Lab06: Assembly Coding

int set_coins(int cents, coins_t *coins);
// Calculate number of quarters, dimes, nickels, pennies of
// change. Param cents is the number of cents, 0-99. Param coins is a
// pointer to a structure which will have its fields set to the number
// of coins of each type. Returns 0 on success.  If cents is out of
// range, no values are changed and 1 is returned.

int total_coins(coins_t coins);
// Returns the total number of cents in the given coins struct.

typedef struct {     // Type for collections of coins
  char quarters;
  char dimes;
  char nickels;
  char pennies;
} coins_t;
// coint_t has the following memory layout and argument passing
// properties:
//
// |          | Pointer | Packed | Packed |
// |          |  Memory | Struct | Struct |
// | Field    |  Offset | Arg#   | Bits   |
// |----------+---------+--------+--------|
// | quarters |      +0 | #1     | 0-7    |
// | dimes    |      +1 | #1     | 8-15   |
// | nickels  |      +2 | #1     | 16-23  |
// | pennies  |      +3 | #1     | 24-31  |
//
// Since the entire struct is less that 64-bits, it is passed in a
// single argument register on 64-bit systems when passed as a value
// such as in the below total_coins() function.

> make
gcc -Wall -Werror -g  -o coins_main coins_main.c coins_funcs_asm.s

> ./coins_main 37
37 cents is...
1 quarters
1 dimes
0 nickels
2 pennies
which is 37 cents

> ./coins_main 91
91 cents is...
3 quarters
1 dimes
1 nickels
1 pennies
which is 91 cents

> ./coins_main 9
9 cents is...
0 quarters
0 dimes
1 nickels
4 pennies
which is 9 cents

# Run the tests
> make test
gcc -Wall -Werror -g  -o coins_main coins_main.c coins_funcs_asm.s
./testy test_coins_main.org 
============================================================
== test_coins_main.org : coins_main Tests
== Running 1 / 1 tests
1)  All Tests            : ok
============================================================
RESULTS: 1 / 1 tests passed

>> make coins_main
gcc -Wall -Werror -g  -o coins_main coins_main.c coins_funcs_asm.s coins_funcs.c

> ./coins_main 27
27 cents is...
1 quarters
0 dimes
0 nickels
0 pennies
which is 25 cents

# looks wrong, fire up gdb to see what's happening in the set_coins() 
# assembly code

>> gdb -tui coins_main

gdb> layout regs                # turn on registers
gdb> break set_coins            # set breakpoint at misbehaving assembly function 
gdb> set args 27                # set commandline arg to 27 cents
gdb> run                        # run to breakpoint
...

                           _________________

                            LAB06 QUESTIONS
                           _________________





Exercise Instructions
=====================

  Follow the instructions below to experiment with topics related to
  this exercise.
  - For sections marked QUIZ, fill in an (X) for the appropriate
    response in this file. Use the command `make test-quiz' to see if
    all of your answers are correct.
  - For sections marked CODE, complete the code indicated. Use the
    command `make test-code' to check if your code is complete.
  - DO NOT CHANGE any parts of this file except the QUIZ sections as it
    may interfere with the tests otherwise.
  - If your `QUESTIONS.txt' file seems corrupted, restore it by copying
    over the `QUESTIONS.txt.bk' backup file.
  - When you complete the exercises, check your answers with `make test'
    and if all is well, create a zip file with `make zip' and upload it
    to Gradescope. Ensure that the Autograder there reflects your local
    results.
  - IF YOU WORK IN A GROUP only one member needs to submit and then add
    the names of their group.


QUIZ Coding Assembly Functions
==============================

  Answer the following questions on basic techniques for assembly
  function coding.

  How are the first 6 arguments to an assembly functions made available
  within the function?
  - ( ) Arguments are passed in registers; the first two are in the
    `%rax' and `%rbx' registers
  - ( ) Arguments are passed in registers; the first two are in the
    `$rdi' and `%rsi' registers
  - ( ) Arguments are in the stack; the first two are available using
    the stack pointer as `0(%rsp)' and `8(%rsp)'
  - ( ) Arguments are in the stack; the first two are available using
    the stack pointer as `8(%rsp)' and `12(%rsp)'

  Which of the following best describes how the kind of arguments passed
  to the FIRST function `set_coins()' function in assembly?
  - ( ) An integer in `%edi' and a pointer to a struct in `%rsi'; to
    get/set fields of the struct syntax like `2(%rsi)' is used
  - ( ) An integer in `%edi' and a packed struct in `%rsi'; to get/set
    fields of the struct bitwise shifts and AND operations are used
  - ( ) An integer in `%esi' and a pointer to a struct in `%rdi'; to
    get/set fields of the struct syntax like `2(%rsi)' is used
  - ( ) An integer in `%esi' and a packed struct in `%rdi'; to get/set
    fields of the struct bitwise shifts and AND operations are used

  Which of the following best describes how the kind of arguments passed
  to the SECOND function `total_coins()' function in assembly?
  - ( ) A pointer to a struct in `%rsi'; to get/set fields of the struct
    syntax like `2(%rsi)' is used
  - ( ) A packed struct in `%rsi'; to get/set fields of the struct
    bitwise shifts and AND operations are used
  - ( ) A pointer to a struct in `%rdi'; to get/set fields of the struct
    syntax like `2(%rsi)' is used
  - ( ) A packed struct in `%rdi'; to get/set fields of the struct
    bitwise shifts and AND operations are used


CODE coins_funcs_asm.s
======================

  Complete the two assembly functions in `coins_funcs_asm.s'. Note that
  until both functions are complete, the tests will likely fail. Correct
  output will appear something like the following:
  ,----
  | >> make
  | gcc -Wall -Werror -g  -o coins_main coins_main.c coins_funcs_asm.s
  | 
  | >> ./coins_main 88
  | set_coins():
  | 88 cents is...
  | 3 quarters
  | 1 dimes
  | 0 nickels
  | 3 pennies
  | total_coins():
  | which is 88 cents
  | 
  `----


  Make sure to complete both assembly functions and test them via
  ,----
  | make test
  `----

  before using
  ,----
  | make zip
  `----

  to create a zip to submit.