Week 1.b
CS3650
01/10 2024
https://naizhengtan.github.io/24spring/

1. Bits, bytes, and ints
2. (continued) Mechanics and admin
3. Crash course of computer organization
------------------------

Recap
 -- course: basics of computer organization + OS
 -- goals & non-goals
 -- basics

1. bits, bytes, and ints

 * bit = binary digit, 0 or 1

   Q: How many bit patterns can a group of 2 bits have?
   [Answer: 2^2 = 4]

   Q: How many bit patterns can a group of 3 bits have?
   [Answer: 2^3 = 8]

   Q: How many bit patterns does a group of n bits have?
   [Answer: 2^n]

 * Digression: any self-respecting CS person must memorize powers of 2

   2^1  = 2
   2^2  = 4
   2^3  = 8
   2^4  = 16
   2^5  = 32
   2^6  = 64
   2^8  = 128
   2^9  = 256
   2^10 = 1024

   [fun fact:
    programmer's day
     -- the 256th of the year (09/12 or 09/13)
     -- 10/24
   ]

 * Q: why bit has only two values?
      does it work for a machine with bits representing three values?

    [Answer: yes! there are ternary computers
      see: https://en.wikipedia.org/wiki/Ternary_computer

      Aside, ternary machines are "optimal" in some sense.
      It provides "optimal coding of numbers".
    ]

 * byte = 8 bits
   - bit is too tiny
   - The term byte is coined by Werner Buchholz (IBM).

   - Q: a bit can represent two numbers, 0 and 1.
     How many numbers can be represent by a byte?
     [Answer: 2^8 = 256]

 * int

   - for *unsigned integer*, a byte can represent: [0, 255]
     -- "unsigned" means "non-negative"

   - for *signed integer*, people use the most significant bit to represent the sign
      -- an analogy: -1 vs. +1
      -- for a byte,
         00000000 = 0
         ...
         01111111 = 127
         ---
         10000000 = V1?
         ...
         11111111 = V2?


      -- Q: V1 and V2, which is smaller? What do you think?
         Also, what're V1 and V2?
        [Answer: 
         for today, V1=-128, V2=-1
         this is called "two's complement".
        ]

 * capacity metric: KB--PB

   2^10  = 1024    ~= 10^3  (Kilo, KB)
   2^20 ~= 10^(3*2) = 10^6  (Mega, MB)
   2^30 ~= 10^(3*3) = 10^9  (Giga, GB)
   2^40 ~= 10^(3*4) = 10^12 (Tera, TB)
   2^50 ~= 10^(3*5) = 10^15 (Peta, PB)
   next are EB, then ZB, then YB

 * memory (DRAM) vs. disk/SSD size

   - Q: if you buy a disk/SSD with 1TB, how many bytes you will get?
     [Answer: 10^12 bytes < 2^40 bytes]

   - Q: if you buy a memory with 16GB, how many bytes you will get?
     [Answer: 16 * 2^30 bytes]

2. (continued) Mechanics and admin
   [from last time]

    c. labs:

      --key piece of the course

      --"Start early"
        "When is early?"
        "Earlier than you think"

      --Regardless, you need to allocate time. 

     --we expect you think through, then ask
       --"Here is my code. It doesn't work. Please debug." won't work.
       --If you get a reply from a TA, and then send email 20 minutes later
         asking a closely related question, that’s probably not great too.

    -- late labs: 120hr slack hours
        -- penalty: -1% per late hour and -50% max
        -- apply slack hours to whichever lab you like.
        -- for example, you have 10 slack hours left,
            * lab2, 60hr late
            * lab3, 10hr late
           What're you going to do?

    d. lectures

      --attending: no roll call, but...will randomly pick students
      to answer questions
        -- get -1 (out of 100) in your final grade
        -- max -1 for each class

    e. integrity policies:

      --Here are some questions:

       Looking at a classmate's solution and then coding it by yourself afterward
       Showing your code to a classmate who has questions
       Modifying code that you find on StackOverflow
       Modifying code for a similar assignment that you find on GitHub

      The correct answer: ALL of these are ruled out by the policy.

      --Please see the policy page, and let me say here:

       --The collaboration and academic integrity policy is real

       --please make sure that you've really thought through your
       question on your own before you ask for help

       --Exams will have questions about labs; and
       "If there are inexplicable discrepancies between exam and lab
       performance, we will overweight the exam, and possibly
       interview you." (see the policy page)

3. Crash course of computer organization

  [draw a mini computer:
    memory, CPU, disk ]

    +--------------+
    | CPU          |            +-------------------+
    | +- ALUs      | <--fetch-- | memory            |
    | +- registers |            | +- array of bytes |
    +--------------+            +-------------------+
               \                   ^
              commands            /
                 \              data
                  v             /
              +--------------------+
              | disk               |
              | +- array of blocks |
              +--------------------+

  * how a CPU works?
    - ALUs
    - registers
      for example, ip tells CPU where's the next instruction

  * how memory works?
    - a sequence of bytes

  * how disk works?
    - a sequence of blocks

  * how a program (helloworld) runs?
    * a file on disk
    * load to memory; have code and data segmentes
    * the code segment is a sequence of instructions
    * instructions are like "add 1 and 1", "multiply x and y"
    * CPU needs to run the instructions
    * how does CPU know which instructions to run? CPU has a register %ip
    * one way to see how a CPU works:
      while (true) {
        instruction <- fetch from memory pointed by %ip
        execute instruction
        %ip <- address of next instruction
      }


 * Demo: executing helloworld
       [use egos-NU as an example]
    - a programmer writes helloworld.c
    - compile helloworld.c to assembly,
      then to object,
      then to executable binary (called ELF, executable and linkable format)
    - run the program


[acknowledgement: Jinyang Li and Tiger Wang]