Fundamentals of computer organisation and architecture

The internal components of a computer normally include CPU, components within a CPU, main memory, buses and I/O controllers. The following block diagram shows how different components of a computer working together as a functional system.

Objective: understand the role of the following components and how they relate to each other:

• Processor

  The processor is made up by the control unit, the arithemetic logic unit(ALU) and several registers.

• The control unit
  • The control unit controls and coordinates all operations.
  • It works through a cycle of fetch, decode, and execution.
    • fetch operation fetches the next instruction from the main memeory
    • decode operation outputs what the ALU needs to perform on the data
    • execution operation causes the instruction to be carried out
  • The role of control unit can be summarised as below:
    • To fetch / decode / execute instructions;
    • To synchronise operation of processor;
    • To marshal/control operation of fetch-execute cycle;
    • To send control signals/commands to other components of fetch-execute cycle;
• Registers
  • Registers are small but very fast memory to hold data before and after being processed by the CPU.
  • All ALU operations are accomplished within registers.
• ALU

  ALU performs addition, subtraction, multiplication, division, comparison(logical ops), and bitwise operations (more later).

• Memory
  • Memory is used to hold temporary instructions and data for manipulation while the system is running.
  • When a block of code or data that is held in memory, it is directly accessible to the CPU for manipulation.
  • Memory is organised in units called words. A word is the maximum number of bits a CPU can process in a single instruction. Typical word size are 8, 16, 32, or 64 bits.
  • Each word has its own address in the memory.

Objectives: Understand the need for, and means of, communication between components. In particular, understand the concept of a bus and how address, data and control buses are used.

Components communicate with each other using buses. A bus is a set of parallel wires connecting different coomponents. The processor is connected to the main memory by three separate buses. The three types of buses:

• Address bus
  • Address bus is a pathway or a set of parallel wires that carries the location of the data to be read from or written to. An address bus is one directional only - from the processor to the memory or an I/O controller.
  • The number of lines for the address bus determines the maximum number of bits it can carry, and in turn determines the maximum possible memory capacity of the computer. To understand this, think if the postal office only allows 3 digits for house numbers, then the maximum addressable houses will be 999. So, if the computer has a 32-bit bus, the maximum addressable memory locations will be 2³², which is 4GB, assuming each memory location stores on byte of data.
• Data bus
  • Data bus is a bi-directional pathway or wires that carries data or instructions between computer components.
  • The width of data bus is a key factor in determining the overall computer performance. Typical data bus is 8, 16, 32 or 64 bits wide. If a computer has a word size of 32 but with a 16-bit data bus, then the data bus has to fetch the word twice from the main memory.
• Control bus
  • Control bus is a bi-directional pathway that carries command, timing and specific status information among components. The following are some of the control information a control bus may carry:
    • write to mempry
    • read from memory
    • write to I/O port
    • read from I/O port
    • request for data bus
    • grant for data bus
    • sync clock
    • reset all components

• I/O controllers
  • An I/O conyroller is a device (an electronic circuit board) that manages the communication between the processor and the I/O device.
  • Each I/O device has a separate controller which connects to the control bus.
  • I/O controllers receive input and output requests from the pocessor, then send device specific control signals to the device.
  • I/O controllers manage the data flow from and to the device.

Objectives: Be able to explain the difference between von Neumann and Harvard architectures and describe where each is typically used.

• arithmetic logic unit: A circuit inside a CPU performs the following functions:
  • ADD, SUBTRACT, MULTIPLY, DIVIDE
  • Arithmetic(signed bit patterns) and logic shift(unsigned bit patterns) on instructions within a register
  • Performs logical operations such as AND, XOR, OR, NOT.
• control unit:
  • It controls and coordinates the activities within the CPU and control the flow of data in and out of CPU.
  • It takes an instruction and decodes it into opcode and operands.
  • It manages the execution of the decoded instrcutions, fetches data from location and stores the resultst in memory or registers.
• clock:
  • The system clock is used to synchronise CPU operations.
  • It generates series of signals of alternating 0 and 1.
  • The CPU takes at least one cycle of time (from 0 to 1 or from 1 to 0) to perform one instruction.
• general-purpose registers:
  • Registers are very fast and small memory inside CPU to hold data before, during and after being processed by the CPU
  • Typically, a CPU can have up to 16 registers
  • Some CPUs only have one general purpose register, called accumulator.
• dedicated registers, including:
  • program counter(PC): a special purpose register that holds the address of the next instruction.
  • current instruction register(CIR or IR): holds the currently being executed instruction.
  • memory address register(MAR): holds the memory address of an instruction to be fetched or written to.
  • memory buffer register(MBR): a.k.a. memory data register(MDR), holds the data fetched from or written to memory.
  • status register(SR): holds the bits to indicate the results of the execution of an instruction, such as oervflow, negative, zero or carry over.

When each instruction of a program is being processed by the CPU, it goes through the cycle of three stages:

1. fetching: retrieves a program instruction from its memory
2. decoding: determines what actions the instruction requires
3. executing: carries out the decoded actions

Registers' roles in the fetch-decode-execution cycle can be illustrated in the following flowchart.(You should get familar with the flowchart).

