Introduction to Digital Computers

Course Outline

• Introduction
• Milestones in Computer Architecture
• The Computer Zoo
• Processors
• Memory
• Input/Output
• The Instruction Set
• Parallel Computer Architectures

Introduction to Digital Computers

• A digital computer follows a sequence of instructions, known as a program, to perform tasks.
• Each computer can execute a limited set of basic instructions directly, such as:
  • Adding numbers
  • Checking if a number is zero
  • Copying data between memory locations

Machine Language

• These basic instructions form a machine language, the fundamental language for communication between people and computers.
• Machine language is simple to keep hardware costs down, but difficult for people to use directly.
• Computers are designed in layers or abstractions to manage complexity. Each layer builds on the previous one.

Structured Computer Organization

• This layered approach to computer design is called structured computer organization.
• It helps designers create systematic, organized computer systems.

Abstractions in Computer Design

• Over time, designers realized organizing computers as a sequence of layers or abstractions helps manage complexity.
• Each layer builds on the one below it, simplifying the design process.
• This layered structure allows computer systems to be systematic, organized, and easier to work with, even as technology grows more complex.

The Human-Computer Gap

• There's a significant gap between what's easy for people and what computers are designed to do. People want to perform complex tasks, but computers can only process simple instructions.
• This difference creates a problem in making computers useful and accessible for human needs.
• The goal of structured computer organization/computer architecture is to bridge this gap. Using layers of abstraction makes computers more convenient and powerful for human use.

Languages, Levels, and Virtual Machines

• To make computers easier for people to use, a new set of instructions (language) is introduced.
  • LO: The machine's built-in language (low-level instructions).
  • L1: A higher-level language designed to be more convenient for people.

Two Approaches to Executing L1 Programs

• Translation: Converts each instruction in L1 into an equivalent sequence of LO instructions. The computer executes this new LO program.
• Interpretation: Uses a program called an interpreter written in LO. The interpreter reads and executes L1 instructions directly without creating a new LO program.

Bridging Human and Machine Needs

• Computers are designed with basic instructions (LO) that are efficient for machines but hard for humans to use.
• People need higher-level languages (L1, L2, etc.) that are closer to human logic to create and manage complex programs.

Virtual Machines as a Solution

• Virtual machines are hypothetical layers that simulate these higher-level languages (L1, L2,...) on a computer that can only process LO.
• They allow people to write in more convenient languages without modifying the hardware.
• A virtual machine enables the execution of L1 code as though it were native to the machine.

Translation and Interpretation

• Virtual machines operate by either translating (converting the entire L1 program to LO) or interpreting (running L1 code step-by-step through an interpreter).
• These techniques make it possible to use advanced languages on hardware designed for basic instructions.

Layered Design for Effective Computing

• By creating multiple virtual machine layers, we can keep adding higher-level languages (e.g., L2, L3), making programming progressively easier and more human-friendly.
• This layered approach allows complex systems to function without requiring prohibitively costly hardware.

A Multilevel Machine

• Virtual machines (e.g. M1, M2, etc.) with different languages.
• Programs in each language can be either interpreted or translated to the language of a lower machine.

Simplifying Programming with Virtual Machines

• Programmers working at a high level don't need to understand the complex layers below.
• The virtual machine structure ensures programs are executed, regardless of whether they run directly on hardware or through layers of interpreters and translators.
• Most developers care only about the top-level language, which is user-friendly and far removed from low-level machine code.
• This abstraction allows developers to concentrate on problem-solving without worrying about hardware specifics.

Contemporary Multilevel Machines in Modern Computers

• Modern computers often consist of multiple levels (upto six).
• Each level represents a layer of abstraction from high-level programming to the machine's physical circuits.
• Level 0 is the hardware level, where circuits execute machine-language instructions from Level 1
• Each successive level builds upon the one below it, allowing complex operations at higher levels while keeping the hardware relatively simple.

Digital Logic Level (Level 0)

• Core Component: Gates (AND, OR, etc.), each built with transistors.
• Function: Gates combine to form memory and the fundamental processing units of the computer.
• Purpose: Foundation for all higher computing functions.

Microarchitecture Level (Level 1)

• Core Components: Registers (8 to 32 registers) and the Arithmetic Logic Unit (ALU).
• Function: Performs arithmetic operations on data, using registers to temporarily hold data.
• Control: May be controlled by a microprogram (software) or hardware directly. This controls the data flow and execution of instructions.

Instruction Set Architecture (ISA) Level (Level 2)

• Definition: The machine language unique to each computer model, e.g., the "language" described in a machine's manual.
• Execution: Programs in the ISA are executed by the microprogram or hardwired circuits.

Operating System Machine Level (Level 3)

• Hybrid Level: Supports instructions from the ISA level and additional features like memory organization and multitasking.
• Execution: Some instructions are interpreted by the operating system, while others are executed by microprogramming.

Assembly Language Level (Level 4)

• Purpose: A symbolic language that translates into lower-level machine code.
• Tool: Programs written in assembly are converted by an assembler for execution by lower levels.

High-Level Languages (Level 5)

• Examples: C, Java, Python, etc.
• Function: Applications-oriented languages that make programming simpler and more accessible.
• Execution: Typically translated to lower levels using compilers or interpreters.

Key Takeaways

• Separation of Concerns: Lower levels focus on machine operations, while upper levels cater to application development.
• Increasing Abstraction: Each level abstracts complexity, making programming more user-friendly at higher levels.
• Role of Systems Programmers: Levels 1-3 are managed by systems programmers to support functionality and translation to higher levels.

Summary

• Computers are designed as a series of layers or levels, each built upon the previous one, with each level representing a distinct abstraction.
• This hierarchical structure helps simplify the complexity of computer systems.

