Lecture Notes: Computer Organization

COMP1003 Computer Organisation

Prof. Haipeng GUO

Lec01

What is a Computer?

• Definition:
  A computer is an electronic, digital, general-purpose machine that follows follows a step-by-step list (computer program) of instructions for solving a problem
  计算机是一个电子的、数字化的通用机器，它按照逐步指令程序解决问题。

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Turing Machine

1936年，Alan Turing。

• Definition:
  Turing machine, which is the abstract model of all computers

• Components:

  • a tape divided into cells
  • a moving read/write head
  • a state register storing the state of the Turing machine
  • a finite table of instruction

The Church-Turing Thesis: All things that can be computed can be computed by a Turing machine

Universal Turing Machine: a Turing machine that could simulate all other Turing machines.

It is programmable – so it is a computer!

A computer is a Universal Turing Machine!

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Historical Development

• Generation Zero: Mechanical Calculating Machines (1642-1945)
• The First Generation: Vacuum Tube Computers (1945-1953)
• The Second Generation: Transistor Computers (1954-1965)
• The Third Generation: Integrated Circuit (IC) Computers (1965-1980)
• The Fourth Generation: VLSI(Very-Large-Scale Integration) Computers (1980-)

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Von Neumann Architecture

Also called stored-program architecture

Both data and program are stored in the memory

• Components:
  CPU (control unit, ALU, registers), memory, and an I/O system.
  CPU（控制单元，算术逻辑单元，寄存器）、内存和I/O系统。

• von Neumann Bottleneck:
  The speed difference between the CPU and memory creates a delay, as the CPU is faster than memory.

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Von Neumann Execution Cycle

• also called the fetch-decode-execute cycle
  • the control unit fetch the next instruction from the memory
  • the instruction is decoded into a language that the ALU understands
  • data operands are fetched from the memory into the registers inside CPU
  • the ALU executes the instruction and places the result into the registers or memory

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Abstraction

The essence of abstraction is preserving information that is relevant in a given context, and forgetting information that is irrelevant in that context.

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Levels of Transformations

Problem –(Software Design)-> Algorithm –(Programming)-> Program –(Compiling/Interpreting)-> Instr Set Architecture

Problem: What you want to do
Algorithm: step-by-step procedure to solve the problem
Program: a computer program that implements the algorithm using some programming languages

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The Machine Levels

• Instruction Set Architecture (ISA): instructions that a CPU can execute
• Microarchitecture: implementation of ISA
• Circuits: Details of electrical circuits
• Devices (transistors): Circuits are built by interconnecting transistors
• Bits: Transistors operate on bits (“0” or “1”) that represent data and information

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Signed Interger Representation

• Sign-magnitude：原码，最高位是符号位，剩下表示大小
• 1’s complement：正数的话同原码。负数的话，在原码基础上，符号位不变，剩下取反。
• 2’s complement：正数的话同原码。负数的话，在原码基础上，符号位不变，剩下取反，再加一。

Real Number Representation

以下是 IEEE754 Single Point Precision Numbers 的示例

单精度浮点数，32 bits。从左到右，1 个 bit 表示 sign，8 个 bit 表示 exponent，23 个 bit 表示 fraction。

$$
(-1)^{sign} \times (1 + fraction) \times 2 ^ {exponent - 127}
$$

• e.g. $(12.375)_{10}$ to float
  • $(12.375){10} = (1100.011){2} = 1.100011 \times 2 ^ 3$
  • Sign bit = 0
  • Exponent: $3 + 127 = 130 = 1000 0010$
  • Mantissa: 1000 1100 0000 0000 000
  • Answer: 0 1000 0010 1000 1100 0000 0000 000