Fetch:

• contents of PC transferred to MAR
• address bus used to transfer this address to main memory
• contents of addressed memory location moved into the MBR by the data bus increment PC

Decode:

• decode instruction held by the CIR by the control unit
• instruction split into opcode and operand

Execute:

• if necessary,data is fetched
• the opcode identifies the instruction to execute
• execute instruction by relevant part of processor
• result stored in accumulator

• Processor instruction set means “all the instructions supported by its hardware”
• different processors have different instruction sets
• typical instruction set includes the following common operations:
  • LOAD, STORE - data handling and memory operations
  • ADD, SUBTRACT, DIVID, MULTIPLY etc - arithmetic operations
  • >, <, = etc - comparison operations
  • AND, OR, XOR, NOR, NOT etc - logical operations
  • Control flow operations - conditional or unconditional
  • Logical shifts
  • Halt

• machine instruction is made of two parts:
  • opcode: the operation to be performed, and a 2-bit code indicate addressing mode used for the operation
  • operands: the value, memory address or register to be operated on

• machine instruction is made of two parts:
  • opcode: the operation to be performed
  • a 2-bit code indicate addressing mode used for the operation
• addressing modes
  • immediate addressing: the operand is the actual value to be operated on.
  • direct addressing: the operand holds the memory address of the value to be operated on.

Suppose that the opcode 010 means LOAD and the addressing mode 1 indicates direct addressing.
Describe the operation that will be performed by the following machine code instruction:
0101 1100

The above instruction means: LOAD the value from the memory location at 12.

barcode reader

• what is barcode?
  • you have seen barcodes appear on all products you buy, and you have seen barcodes being scanned at grocery checking out point. Barcodes have been used to keep track of anything accurately since 1970s. There are two types of barcodes:
    • linear barcode (1D): the ones with vertical stripes of various thickness.
    • 2D barcode: the quick response (QR) code can hold more information than a linear barcode. You normally seen those on train tickets, flight tickets, and concert tickets. Smartphones have apps to allow users to scan QR code to take them to a website or other informations.
• barcode reader
  • There are four types of barcode readers:
    1. laser scanners
       • Often seen in supermarket checkout counters. They use laser beams as the light source to digitise the code. The intensity of the light reflected back from the light source and generates a waveform that is used to measure the widths of the bars and spaces in the barcode. Dark bars in the barcode absorb light and white spaces reflect light so that the voltage waveform generated is an exact duplicate of the bar and space pattern in the barcode. This waveform is decoded by the scanner in a manner similar to the way Morse code dots and dashes are decoded.

         

    2. Camera-based readers
       • Camera based readers that use a small video camera to capture an image of a barcode. The reader then uses sophisticated digital image processing techniques to decode the barcode.
    3. Charge-couple Devices(CCD) readers
       • CCD (Charge Coupled Device) readers use an array of hundreds of tiny light sensors lined up in a row in the head of the reader. Each sensor measures the intensity of the light immediately in front of it. The important difference between a CCD reader and a pen or laser scanner is that the CCD reader is measuring emitted ambient light from the barcode whereas pen or laser scanners are measuring reflected light of a specific frequency originating from the scanner itself.– from http://www.taltech.com/
    4. Pen-type readers
       • Pen type readers consist of a light source and a photo diode that are placed next to each other in the tip of a pen or wand. To read a barcode, you drag the tip of the pen across all the bars in a steady even motion. The photo diode measures the intensity of the light reflected back from the light source and generates a waveform that is used to measure the widths of the bars and spaces in the barcode.

digital camera

• digital cameras use a grid of tiny light sensors to measure the amount of light passing through the lens when the shutters are open.
• each sensor measures the colour and brightness of the each pixel.
• each sensor turnes the light into electricity and the amount of charge is stored as binary data.
• the binary data are then recorded onto the camera's memory card.
• images then can be produced using appropriate image software.

laser printer

• A laser printer uses powdered ink, called toner.
• A computer sends data to be printed to a laser printer and the laser printer's onboard circuit works out how to print the data.
• The printer generates a bitmap of the data.
• A photoreceptor drum is given a positive charge spread uniformly across its surface.
• the circuit activates the laser to make it draw the image of the page onto the drum. The laser beam doesn't actually move: it bounces off a moving mirror that scans it over the drum. Where the laser beam hits the drum, it erases the positive charge that was there and creates an area of negative charge instead. Gradually, an image of the entire page builds up on the drum: where the page should be white, there are areas with a positive charge; where the page should be black, there are areas of negative charge.
• An ink roller touching the photoreceptor drum coats it with tiny particles of powdered ink (toner). The toner has been given a positive electrical charge, so it sticks to the parts of the photoreceptor drum that have a negative charge.
• A sheet of paper from a hopper on the other side of the printer feeds up toward the drum. As it moves along, the paper is given a strong positive electrical charge.
• When the paper moves near the drum, its positive charge attracts the negatively charged toner particles away from the drum. The image is transferred from the drum onto the paper but, for the moment, the toner particles are just resting lightly on the paper's surface.
• The inked paper passes through two hot rollers (the fuser unit). The heat and pressure from the rollers fuse the toner particles permanently into the fibers of the paper.
• Laser printer is best suited for text printing and for photo-realistic printing, it is best to use inkjet printers.

RFID

• RFID - Radio Frequency Identification.
• Used for tracking animals, bank cards, passports and other products.
• It can be read remotely, up to 300 metres away.
• RFID contains a tiny microchip transponder and an antena.
• Very small in size, therefore can be implanted in animals.

• hard disk
• optical disk
• solid-state disk (SSD) SSD = NAND flash memory + a controller that manages pages, and blocks and complexities of writing. Based on floating gate transistors that trap and store charge. A block, made up of many pages, cannot overwrite pages, page has to be erased before it can be written to but technology requires the whole block to be erased. Lower latency and faster transfer speeds than a magnetic disk drive.