Processor

• Processor
  • Fetches instructions from memory, decodes and executes them
  • Set of instructions is processor specific
  • Instructions include:
    • load value from memory into register
    • combine operands from registers or memory
    • branch
  • All CPU's have registers to store
    • key variables and temporary results
    • information related to control program execution
• Processor Registers
  • Data and address registers
    • Hold operands of most native machine instructions
    • Enable programmer to minimize main-memory references by optimizing register use
    • user-visible
  • Control and status registers
    • Used by processor to control operating of the processor
    • Used by operating-system routines to control the execution of programs
    • Sometimes not accessible by user (architecture dependent)
• User-Visible Registers
  • May be referenced by machine language instructions
  • Available to all programs - application programs and system programs
  • Types of registers
    • Data
    • Address
      • Index
      • Segment pointer
      • Stack pointer
    • Many architectures do not distinguish different types
• Control and Status Registers
  • Program Counter (PC), Contains the address of an instruction to be fetched
  • Instruction Register (IR), Contains the instruction most recently fetched
  • Processor Status Word (PSW)
    • condition codes
    • interrupt enable/disable
    • supervisor/user mode
  • Condition Codes or Flags
    • Bits set by the processor hardware as a result of operations
    • Can be accessed by a program but not altered
    • Examples: positive/negative result, zero, overflow
• Instruction Fetch and Execute
  • Program counter (PC) holds address of the instruction to be fetched next
  • The processor fetches the instruction from memory
  • Program counter is incremented after each fetch
  • Overlapped on modern architectures (pipelining)
• Instruction Register
  • Fetched instruction is placed in the instruction register
  • Types of instructions
    • Processor-memory, transfer data between processor and memory
    • Processor-I/O, data transferred to or from a peripheral device
    • Data processing, arithmetic or logic operation on data
    • Control, alter sequence of execution

Figure 5: Fetch and Execute