COMP-1200 Computer Models Notes

Layers of a Computer System

Components of a Computer System:

• Program Instructions: Refers to the sequences of coded operations that a computer can perform.

• Computer Model: Represents the conceptual framework for understanding how computers function based on theories and design principles.

• Digital Logic: The foundation of data and electronics, focusing on binary operations (0s and 1s) that underpin computational tasks.

• Operating System: The software that manages computer hardware, provides common services for computer programs, and acts as an intermediary between users and the computer hardware.

• Data Representation: Binary bits (e.g., 1010) are utilized to represent various forms of data, including numbers, characters, and instructions.

Learning Objectives

• Computer Models: Understand the concept of computer models, focusing on the Von Neumann Machine, which illustrates how data and instructions are handled.

• Logic Circuits: Illustrate how logic circuits combine to form the fundamental operations of a computer, including the use of gates (AND, OR, NOT) to perform computations.

What is a Computer System?
Essential Components:

• DRAM (Dynamic Random Access Memory): A type of volatile memory that is used to store data temporarily while the computer is in operation, allowing for faster access than other memory types.

• Power Supply: Essential for operation—converts AC from the wall outlet to usable DC power and regulates voltage for stability.

• Storage Devices: Both volatile (like RAM) and non-volatile (like hard drives or SSDs) storage allow for data retention, with SSDs offering faster speeds than traditional hard drives.

• Processor: The central processing unit (CPU) is where most computations are carried out, executing instructions from programs and managing the flow of information through the system.

• I/O Devices: Keyboard, Mouse, Screen—peripherals that facilitate user interaction with the computer and the display of output data.

History of Computers

• ENIAC (1943):

  • First Electronic Computer: Utilized approximately 18,000 vacuum tubes, making it a colossus of its time.

  • Lacked transistors, which limited reliability and performance.

  • Programming was hard-wired through physical connections, making it inflexible and labor-intensive.

  • Difficult to change programs (plugboard system) resulted in lengthy initialization times.

• EDVAC:

  • Introduced the revolutionary idea of storing programs directly in memory, thereby moving away from hard-wired systems that were cumbersome.

  • Allowed for more efficient program execution, since programs could be saved to magnetic tape, making retrieval and execution faster.

  • John von Neumann contributed significantly to this concept in his 1945 report, which laid the foundations of modern computing architecture.

The Von Neumann Model
Describes a computer architecture consisting of:

• Processing Unit: Performs arithmetic and logical operations, serving as the brain of the computer.

• Memory: Stores both instructions and data, allowing for efficient data retrieval and execution.

• Control Unit: Interprets the instructions in memory, directing operations and managing data flow within the system.

• Key Innovation: Storing instructions alongside data in a unified memory space allows for easy program modification and scalability of computing tasks.

Instruction Set Architecture (ISA)
Defines how instructions and data are represented in a computer system.
Fundamental Elements:

• Opcodes: Indicates the operation to be performed, essential for executing instructions.

• Operands: Data or locations that the operation will utilize, establishing the context of the operation.

Sample Instructions

• Hypothetical ADD Instruction:

  • Format: add r1, r5, 10

  • Opcode: add, signifies the specific operation to be carried out.

  • Destination: r1 (where the result goes, identifying the storage location for the outcome).

  • Source: r5 and 10 (the values to be added, showing where to pull the operands from).

Types of Operations (Opcodes)
Categories:

• Arithmetic: Includes Addition, Subtraction, Multiplication, and Division operations essential for numerical computing.

• Logic Operations: Such as AND, OR, and NOT, are fundamental for decision-making processes in programming.

• Data Movement: Loading and storing data, which facilitates interaction with memory and I/O devices.

• Control Operations: Branching, subroutine calls, and other functions that manage program flow and execution states.

Processors and ALU
Arithmetic Logic Unit (ALU):

• Core of computing processes, managing key arithmetic and logic operations central to program execution.

• Can consist of multiple ALUs specialized for specific tasks (e.g., advanced multiplication, division, and floating-point operations) to enhance performance and efficiency.

Word Size
Definition: The number of bits a processor can handle in one operation, significantly impacting the system's capacity and performance.
Importance: Affects processing power and memory addressing capabilities; typically defined in the ISA:

• LC-3: 16 bits

• ARM/RISC-V: 32 bits

• Intel x86: 64 bits

Register Systems
Registers: Small, fast storage areas located within CPUs, enabling rapid access to data needed for processing.
Used to hold operands for the ALU and results of operations, enhancing the efficiency of computations.

• Register File: Represents a collection of registers as defined by the ISA, often organized to optimize access patterns for frequent operations.

Control Unit Operations
The control unit manages the execution of instructions by controlling the on/off status of components and generating signals based on:

• Instruction Register (IR): Holds the current instruction being executed, allowing the CPU to track its progress through the instruction cycle.

• Program Counter (PC): Indicates the address of the next instruction to be fetched and executed, ensuring smooth instruction flow.

Memory Interface

• Memory Address Register (MAR): Holds the address of the memory location designated for reading or writing, serving as a bridge between the CPU and memory units.

• Memory Data Register (MDR): Contains the actual data being transferred, supporting communication between the CPU and memory.

Input and Output Devices

• Input: Devices that send data to the computer (e.g., keyboard, mouse, scanners), allowing users to perform actions and submit data.

• Output: Devices that receive data from the computer (e.g., monitor, printer), translating digital information into usable formats for users.

• Some devices, like touchscreens, can perform both input and output functions, enhancing interactivity.

Clock Signal
Role of the Clock: Coordinates operations within the CPU by providing a regular timing signal that synchronizes processes.
Defines operational speed and performance efficiency, determining how many instructions a processor can execute per second.

System Bus
Definition: A communication system that transfers data between different components of a computer, facilitating coherent system function.
Types of Buses:

• Address Bus: Carries memory addresses from the processor to other components, crucial for memory access.

• Data Bus: Carries actual data to/from memory and I/O devices, acting as the main channel for data flow during processing.

Instruction Cycle
Overview: The sequence of steps for processing an instruction typically includes:

1. Fetching: Retrieve instruction from memory.

2. Decoding: Interpret the fetched instruction.

3. Executing: Perform the specified operation.

4. Write Back: Store the result back in memory or a register.
   Cycle Repetition: The cycle is repeated for subsequent instructions as the CPU processes a program continuously, optimizing performance through pipelining and parallel operation where