CS208 Introduction to Computer Systems
Wednesday, 7 February 2024

+ Today
- Exam returned
- Lots more assembly language

+ Exam
- Stats
    Possible: 50
    High score: 50
    Median: 33.5
    40s: 9
    30s: 7
    20s: 10
    Other: 2
- Submit corrections by class time Monday
    - no extensions
    - submit only corrections, on fresh paper
    - submit stapled to original exam
    - final score = ((original score) + (corrected score)) / 2
- I'll send course grade status reports Monday or Tuesday
    after I grade the corrections
- Quick run-through of major issues

+ Break

+ How to learn assembly language
(0) Take notes. Try to remember patterns of code. Think!

(1) Read assembly on its own. Ask:
- where does this instruction get its data?
- where does this instruction put its result?
- how is the EFLAGS register affected?
- what addressing modes does this instruction use?

(2) Write tiny functions in Compiler Explorer
- what does a very simple if-statement turn into? if(a > 0)
- what does a more complex if condition turn into? if(a > b && b > 0)
- what does an if-else turn into?
- what does a simple loop turn into? int k=10; while(k > 0) { k = k - 3; }
- what does a function call look like?
- what does a simple index-based traversal of an array look like?
    char buffer[10] = "goat";
    int length = 0;
    for (int k = 0; buffer[k] != '\0'; k++) {
        length++;
    }
- what does a pointer-based traversal of the same array look like?
    for (char *p = buffer; *p != '\0'; p++) {
        length++;
    }

+ Hints for the homework
- what kinds of structures would I want you to see?
- get started on a puzzle
    - what registers are used without initialization?
      those are your function's parameters
      (in order, they're rdi, rsi, rdx, rcx,...)
    - identify familiar assembly structures
        if-conditions
        loops (somewhere, there's a jump that goes backwards)
        assignment statements
        ...
- one puzzle function takes 3 parameters; the others all take 1 or 2
- two of the puzzle functions compute the same thing--one iteratively
    and one recursively
- you can ask questions on Slack
- intention: get you used to paying attention to the details in
    assembly code; you're gonna need that habit for multiple
    future assignments

======

// Returns the correct length, but destroys the list
// and leaks memory by not freeing the individual nodes.
int linked_list_length(linked_list *the_list) {
    int count = 0;
    while (the_list->head != NULL) {
        count++;
        the_list->head = the_list->head->next;
    }

    return count;
}

// Better
int linked_list_length(linked_list *the_list) {
    int count = 0;
    node *current = the_list->head;
    while (current != NULL) {
        count++;
        current = current->next;
    }

    return count;
}