CS UNIT 4

• The central processing unit (CPU) architecture is the fundamental structure of a computer's processor.
• The Von Neumann architecture is a computer architecture that introduced the concept of the stored program in the 1940s.
• The arithmetic logic unit (ALU) is a component in the processor that carries out all arithmetic and logical operations.
• The control unit ensures synchronization of data flow and programs throughout the computer by sending out control signals along the control bus.
• The system clock produces timing signals on the control bus to ensure synchronization takes place.
• The immediate access store (IAS) holds all data and programs needed to be accessed by the control unit.
• An accumulator is a temporary general-purpose register that stores numerical values at any point in a given operation.
• A register is a temporary component in the processor, which can be general or specific in its use and holds data or instructions as part of the fetch-execute cycle.
• The status register is used when an instruction requires some form of arithmetic or logical processing.
• A flag indicates the status of a bit in the status register; for example, N = 1 indicates the result of an addition gives a negative value.
• The address bus carries the addresses throughout the computer system.
• The data bus allows data to be carried from the processor to memory (and vice versa) or to and from input/output devices.
• The control bus carries signals from the control unit to all other computer components.
• Unidirectional describes a bus in which bits can travel in one direction only.
• Bidirectional describes a bus in which bits can travel in both directions.
• Word is a group of bits used by a computer to represent a single unit.
• Clock cycle refers to clock speeds measured in GHz, the vibrational frequency of the clock which sends out pulses along the control bus or a 3.5GHZ clock cycle which means 3.5 billion clock cycles a second.
• Overclocking is changing the clock speed of a system clock to a value higher than the factory or recommended setting.
• BIOS is a basic input/output system.
• Cache memory is high-speed auxiliary memory that permits high-speed data transfer and retrieval.
• A core is a unit made up of ALU, control unit and registers which is part of a CPU, a CPU may contain a number of cores.
• Dual-core refers to a CPU containing two cores.
• Quad-core refers to a CPU containing four cores.
• Port is an external connection to a computer which allows it to communicate with various peripheral devices; a number of different port technologies exist.
• Universal Serial Bus (USB) a type of port connecting devices to a computer.
• Asynchronous serial data transmission in serial which refers to a single wire being used to transmit bits of data one after the other; asynchronous refers to a sender using its own clock/timer device rather sharing the same clock/timer with the recipient device.
• High-definition multimedia interface (HDMI) is a type of port connecting devices to a computer.
• Video Graphics Array (VGA) is a type of port connecting devices to a computer.
• High-bandwidth digital copy protection (HDCP) is part of HDMI technology, which reduces the risk of software and multimedia piracy.
• The fetch-execute cycle is a continuous loop in which instructions and data are fetched from memory, then decoded, and finally executed.
• The program counter (PC) is a computer's register to store the address of the instruction that is currently being executed.
• The current instruction register is a register used to contain the instruction that is currently being executed or decoded.
• Register Transfer Notation (RTN) is a shorthand notation used to show the movement of data and instructions in a processor, representing the operation of the fetch-execute cycle.

Von Neumann Architecture

• The Von Neumann architecture consists of a single, shared memory for programs and data.
• The control unit directs and coordinates other parts of the computer system.
• The control unit controls operations of the computer system.
• The control unit fetches computer instructions and decodes each instruction.

Arithmetic Logic Unit

• A unit which performs arithmetic operations (+, -, /, *)

• And bit shifting operations

• Performs logic operations (AND, OR, XOR)

• Designed to perform integer calculations

• OPERAND: part of the computer instruction which specifies which data is to be processed

• OPCODE: instruction executed by the CPU

• Small piece of memory

• Part of the processor

• Storing temporary data that is about to or has been processed

• GENERAL PURPOSE REGISTER: holds the temporary data whilst carrying out different operations

• SPECIAL PURPOSE REGISTER: holds the status of the program

• Program Counter: stores the address of the next instruction to be fetched

• MAR: stores memory address where data is to be read from

• MDR: stores data that has just been read from memory

• CIR: stores the instruction that is currently being executed

• IX: used for index addressing

• ACC: is the register where the data is worked on

• Status Register: it's interpreted as independent flags, each flag is set depending on an event

• System Clock

System Clock

• Regulates the rate at which instructions are executed
• CPU requires a fixed number of clock cycles to execute each instruction
• Used to synchronise all operations

