Hardware Organization of a System

You may be wondering why we care about binary and number representation. It turns out that understanding the relationship between bits, bytes, data representation, and integer calculations is crucial because all modern computer hardware is fundamentally built on the binary system, where bits (binary digits) are the smallest unit of data. Whether it's representing simple integers or complex data structures, these bytes are processed and manipulated by the CPU, specifically within its Arithmetic Logic Unit (ALU) for calculations and its Control Unit (CU) for orchestrating the sequence of operations. Let's dive into the architecture of these systems!

The von Neumann Architecture

The von Neumann architecture lays out the basic blueprint for constructing a functional computer system. It consists of four main subsystems: the arithmetic logic unit (ALU), the control unit (CU), memory, and input/output (I/O) interfaces. All these are interconnected by a system bus and operate cohesively to execute programs.

Von Neumann Architecture.svg
By Kapooht - Own work, CC BY-SA 3.0, Link

Here's a description of each component:

Central Processing Unit (CPU):

  • Arithmetic Logic Unit (ALU): Think of the ALU as a chef in a kitchen. Just as a chef precisely follows recipes to prepare a variety of dishes (arithmetic and logical operations), the ALU follows instructions to perform various calculations and logical decisions. Arithmetic operations include basic computations like addition, subtraction, multiplication, and division. Logical operations involve comparisons, such as determining if one value is equal to, greater than, or less than another.
  • Control Unit (CU): The CU can be likened to an orchestra conductor. An orchestra conductor directs the musicians, ensuring each plays their part at the right time. Similarly, the CU directs the operations of the computer, ensuring each process happens in the correct sequence and at the right time. The CU fetches instructions from memory, decodes them to understand the required action, and then executes them by coordinating the work of the ALU, memory, and I/O systems.
  • Registers: These are small, fast storage locations within the CPU used to hold temporary data and instructions. Registers play a key role in instruction execution, as they store operands, intermediate results, and the like. Registers are like the small workbenches in a workshop, where tools and materials are temporarily kept for immediate use. These workbenches are limited in size but allow for quick and easy access to the tools (data and instructions) that are needed right away.

Memory:

  • Primary Memory: This includes Random Access Memory (RAM) that stores data and instructions that are immediately needed for execution. RAM is volatile, meaning its contents are lost when the power is turned off. Think of RAM as an office desk. Items on the desk (data and programs) are those you are currently working with. They are easy to reach but can only hold so much; when the work is done, or if you need more space, items are moved back to the storage cabinets (secondary storage).
  • Cache Memory: Located close to the CPU, cache memory stores frequently accessed data and instructions to speed up processing. It is faster than RAM but has a smaller capacity. Cache memory is like having a small notepad or sticky notes on the desk. You jot down things you need frequently or immediately so you can access them quickly without having to search through the drawers (RAM) or cabinets (secondary storage).

Input/Output (I/O) Mechanisms:

  • Input Devices: These are peripherals used to input data into the system, such as keyboards, mice, scanners, and microphones. These are like the various ways information can be conveyed into a discussion or meeting – speaking (microphone), writing (keyboard), or showing diagrams (scanner).
  • Output Devices: These include components like monitors, printers, and speakers that the computer uses to output data and information to the user.These are akin to how information is presented out of a meeting – through a presentation (monitor), printed report (printer), or announcement (speaker).
  • I/O Controller: This component manages the communication between the computer's system and its external environment, including handling user inputs and system outputs.

Storage: Storage can be thought of as a special case of input/output devices.

  • Secondary Storage: Unlike primary memory, secondary storage is non-volatile and retains data even when the computer is turned off. Examples include hard disk drives (HDDs), solid-state drives (SSDs), and optical drives (like CD/DVD drives). This is like the filing cabinets in an office where documents are stored long-term. Unlike the items on the desk, these files remain safe even if the office closes for the day (computer is turned off).
  • Storage Controllers: These are akin to the office clerks who manage the filing cabinets, responsible for filing away documents (writing data) and retrieving them when needed (reading data).

❓ Test Your Knowledge

Consider a simple computer task such as opening a music file from a hard drive and listening to it it. Explain how each of the following components of a computer contributes to this task: ALU, Control Unit, Registers, Primary Memory, Cache Memory, Input/Output Mechanisms, and Secondary Storage. Include the roles they play and how they interact with each other during the process.

The Fetch-Execute Cycle

The fetch-execute cycle is the process by which a computer carries out instructions from a program. It involves fetching an instruction from memory, decoding it, executing it, and then repeating the process. This cycle is the heartbeat of a computer, underlying every operation it performs. This video walks you through the fetch-execute cycle:

Here's a summarized overview of the steps involved in this cycle:

  1. Fetch:

    • The CPU fetches the instruction from the computer's memory. This is done by the control unit (CU) which uses the program counter (PC) to keep track of which instruction is next.
    • The instruction, stored as a binary code, is retrieved from the memory address indicated by the PC and placed into the Instruction Register (IR).
    • The PC is then incremented to point to the next instruction in the sequence, preparing it for the next fetch cycle.

  2. Decode:

    • The fetched instruction in the IR is decoded by the CU. Decoding involves interpreting what the instruction is and what actions are required to execute it.
    • The CU translates the binary code into a set of signals that can be used to carry out the instruction. This often involves determining which parts of the instruction represent the operation code (opcode) and which represent the operand (the data to be processed).

  3. Execute:

    • The execution of the instruction is carried out by the appropriate component of the CPU. This can involve various parts of the CPU, including the ALU, registers, and I/O controllers.
    • If the instruction involves arithmetic or logical operations, the ALU performs these operations on the operands.
    • If the instruction involves data transfer (e.g., loading data from memory), the necessary data paths within the CPU are enabled to move data to or from memory or I/O devices.
    • Some instructions may require interaction with input/output devices, changes to control registers, or alterations to the program counter (e.g., in the case of a jump instruction).

  4. Repeat:

    • Once the execution of the instruction is complete, the CPU returns to the fetch step, and the cycle begins again with the next instruction as indicated by the program counter.

This cycle is fundamental to the operation of a CPU, enabling it to process instructions, perform calculations, manipulate data, and interact with other components of the computer system. The speed at which this cycle operates is a key factor in the overall performance of a computer.

❓ Test Your Knowledge

In a typical fetch-execute cycle of a CPU, describe the specific role of the Program Counter (PC) and the Instruction Register (IR). How would the cycle be affected if the PC does not increment correctly after fetching an instruction?