Unit 5

• A system block diagram is?


  A graphical representation of a system, project, or scenario
• Concept of registers

  ...

  • Small, permanent storage locations within the CPU, used for a particular purpose
  • Manipulated directly by the control unit
  • Wired for specific function
  • Size in bits/bytes
  • Holds data, address, or instruction
• Use of registers


  It holds data that may be needed quickly or frequently, and stores information about the status of the CPU and running programs
• General purpose registers


  They are user-visible registers that hold intermediate results or data values. There are typically a several dozen in current CPUs

  
  

  The functions of the registers can be modified by programmers to suit their program requirements
• Special purpose registers


  User invisible (register functionality cannot be changed)

  • Program counter register (PC) - keeps track of the memory addresses of the next instruction to be executed
  • Instruction register (IR) - stores instructions fetched from the memory
  • Memory address register (MAR) - holds the memory address of the data that needs to be accessed or stored
  • Memory data register (MDR) - holds the data that is being transferred to or from the computer's memory
  • Status register - holds flags indicating the outcome of arithmetic and logic operations
• Register operations

  ...

  • Stores values from other locations (registers and memory)
  • Addition and subtraction
  • Shift or rotate data
  • Test contents for conditions such as zero or positive
• Operation of memory

  ...

  • Every location has its own address
  • The address of an instruction is copied to the MAR which finds the location in the memory
  • CPU will determine if it is a store or retrieval
  • Transfer takes place between the MDR and memory
  • MDR is a two-way register (data travels in both directions), MAR is not
• Relationship between MAR, MDR and Memory

  MAR and MDR facilitate CPU communication with the memory, which allows the CPU to retrieve and store data to and from specific memory locations
• Memory capacity

  Determined by 2 factors:

  1. Number of bits in the MAR - 2^k where k = width of the register in bits
  2. Size of the address portion of the instruction - 4 bits allow 16 locations, 8 bits allow 256, 32 bits allow ~4GB
• Types of RAM

  DRAM (dynamic RAM):

  • Most common, cheap, uses less power/heat/space
  • Volatile - must be recharged thousands of times/sec

  SRAM (static RAM):

  • Faster but more expensive than DRAM, volatile, small amounts are often used in cache memory for high-speed memory access
• Non-volatile memory


  ...

  • ROM
  • EEPROM - Electrically Erasable Programmable ROM
  • Flash memory - faster but more expensive than disks, slow rewrite time compared to RAM
• Fetch-execute-cycle

  Two-cycle process because both instructions and data are in memory

  • Fetch - decode or finds instruction, loads from memory into register and signals ALU
  • Execute - moves/transforms data, performs operations that instruction requires
• Load fetch/execute cycle

  PC --> MAR = Transfers the address from PC to MAR

  
  

  MDR --> IR = Transfers instructions to IR

  
  

  IR[address] --> MAR = Address portion of the instruction loaded into MAR

  
  

  MDR --> A = Actual data copied into accumulator

  
  

  PC + 1 --> PC = Program counter incremented
• Store fetch/execute cycle

  PC --> MAR = Transfers the address from PC to MAR

  
  

  MDR --> IR = Transfers instructions to IR

  
  

  IR[address] --> MAR = Address portion of the instruction loaded into MAR

  
  

  A --> MDR = Accumulator copies data into MDR

  
  

  PC + 1 --> PC = Program counter incremented
• Add fetch/execute cycle

  PC --> MAR = Transfers the address from PC to MAR

  
  

  MDR --> IR = Transfers instructions to IR

  
  

  IR[address] --> MAR = Address portion of the instruction loaded into the MAR

  
  

  A + MDR --> A = Contents of MDR added to contents of accumulator

  
  

  PC + 1 --> PC = Program counter incremented
• What is bus?

  An electronic pathway through which data can be transferred from one location in the computer system to another

  Groups of electrical or optical conductors for carrying signals from one location to another. Buses can be wires or conductors printed on a circuit board or a line

  4 kinds of signals

  • Data
  • Addressing
  • Control signals
  • Power (sometimes)
• Bus characteristics

  ...

  • Number of separate conductors
  • Addressing capacity
  • Type of control required
  • Defined purpose
  • Features and capabilities
• Bus categorizations

  ...

  • Parallel v.s serial buses
  • Direction of transmission - simplex is unidirectional, half duplex is bidirectional but only one direction at a time, full duplex is bidirectional simultaneously
  • Method of interconnection - point-to-point is a single source to single destination (cables are point-to-point buses that connect an external device)
  • Multipoint bus - broadcast bus or multidrop bus (connect multiple points to one another)
• Parallel v.s serial buses


  Parallel:

  • Expensive and uses lots a space
  • Generally used for short distances (e.g CPU buses & computer motherboards)
  • High throughput because all bits of a word are transmitted simultaneously

  Serial:

  • 1 bit transmitted at a time
  • Single data line pair and a few control lines
  • For many applications, throughput is higher than parallel due to the lack of electrical interference
• Classification of instructions

  ...

  • Data movement (load, store)
  • Arithmetic
  • Boolean logic
  • Bit manipulation instructions
  • Shift and rotate
  • Program control
  • Stack instructions
  • I/O and machine control
• Instructions

  They are directions given to a computer, it causes electrical or optical signals to be sent through specific circuits for processing

  
  

  Instruction set defines the functions performed by the processor, it differentiates computer architecture by the format, number of instructions, use of registers, and addressing