Central Processing Unit (CPU) Architecture (Copy)

Von Neumann Model

• The model consists of
  • Central processing unit (CPU or processor)
  • Processer’s direct access to memory
  • Computer memories can store program data
  • Stored programs have instructions that are executed in sequential order

Components of The Processor (CPU)

• Arithmetic Logic Unit (ALU)
  • Carries out arithmetic and logic operations
  • More than one ALU possible
  • One does fixed point operations and the other floating point operations
  • Multiplications and Division
    • Sequences of addition, subtraction, and then left right shifting operations
  • Accumulator (ACC)
    • Temporary register when carrying out ALU calculations
• Control Unit (CU)
  • Reads instruction from memory
  • Address of the location of the instruction
    • Program Counter (PC)
  • Interpreted
  • Signals are generated along the Control Bus
  • CU ensures synchronization of data flow and program instructions throughout the computer
• System Clock
  • Timing signals
  • Control bus ensures vital synchronization
  • Computer will crash without it
• Immediate Access Store (IAS)
  • IAS has the data and programs that the CPU needs to access
  • The program and data is placed n the IAS temporarily
  • IAS are considerably faster read and write than backing store
  • Its’s another name for primary memory – RAM

Registers

• General and specific purpose.
• General
  • Data frequently used by CPU
  • Addresses CPU directly
  • Accumulator as an example
• Specific
  • Specific function within the CPU and hold the program state
• Current instruction register (CIR)
  • Stores the current instruction being decoded and executed.
• Index Register (IX)
  • Index addressing operations (assembly code)
• Memory address Register (MAR)
  • Stores the address of memory location currently being read and written
• Memory data/ buffer register (MDR/ MBR)
  • Data read from memory or data about to be written to memory
• Program Counter (PC)
  • Address where the next instruction is present
• Status Register (SR)
  • Bits can be set or cleared depending on the operation
  • Where there is some arithmetic or logic progressing is required
  • Each bit called a flag
    • Mostly 4 flags
    • Carry flag (C)
      • Set to 1 if a CARRY following an addition operation
    • Negative flat (N)
      • Set to 1 if the result of a calculation yields a negative value
    • Overflow flag (V)
      • Set to 1 if an arithmetic operation results in an OVERFLOW bring produced
    • Zero flag (Z)
      • Set to 1 if the result of an arithmetic or logic operation is ZERO

System Buses

• Parallel transmission component
  • Each wire transmits one bit of data
• Address Bus
  • Carries addresses throughout the computer system
  • Between CPU and memory it is unidirectional
  • So address not caried back to CPU
  • Width of bus is important
    • Wider bus more memory locations addressed directly
  • More bits, more bus width
    • Exponent of 2 for the bits number
• Data Bus
  • Bidirectional
  • To and from CPU to memory and input/ output devices
  • Data can be address, instruction or numerical value.
  • Word means a group of bits which can be considered a single unit.
    • Such as a 64-bit word
  • Larger words can improve overall performance.
• Control Bus
  • Bidirectional
  • Carries signals from CU to all other computer components
  • 8-bit wide
  • Control signals only
  • Clock defines clock cycle
    • • Synchronizes computer operations
    • Control bus transmits timing signals
    • Increasing clock speed will increase processing speed
    • Overall performance may still not be increased
  • Other factors affecting CPU performance
    • Width of address bus and data bus
    • Overclocking
      • Clock speed can be changed by accessing the Basic Input/ Output System (BIOS)
      • Execution of instructions outside design limit can result in unsynchronized operations
      • Overheating of CPU may occur
    • Cache memory
      • Faster access time
      • No need to keep refreshing
      • Stores frequently used instructions
    • Use of different number of cores
      • Improve performance
      • Dual core
      • Quad core
      • Different cores have different channels and can perform accordingly

Computer Ports

• Input and Output devices are connected to computer via ports
• USB Ports
  • Universal Serial Bus (USB)
  • Asynchronous Serial Data Transmission
  • Standard method for transferring data between a computer and a number of devices
  • Four-wired shielded cable.
    • Two wires for power and the earth, and two wires for data transmission.
    • If one USB port for plugging
      • Computer automatically detects that the device is present
      • Automatically recognized
      • Device driver installed
  • Pros
    • Automatically detected and loaded up
    • Fits one way only
    • No incorrect connections
    • Industry standard with considerable support
    • Different transmission rates supported
    • Newer USB standards are backward compatible with older USB standards
  • Cons
    • Present transmission rate is limited to less than 500 megabits per second
    • Cable length about 5 metres
    • Older USB standard may not be supported in future
• High-Definition Multimedia Interface (HDMI)
  • Output both audio and visual
  • High-end definition signals
  • Video Graphics Array (VGA) replaced by it.
  • It allows widescreen format, greater number of pixels and range of colors
  • More data transfer required
  • Some protected against piracy
  • High-bandwidth digital copy protection (HDCP) provides protection against piracy
• VGA (Video Graphics Array)
  • VGA supports only 640 x 480 pixel resolution
  • Refresh rate up to 60 Hz
    • 60 frames per second
  • Analogue
• Pros of HDMI
  • Current standard
  • Fast data transfer
  • Improved security
  • Modern digital systems
• Cons of HDMI
  • Robust connection
  • Limited cable length
  • Five cable/ connection standards
• Pros of VGA
  • Simpler technology
  • One standard available
  • Split the signal and connect multiple devices
  • Connection is very secure
• Cons of VGA
  • Out-dated Analogue
  • Pin bending
  • Cable high grade to avoid distortion of signal

Fetch-Execute Cycle

• Fetch
  • Next instruction fetched from memory address
  • Currently stored in Program Counter (PC)
  • Stored in Current Instruction Register (CIR)
  • PC is increased by 1
  • So next instruction processed
  • Interpreted next part of cycle
• Execute
  • Processor decodes instruction
  • Passes it as control signals to appropriate components
  • Carried out in logical sequence
• Register Transfer Notation (RTN)
  • Describe what is happening.
  • Uses generic/ abstract or concrete/ specific notation
• Use of Interrupts in the Fetch-Execute Cycle
  • Interrupt Register
  • In middle of a cycle, interrupt can tell the system that another work needs to be performed
  • Before the next fetch-execute cycle, interrupt register is checked bit by bit.
  • If the interrupt occurred, it needs servicing, Control transferred to interrupt handler
  • After servicing, the register is reset and contents of register are restored

Interrupts

• Sent by device or software to processor
• Causes
  • Timing signal
  • Input output processes
  • User interaction
  • Software error cannot be ignored
• Interrupt priority
  • Identification of interrupt type
• Interrupts allow computers to carry out many tasks or to have several windows
• Interrupt service routine (ISR) is executed by loading the start address to program counter
• After servicing, the status of the interrupted task is reinstated