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How the CPU Works

How the CPU Works. The “Little Man Computer” Version. Download the tutorial. You can download your own copy of the Little Man Computer simulation from the University of Hertfordshire Start the download now Save it to your hard drive We will use it shortly

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How the CPU Works

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  1. How the CPU Works The “Little Man Computer”Version

  2. Download the tutorial • You can download your own copy of the Little Man Computer simulation from the University of Hertfordshire • Start the download now • Save it to your hard drive • We will use it shortly • The link: http://www.herts.ac.uk/lis/ltdu_temp/ltdu/projects/mm5/# • You can access this link directly from my website

  3. The Little Man Computer

  4. LMC Instruction Codes • LOAD instruction – op code 1 • The Little Man goes to the mailbox specified • Reads the three-digit number in the mailbox • Puts that into the Calculator • Note: • The number in the mailbox is unchanged • The number in the calculator is replaced by the new number

  5. LMC Instruction Codes • STORE instruction – op code 2 • The Little Man goes to the Calculator and reads the number there • He writes the number down on a slip of paper and puts it into the mailbox specified in the instructions • Note: • The number in the Calculator does not change • The number in the Mailbox is replaced with the new value

  6. LMC Instruction Codes • ADD instruction – op code 3 • The Little Man walks over to the mailbox address specified in the instructions • He reads the number and adds this number to the number already in the calculator. • Note: • The number in the mailbox is unchanged

  7. LMC Instruction Codes • SUBTRACTION – op code 4 • Same as the ADD instruction, except the number is subtracted • For this model we will assume that the Little Man handles negative numbers correctly, though we will not deal with it here. • Note: • The number in the mailbox is left unchanged

  8. LMC Instruction Codes • INPUT (or READ) – op code 5, “address” 00 (the 00 is ignored for this special instruction code) • The Little Man gets a slip of paper in from the in-basket • He types this number into the calculator • Each INPUT instruction handles only 1 slip of paper • The Little Man will ignore the address portion of this code

  9. LMC Instruction Codes • OUTPUT (or PRINT) – op code 6, “address” 00 (the 00 is ignored for this special instruction code) • The Little Man walks to the calculator and writes down the number on a slip of paper • This slip is put in to the out basket • Each OUTPUT instruction places 1 piece of paper in the out basket • The Little Man will ignore the address portion of this code

  10. LMC Instruction Codes • COFFEE BREAK (or HALT) – op code 7 • The Little Man takes a break. • He will ignore the address portion of the instruction

  11. A Simple Program • A few items • We need to store the instructions somewhere • We need a method to tell the Little Man where to find the particular instructions that he is supposed to perform at any given time

  12. A Simple Program • Assumed Solutions • Assume that the instructions are stored in the mailbox starting at mailbox number 00 • The Little Man will perform instructions by looking at the value in the instruction location counter and execute the instruction found in the mailbox whose address has that value

  13. A Simple Program • Assumed Solutions • Each time the LM completes and instruction, he will increment the Instruction Location Counter and do the instruction specified by the counter

  14. A Simple Program • Look at a program where the USER uses the Little Man Computer to add two numbers. • The User will place two numbers in the in basket • The sum of the two will appear as a result in the out basket

  15. A Simple Program • INPUT 500 • The first step is to give the LM access to the data • This instruction has the LM read the first number from the in basket in to the calculator

  16. A Simple Program • STORE 99 299 • It is not possible for the LM to simply read another number into the calculator. • It would destroy the first number • We must save the first number somewhere • Mailbox 99 is chosen so that we don’t have any conflicts with other data or programs • Discuss this

  17. A Simple Program • INPUT 500 • The LM reads the second number into the calculator • ADD 399 • We do not need to save the 2nd number in the example, but we could have. • The 1st number from mailbox 99 is added to the 2nd number in the calculator

  18. A Simple Program • OUTPUT 600 • The LM puts the result in to the out basket • COFFEE BREAK 700 • The program terminates • Note: • These instructions are stored sequentially starting from mailbox 00. • This is so we could run the program again

  19. A Simple Program • Code Summary Mailbox Instruction Step code description 00 500 INPUT 01 299 STORE DATA 02 500 INPUT 2ND # 03 399 ADD 1ST # TO it 04 600 OUTPUT RESULT 05 700 STOP 99 DATA

  20. Run the CPU Simulator • Now you try it. • Do the sample program included with the simulation • Write your own program (and show me that they work) • Write a program to add 3 numbers • Write a program that correctly calculates x + y + z – a – b (Ex. 1 + 2 + 3 – 4 – 5 = -3) • For the bold and intelligent ones: • Write the last program only using 2 mailboxes!

  21. How the CPU Works A More Complex View

  22. Overview • We saw in the “Little Man Computer” a simplified version of how a CPU works. • Memory, the Control Unit, the Accumulator (calculator) were all located in the same area • Decimal system used for communication of instructions • Memory was limited to 100 “mailboxes”

  23. CPU Components • 3 Major Parts • Arithmetic/Logic Unit (ALU) • Control Unit (CU) • Memory • The ALU & CU are known as the Central Processing Unit

  24. CPU Memory ALU Control UnitProgram Counter I/O Block Diagram of System

  25. Direct Comparison of LMC & CPU ControlUnit ALU I/OInterface Memory ProgramCounter

  26. CPU Memory ALU Control UnitProgram Counter I/O Arithmetic/Logic Unit (ALU) • The component where data is held temporarily • Calculations occur here • It knows how to perform operations such as ADD, SUB, LOAD, STORE, SHIFT. • It knows the commands that make up the machine language of the CPU • It is the calculator

  27. CPU Memory ALU Control UnitProgram Counter I/O Control Unit • Interprets and controls the execution of instructions • This control corresponds to the “fetch-execute” cycle • The fetch cycle gets the instructions • The execute cycle does the work specified in the instruction Note

  28. CPU Memory ALU Control UnitProgram Counter I/O Registers • “A Register is a single, permanent storage location within the CPU used for a PARTICULAR, defined purpose.” • “A register is used to hold a binary value temporarily for storage, for manipulation, and/or for simple calculations.” • Registers have special addresses

  29. The End…

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