The CPU

• We are going to look at a very simple machine in this section.
• In our simple machine, we will have a single register, that we will call the accumulator.
  • There were machines built on this model.
  • Most operations involve the accumulator in some way.
  • To program c=a+b
    • Load the accumulator with M[a]
    • Multiply the accumulator by M[b]
    • Store the accumulator at M[c]
• Our instruction set.
  • ADD addr
  • SUB addr
  • MPY addr
  • DIV addr
  • LOAD addr
  • STORE addr
  • Assume memory holds 32 bytes. Write a program that converts a temperature from Fahrenheit to Celsius.
    • Assume that the Temp if F is stored at M[1F]
    • Assume 32 is stored at M[1E]
    • Assume 9 is stored at M[1D]
    • Assume 5 is stored at M[1C]
    • Assume we want to store the result at M[1B]

      LOAD 31
      SUB  30
      DIV  29
      MPY  28
      STORE 27
      	   

  • We will need to encode our assembly language instructions for the machine.
  • We can create our own code here, but if we match the book, the examples work out.
  • In other Architectures, there is more reason for the encoding. (Arch II , Spring 04)

  • Operation	Encoding
    ADD	000
    SUB	001
    MPY	010
    DIV	011
    LOAD	100
    STORE	101

  • This encoding is called an op-code, and is the machine language representation of the instruction.
  • Note, the instructions also need to carry an address, this is called the operand of the instruction, and we will encode in 5 bits.
  • We will encode our instructions (op code and address) in 8 bits.
  • As you know, the first list of instructions were assembly language
  • The process of turning assembly language into machine language is know as assembly
  • Hand assemble the FtoC program above

  • Assembly           Machine    Hex
    LOAD 31            100 11111   9F
    SUB  30            001 11110   3E
    DIV  29            011 11101   7D
    MPY  28            010 11100   5C
    STORE 27           101 11011   AB
    

  • What other instructions might we want to add?
  • How many more can we add?
  • Can we use a larger address space?
• A simple Machine
  • 
• ACC - the ACCumulator
• ALU - the Arithmetic Logic Unit
• IR - Instruction Register
• MAR - Memory Address Register
• MDR - Memory Data Register
• PC - Program Counter
• Which parts are registers? (Sequential Circuits)
• Which parts are combinatonial circuits?
• How big are each of the registers?