CS 177 Week 13 Recitation Slides

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CS 177 Week 13 Recitation Slides
Inside The Von Neumann Computer
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Announcements

• REMINDER Project 4 due Wednesday Dec 3rd right
  after Thanksgiving Break. Plan accordingly!
• Evening consultant hours next week on Monday and
  Tuesday only (Tuesday will only have one TA
  present)
• Final recitation sessions will be the week before
  finals
• Review of course material
• Recitation evaluations will be completed

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Project 4 Item Drop Down Menu

• Some students are confused by how to set up the
  drop down menu to select the course.
• Below, you will find the code you should use to
  create the drop down menu for project 4

ltselect id"course" size"4" onchange"choose_cour
se()"gt  ltoption value"1" selectedgtCS 177
Programming with Multimedia Objectslt/optiongt 
ltoption value"2"gtCS 180 Programming Ilt/optiongt 
ltoption value"3"gtCS 110 Introduction to
Computerslt/optiongt  ltoption value"4"gtCS 182
Foundations of Computer Sciencelt/optiongtlt/selectgt

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Project 4 Item For Loops

• Some students were confused when seeing a for
  loop used in step 8 of the project spec since we
  only covered while loops in the course.
• For loops are an alternative to while loops that
  can be used in the same way but are coded with
  different syntax
• The general form of a for loop is as follows

FOR (INITIALIZATION BOOLEAN_TEST UPDATE)
STATEMENTS_EXECUTED_AS_LONG_AS_TRUE

• Initialization set default value for variables
  or counters
• Boolean_test same as in the while loop. Loop
  will execute as long as this test results in
  true
• Update update variables or counters to prevent
  infinite loops.

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For Loop vs. While Loop

• Observe the following example for how to
  implement a simple loop to print all numbers
  between 1 and 10 inclusive

For loop
While loop

• var count 1
• while (count lt 10)
• document.write(Count is count)
• count

• for (var count 1 countlt10 count)
• document.write(Count is count)

• Note how the for loop achieves the same
  functionality with less code

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Project 4 Item Dynamic Table

• Many students have noted that the dynamic table
  for project 4 cannot be created in the same way
  as in project 2. Particularly, how can you add
  columns dynamically to the table when we do not
  know how many we will need in advance?
• Lets say you want to create the top row in the
  table with the items to be graded in the
  specified course. The following code will achieve
  this

table document.getElementById("table_id") new_r
ow table.insertRow(0) new_row.innerHTML
lttdgtlt/tdgt var num0 while (num lt
itemsArray.length) new_row.innerHTML
lttdgtitemsArraynumlt/tdgt num
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• OTHER QUESTIONS???

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CPU Subunits and Datapath

• What is the CPU?
• Brain of the computer
• Executes instruction obtained from memory and
  writes results back to memory
• Program instructions are in the machine language
  of that CPU
• High-level instructions must be translated into
  machine language before executing

von Neumann architecture
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CPU Subunits

• Arithmetic Logic Unit (ALU) performs the
  operations on data (addition, subtraction, bit
  manipulations)
• Registers memory locations built into the CPU
• data transferred between registers and main
  memory via a bus
• Control Unit (CU) circuitry in charge of
  fetching data and instructions from main memory,
  as well as controlling the flow of data between
  registers and the ALU

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CPU Datapath Cycles

• Datapath - path that data follows within the CPU,
  traveling along buses from registers to the ALU
  and then back to registers
• Datapath cycle - a single rotation around the
  CPU datapath
• CPU speed - the number of CPU cycles per second
• e.g.., a 3.0GHz CPU can perform 3.0 billion
  cycles per second
• Not all computers are created equal!
• CPUs have different instruction sets and one may
  be able to complete more complex tasks in a
  single cycle
• so, a slower CPU with a richer instruction set
  may complete some tasks faster

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CPU and Main Memory

• CPU is connected to the computers main memory
  via a bus
• main memory is comprised of a large number of
  memory locations accessed by its address
• main memory stores all active programs and data
• data and instructions are copied to registers and
  the results are then copied back to main memory

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Main Memory Data Transfer

• Control unit processes program instructions and
  identifies data needed to carry out tasks
• Data is then fetched from main memory along the
  main memory bus and copied into registers. This
  MUST occur before the data can be operated on by
  the ALU
• The data transfer takes much longer than a single
  datapath cycle due to the distance that must be
  covered
• Modern CPUs compensate by fetching multiple
  instructions at once

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Stored-Program Computer

• A stored-program computer allows tasks to be
  represented as instructions stored in main memory
  along with data
• control unit fetches, decodes, and executes those
  instructions
• Each CPU has its own machine language
  instructions represented as binary strings where
  the bits correspond to hardware settings
• These encoded instructions will tell the computer
  what simple commands to execute on the data (e.g.
  add, subtract, load etc)
• Why is this approach better than the previous
  examples?

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Examples of Machine Language Instructions

• 1st two instructions select ALU operation and
  registers to operate on
• Next 3 instructions control flow of data
  between main memory and datapath
• Last instruction mark the end of instruction
  sequence

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Fetch-Execute Algorithm

• Essentially the algorithm can be broken down into
  the following steps
• Control unit fetches machine language
  instructions from memory
• It then decode the instruction and interprets its
  meaning
• Next it executes the required CPU datapath cycle,
  
• Finally, it moves on to the next instruction and
  repeats the previous steps

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Fetch-Execute Algorithm

• In the above algorithm, how does the control unit
  know when the sequence of instructions is over?
• The HALT instruction indicates end of sequence
• How does the control unit know in a real
  computer?
• The operating system is responsible for keeping
  track of the programs location

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The Role of I/O Devices

• I/O devices enable the user to interact with the
  computer
• For the first programmable computers, the user
  triggered the single programs execution
• User entered program instructions and data into
  main memory via input devices
• User instructed the CPU to fetch program
  instructions and execute them in sequential order
• User viewed results by sending the contents of
  the memory to an output device
• Most modern computers allow for multitasking
• separate portions of main memory for program
  instructions and data
• during program switches, CPU must save the state
  of the current program and locate the portion of
  memory for the new program (handled by OS)

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Machine vs. Assembly Language

• machine code is nearly impossible to understand
  and debug (difficult to program)
• assembly languages simplify programming
• assembly languages allow for words to substitute
  for bit patterns
• also, allow for variable names instead of
  numerical addresses

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QUESTIONS???