released: 03/30, 12:00
due: 04/05, 23:59

Answer the following questions.
Submit your answers to Canvas assignments.
There is an entry for this homework.


1. MMIO and WeensyOS (4 points)

Below is a snippet of code from "lib.cc".
This is the new version piece of code in the final panel of handout week11b
(https://naizhengtan.github.io/23spring/notes/handout_w11b.pdf).

Read it and answer the following questions.

   1  inline void console_printer::putc(unsigned char c, int color) {
   2      while (cell_ >= console + CONSOLE_ROWS * CONSOLE_COLUMNS) {
   3          scroll();
   4      }
   5      if (c == '\n') {
   6          int pos = (cell_ - console) % 80;
   7          while (pos != 80) {
   8              *cell_++ = ' ' | color;
   9              ++pos;
  10          }
  11      } else {
  12          *cell_++ = c | color;
  13      }
  14  }


1.a If we want to replace all empty spaces after a new line with '_', how
    should we modify the code?
    That is, if printing "abc\n123" which is used to look like:
        """
        abc
        123
        """
    now should be:
        """
        abc_______________
        123
        """

    (2 points) Write down the line numbers and your modifications to
    make this happen.




1.b Then, we want to fix the color of the characters to be yellow and we will
    ignore the input argument "color" (line 1), meaning whatever input "color" is,
    the color of the characters will be yellow.

  [ A crash course of 16-color coding
    ----

    In WeensyOS, the input "color" will look like this:

      0x0F00
        ^^^^
        ||++-> these two should be zeros for color.
        |+-> this hex digit means the color of the character.
        +-> this hex digit means the color of the background.

    WeensyOS uses a 16-color system.
    (why 16? a digit of hex number can only represent 16 numbers, 0x0 to 0xf.)
    And here are the colors and their decimal code:

             0    black
             1    blue
             2    green
             3    cyan
             4    red
             5    magenta
             6    brown
             7    light gray
             8    dark gray
             9    bright blue
            10    bright green
            11    bright cyan
            12    bright red
            13    bright magenta
            14    bright yellow
            15    bright white
    ]

    (2 points) Write down the line numbers and your modifications to always
    print yellow characters.



    [Aside, why the main body of the console doesn't change to yellow? because
    main body's print does not use the above code. See k-memviewer.cc for how
    WeensyOS prints the main body.]




2. Disk performance (6 points)

CS5600 staff designs a disk, CS5600-Disk:

  --Spindle Speed: 6000 RPM
    note:
     * this speed number is per minute.
     * on average, it takes the disk to rotate half a circle to reach a sector.
     * So, on average, how long does CS5600-Disk rotate to a sector?
       [this is not a question you need to answer but one you should think of.]

  --Avg Seek Time, read/write: 5ms / 10 ms
    (note: this is the time moving heads to the right track.)

  --Transfer rate: 64 MB/s
    (note: this is the rate of reading/writing sequential data.
     Assume 1MB=10^6B here. Again, this is 10^6B, not 2^20B.)

When CS5600-Disk reads/writes a sector (512 Bytes), it needs to
(1) move the head to the right track (seeking),
(2) wait rotating to the right sector (rotation),
and (3) read/write the data (data transfer).
This means, the time finishing a job (like reading one sector)
equals to the sum of time spending on the above three phases.

Questions:

2.a How long would it take to do 500 sector reads, spread out
randomly over the disk (and serviced in FIFO order)?  (2 points)
Write down your calculation and write the final result in milliseconds.


(b) How long would it take to do 500 sector writes, spread out
randomly over the disk (and serviced in FIFO order)?  (2 points)
Write down your calculation and write the final result in milliseconds.



(c) How long would it take to do 500 sector reads, SEQUENTIALLY on the
CS5600-Disk? (FIFO order once more) (2 points)
  [notice that seek and rotation will appear only once in sequential reads.]
Write down your calculation and write the final result in milliseconds.