Key Points of Abstraction

• Each level provides a different layer of functionality and operations, making the system easier to understand and work with.
• The lower levels (hardware, circuits) are complex but crucial for the operation of higher levels.

Architecture

• The architecture of a level defines its visible features, such as data types, operations, and programming interfaces. These elements are the programmer interacts with, like the instruction set or memory organization.

Implementation vs. Architecture

• Architecture deals with how the system is used by the programmer.
• Implementation refers to how the system is built (e.g., technology used).
• Implementation details are NOT part of the architecture.
• Computer architecture is the field of study focusing on designing the parts of a computer system, visible to programmers (instruction set, memory management, component interaction), to determine how efficiently a computer can perform tasks.

Virtual Machines

• In modern computing, virtual machines (VMs) often act as a layer above hardware to provide an abstraction. This makes systems more flexible and easier to work with.
• Different programming languages may target different virtual machines, improving compatibility and simplifying the development process.
• Computer systems are composed of multiple interdependent layers, each with its own purpose; architecture defines how these layers interact.

Hardware vs. Software

• Hardware refers to the physical components (circuits, memory, input/output devices). It's tangible and directly executes machine-level instructions.
• Software, a set of algorithms and instructions, tells hardware what to do. It is stored on various media (discs, etc.), and its essence are the instructions making up programs.

Historically Clear Boundary

• In the early days of computing, the line between hardware and software was clearly defined.
• With the evolution of multilevel machines, some operations that were once embedded in hardware are now handled by software, and vice versa.

Hardware and Software Equivalence

• In modern computing, hardware and software are logically equivalent. Any function performed by hardware can also be implemented in software, and vice versa.

Decisions Between Hardware and Software

• The decision of implementing a function in hardware or software depends on factors including cost, speed, reliability and expected changes. These decisions are not fixed but change based on technological trends, economic considerations and evolving user demands.
• As technology evolves, hardware and software roles change, and the number of layers between hardware and highest-level programming languages grows.

Trends in Technology

• As technology evolves, the roles of hardware and software change.
• This ongoing evolution leads to multilevel machines where the number of layers between hardware and the highest-level programming languages continues to grow.

The Invention of Microprogramming

• The concept of microprogramming emerged as a way to simplify the hardware design of early computers. Early computers had a simpler two-level architecture (ISA level and digital logic level). This design was challenging due to the complexity and unreliability of vacuum tubes.

Maurice Wilkes and the Three-Level Machine (1951)

• Maurice Wilkes proposed a radical change- introducing a third level, called the microprogramming level.
• The key idea was designing a microprogram to serve as a built-in interpreter to execute ISA-level programs.

Impact of Microprogramming

• The introduction of microprogramming significantly reduced hardware complexity by offloading the interpretation of complex ISA instructions to simplified microprograms.
• By 1970, microprogramming was widely adopted in major computers.

The Invention of the Operating System (OS)

• Early computers required programmers to manually operate the machines, leading to idle time and inefficiencies.
• Operating systems (OSs) were introduced to automate tasks, like managing job queues and loading compilers.

Evolution of OS

• As OSs evolved, they added new instructions and features that resembled a new level of abstraction beyond the ISA.

The Migration of Functionality to Microcode

• With the rise of microprogramming, machine designers could add new instructions via software. This included features like multimedia processing.

Key Additions to Instruction Sets

• Many new instructions were not strictly necessary but provided performance optimizations (e.g., increment instead of addition).
• Included floating-point, integer multiplication, procedures, loops, and character strings.

Microprogramming's Decline

• As microprograms grew complex in the 1960s and 1970s, they slowed down.
• Researchers proposed eliminating microprogramming and reducing instruction sets to lead to direct execution by hardware, thus speeding up performance.

Milestones in Computer Architecture

• Zeroth Generation - Mechanical computers
• First Generation - Vacuum tubes
• Second Generation - Transistors
• Third Generation - Integrated circuits
• Fourth Generation - VLSI
• Fifth Generation - Low-power, invisible computers

The Computer Zoo

• Moore's Law: Computer industry's growth driven by increasing transistors on chips every year. This results in faster processing, larger memories and lower costs.
• Gordon Moore's Prediction: Transistor numbers double approximately every 18 months leading to increased computing power.
• Industry Transformation: This growth shaped the personal computer, mobile phone, and semiconductor industries.

Technological and Economic Forces

• Moore's Law: The increasing number of transistors on chips leads to significant advancements in chip technology.
• Limitations and Future Challenges: Shrinking transistor size is approaching physical limits, posing challenges like energy dissipation

Economic Impact and the Virtuous Circle

• Economic growth from technological advancements: Increasing processing power and falling prices fuel new applications and markets, creating higher demand and business opportunities.
• Nathan Myhrvold's Law of Software: Growing software features drive further advances in processors and memory technology.

Memory and Storage Evolution

• Storage Growth: Dramatic improvements in disk storage over the past few decades have led to larger storage capacities and increased speeds for devices.
• Shift to Flash Memory: Traditional hard disks are increasingly replaced by faster and more reliable flash memory.

Telecommunications and Networking

• Exponential Growth in Networking: Extraordinary progress has seen communication technologies evolve from slow modems to high-speed fiber optics.

The Computer Spectrum

• Over the past four decades, computer technology increased by factors of millions in processing power, storage and connectivity.

The Current Spectrum of Computers Available

• Disposable computer
• Microcontroller
• Mobile and game computers
• Personal computer
• Server
• Mainframe

Examples of Computer Families

• x86 Architecture
• ARM Architecture
• AVR Architecture

This quiz covers the fundamental concepts of digital computers, including their architecture, machine language, and structured organization. You will explore how computers execute instructions and manage complexity through various layers of abstraction. Test your understanding of these essential topics in computer science!