 x3
MUX
out
selector
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Computer Systems: Introduction

20. Computer Systems: Introduction
Data Path
Bus consists of wires and buffers.
Wires carry data (signal).
Buffer controls the traffic of data from any element to a bus.
A bus can be shared to reduce the number of wire within a circuit.
A bus can broadcast data to many receivers limited by the fan-out electrical characteristic of the bus, the ability to drive other circuits.
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Computer Systems: Introduction

21. Levels of descriptions of computer systems
Applications
Operating systems
Instruction set
Functional units
Finite state machine
Logic gates
Electronics
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Computer Systems: Introduction

22. Levels of descriptions of computer systems
Applications
Application level is what a user typically sees a computer system, running his/her application programs.
An application is usually written in a computer language which used many system functions provided by the operating system.
Operating systems
Instruction set
Functional units
Finite state machine
Logic gates
Electronics
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Computer Systems: Introduction

23. Levels of descriptions of computer systems
Applications
An operating system is abstraction layers that separate a user program from the underlying system-dependent hardware and peripherals.
Operating systems
Instruction set
Functional units
Finite state machine
Logic gates
Electronics
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Computer Systems: Introduction

24. Levels of descriptions of computer systems
computer architecture begins at the instruction set.
An instruction set is what a programmer at the lowest level sees of a processor
one instruction set with different level of performance for many models, based on the implementation of a control unit via “microprogram”.
Present day processor designs converge, their instruction sets become more similar than different.
Applications
Operating systems
Instruction set
Functional units
Finite state machine
Logic gates
Electronics
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Computer Systems: Introduction

25. Levels of descriptions of computer systems
Units are divided by their functions, e.g. ALU, control unit, etc.
The interconnection between units are described.
Applications
Operating systems
Instruction set
processor
registers
ALU
Control units
Instruction register
Program counter
Functional units
data
Finite state machine
memory
Logic gates
address
Electronics
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Computer Systems: Introduction

26. Levels of descriptions of computer systems
Applications
Mathematical description of the behavior of a system.
Important tool for verification of the correct behavior of the hardware.
Operating systems
Instruction set
Functional units
Finite state machine
Logic gates
Electronics
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Computer Systems: Introduction

27. Levels of descriptions of computer systems
Applications
Logic gates (e.g. and, or, not gates) are used to build functional units.
Operating systems
Instruction set
Functional units
Finite state machine
Logic gates
Electronics
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Computer Systems: Introduction

28. Levels of descriptions of computer systems
Applications
Electronic circuits are used to build logic gates.
Operating systems
Instruction set
Functional units
Finite state machine
Logic gates
Electronics
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Computer Systems: Introduction