The CPU

• Read chapter 2 of Jorgensen
  • Don't get too caught up in x86 details yet.
  • But try to get the terms down and the relationship of the x86 chip to these terms.
• For these notes, we will take a generic look at the CPU
  • The CPU is the "brains" of the computer.
  • But this implies WAY too much intelligence.
  • It is the part that "runs" the program.
• A Register
  • Is a basic computer component.
  • It will store data (as long as the power is on)
  • So, it makes sense that registers are used to store data.
• The Fetch Decode Execute cycle.
  • This is the algorithm that most CPUs run.

  • While not HALT
       Fetch the next instruction from memory
       Decode the instruction and decide what actions to take
       Execute the instruction (or take the actions)
           

• There are two types of registers
  • General Purpose Registers
    • These registers can hold any data.
    • They are usually controlled by the programmer.
    • But some of them have restricted use.
  • Special purpose registers
    • These registers contain a specific piece of data.
    • They are generally not controlled directly by the program.
• A SISD CPU consists of the following components or functional units.
  • A program counter (PC)
    • This is a special purpose register.
    • This is sometimes called the IP or instruction pointer.
    • It is a special purpose register.
    • It holds the address of:
      • The instruction to be executed at the beginning of the Fetch Decode Execute cycle.
      • The next instruction to be executed at the end of the Fetch Decode Execute cycle.
    • The program counter is usually initialized to zero.
    • The program counter is usually incremented by the size of the current instruction.
    • The program counter is set to a value for jump type instructions.
    • Think of this as a finger pointing to the words a child is reading in a book.
  • An Instruction Register (IR)
    • This is a special purpose register.
    • It holds the instruction that the CPU is currently executing.
    • Other than the instructions that the programmer writes, they have no control over the contents of this register.
    • Think of this as the word the child just read.
  • The General Purpose Register(s)
    • These are registers that the programmer can use as they wish.
    • There may be as few as one
    • Or very many (some power of 2 like 16 (x86) or 32 (mips)
    • Think of these as variables in your program.
    • In x86 these are rax, eax, a, al, rbx, ...
  • The ALU or Arithmetic Logic Unit
    • This contains the logic to
      • do math (add, subtract, multiply, ...)
      • Compare numbers
      • Perform logical operations (and, or, not)
    • These might be very sophisticated or relatively simple.
    • Think of this as a simple calculator with no memory.
  • Memory
    • Not part of the CPU but I need to discuss it now.
    • It is a huge array
    • Indexed by an address.
    • At each location, data is stored.
    • We will discuss this in more detail later.
  • The Memory Management Unit (MMU)
    • This is the interface between memory and the CPU
    • We will discuss memory soon, but
      • The CPU is very fast
      • Memory can be very slow
      • The MMU handles the problems the difference in speed causes.
    • The MMU has two Special Purpose Registers
      • The MAR or memory address register
      • The MDR or memory data register.
    • To read from memory
      • Put the address of the data we want in the MAR
      • Activate memory to read
      • The data we want will show up in the MDR
    • To write to memory
      • Put the address we want to store data to in the MAR
      • Put the data we want to store in the MDR
      • Active memory to write
  • The Control Unit (CU)
    • For now this is an abstraction.
    • It is the thing that decodes the instruction
    • And carries out the execution of the instruction.
  • The Bus
    • The bus connects all components.
• 
• Instructions
  • The CPU executes instructions.
  • Some typical instructions are
    • Load a register from memory
    • Store a register to memory
    • Add the value of two registers and store this in another register.
  • A made up assembly language might look something like this
    • load RD, ADDRESS: load register RD from the data at address ADDRESS
    • store RS, ADDRESS: store the value at RS to the memory location ADDRESS
    • add RD, RS1, RS2: add the contents of register RS1 to the contents of register RS2 and store the result in RD
    • sub RD, RS1, RS2: subtract the contents of register RS2 from the contents of register RS1 and store the result in RD
    • jump ADDRESS: Set the value of the PC to ADDRESS
    • jumpz R, ADDRESS : If the contents of register R is 0, set the PC to ADDRESS
    • jumpn R, ADDRESS : If the contents of register R are less than 0, set the PC to ADDRESS
    • HALT stop the execution of the program.
  • Assume that the following is stored in memory:

  • Address	Value
    0	load R1, 10
    1	load R2, 9
    2	sub R3, R1, R2
    3	jumpn R3, 5
    4	load R1, 9
    5	store R1, 8
    6	halt
    7	0
    8	0
    9	23
    10	35

  • What will happen when this is run?