Buses

• Address Bus:
  • Uni-directional bus
  • Carries signal related to memory address
  • B/w processor and memory
• Control Bus:
  • Bi-directional
  • Transmits signals b/w control unit and all other components
• Data Bus:
  • Carries data b/w processor and memory
  • Bi-directional bus
  • Used to exchange data

Factors that affect computer performances

• Data Bus Width:
  • Determines the number of bits that can be simultaneously transferred
  • Increasing the width of the data bus increases the number of bits that can be transferred
  • Hence improving processing speed as fewer transfers are needed

Clock speed:

• One F-E cycle is run on each clock pulse

• Dictates the number of instructions that can be run per second

• Faster clock speed = more instructions run per second

• Processing speed increases

• Cache memory: Cache uses SRAM so they have faster access time as there's no need to keep refreshing. Cache memory stores frequently used instructions and data that need to be accessed faster, this improves processor performance.

• Number of cores:

  • Each core processes one instruction per clock pulse
  • More cores= sequence of instructions that can be split more than one instruction executed per clock pulse
  • More cores decrease the time taken to complete the task

• Ports Act as an interface b/w computers and other peripheral devices

USB: has the following benefits:

• Fast data transfer, useful when transferring large files such as video files
• Automatic connection, so usually there's no need to install separate devices
• Industrial standard, all computers have a USB port
• The device may be charged through USB, external devices can be charged while working

HDMI:

• Current standard for modern TVs and monitors
• Very fast data transfer rate
• Improved security, helps prevent piracy by the help of HDCP
• Supports modern digital systems Disadvantages
• Not a very robust connection
• Limited cable length

Video Graphics Array (VGA)

• Simpler technology
• Only one standard is available
• It is easy to split the signals and connect several devices from one source
• The connection is very secure
• Supports a higher refresh rate
• Doesn't support audio Outdated as modern laptops and computers rarely have a VGA port
• Easy to bend the pins when making connections

Fetch-Execute Cycle

• PC holds the address of the next instruction to be fetched
• The address in PC is copied into MAR
• PC is incremented
• The instruction is copied from address stored in MAR to MDR
• Then instruction from the MDR is copied into CIR

Interrupts

• It's a signal from a source/device that tells the processor that its attention is needed
• ex. Printer out of paper, no CD in drive
• Software interrupt: ex. Run-time errors, a running program needs input

How processors handle interrupts:

1. At the start of each F-E cycle the processor checks for interrupts
2. Checks if interrupt flag is set
3. Identifies source of interrupts
4. Checks priority of interrupts
5. If interrupt's priority is high enough the lower priority interrupts are disabled
6. Processor saves current content of registers
7. Processor calls ISR (Interrupt Service Routine)
8. Address of ISR is loaded in PC
9. When servicing the interrupt is complete, processors restores registers
10. Lower priority interrupts are reenabled
11. Processor continues with F-E cycle

Assembly Language

• Assembly language is a low-level language that can be used to directly manipulate hardware, such as reading/writing to specific memory addresses.
• Assembly language is one step above machine code, which is a series of binary numbers (0s and 1s).

Addressing Modes

• Methods of accessing memory are known as addressing modes:
  • Immediate Addressing: the value that is shown is loaded
  • Direct Addressing: the content in the given address is loaded
  • Indirect Addressing: the contents of the contents of the address are loaded
  • Indexed Addressing: the initial address is added with IX to create a new address. The contents in the new address are loaded.
  • Symbolic address: uses a label

Assembler

• Before a program, written in assembly language, it needs to be translated into machine code. The translation is performed by an assembler.
• An assembler translates each assembly language instruction into a machine code instruction.
• Two types:
  • Pass 1:
    • Read the assembly language program one line at a time.
    • Ignore anything not required, such as comments.
    • Allocate a memory address for the line of code.
    • Check the opcode is in the instruction set.
    • Add any new labels to the symbol table with the address, if known.
    • Place the address of labeled instruction in the symbol table.
  • Pass 2:
    • Read the assembly language program one line at a time.
    • Generate object code, including opcode and operand, from the symbol table generated in Pass 1.
    • Save or execute the program.

Explore fundamental CPU architecture concepts. Understand the functions of components like the ALU and Program Counter. Learn about data transmission methods and the role of the BIOS.