CS208 Introduction to Computer Systems
Wednesday, 25 Jan 2023

+ In-class exam, Mon Jan 30
- General info
    - Closed-everything-except-your-brain
    - In Olin 304/306/308
    - If you have official accommodations related to test-taking conditions,
        I'd be grateful for a Slack DM reminder
    - If you won't be able to attend due to COVID restrictions, let me know
        as soon as practical

- General topics
    - data representation
    - C
    - x86 assembly very beginning basics

- My philosophy of in-class exams
    - Stuff that I want you to have in your head, not just your notes
    - Mostly "what" and "how", only a little bit of "why"
    - No tricks
    - I try not to make it long (occasionally, I screw this up, but mostly not)
    - I indicate what I think is most important for you right now via classroom topics,
        homework assignments, sample programs, and quizzes

- What to study
    - homework assignments
    - quiz
    - sample code
    - class notes
    - backup reference material: textbook readings

- Types of questions
    - Short answers
    - Here's some code; what's the output?
    - Write this tiny amount of code to do X
    - That memory grid from the quiz
    - Wildcard (I reserve the right to come up with a different
        type of question, but see philosophy above)

- Integers
    - binary/decimal/hexadecimal notation
    - two's complement
    - byte order (little-endian vs. big-endian)

- Characters
    - difference between characters and C char
    - codepoints vs. encodings
    - UTF-8 encoding (with chart in hand)
    - UTF-16 BE and UTF-16 LE

- [General C advice]
    - You can write your own experimental programs
        to see what happens when you do this or that

- C types
    - int vs. long
    - char *p vs. char p[10]
    - string literals: "Hello"
    - char literals: 'A', '\0', '\\', '\''
    - sizeof (don't confuse with strlen!)

- C I/O (input/output)
    - printf, %c, %s, %d, %x, %X, %p
    - fgets (including what if line is longer than buffer?)

- Null-terminated char strings
    - printf %s vs. %p
    - strlen
    - strcpy vs. strncpy (including what if source is longer than buffer in strncpy)
    - strcmp

- C pointers
    - & as unary "address-of" operator
    - * in type declarations
    - * as unary operator for dereferencing
    - assignment statements with pointers

- C bitwise operators
    - ~ (not)
    - & (and)
    - | (or)
    - ^ (exclusive or / xor)

- C stuff that's pretty much the same as Java
    - if, else
    - &&, ||, !
    - while, for
    - {}, ()
    - +, -, / (int div), %
    - function signatures

- Basics of x86_64 assembly
    - registers (what are they?)
    - mov, add, sub, push, pop
    - test, cmp
    - j, js, jz, jl, jle
    - call, ret
    - how do you loop?
    - how do you do if/else?
    - how do you call a function?

-----------

+ Last week's quiz
- str1 vs. pointers
    generally, array declaration/definition
        vs. pointer declaration/initialization
- problem #6
- other?

+ x86_64 essentials ("*" means "kind of weird")
- register naming *(r, e, x, l, h)
    8086 - registers a, b, c... (all 8 bits)
    80186 - registers ax, bx, cx, ... (16 bits)
    80386 - registers eax, ebx, ecx,... (32 bits)
    IA-64 - registers rax, rbx, rcx, ... (64 bits)
- register conventions
    (https://web.stanford.edu/class/archive/cs/cs107/cs107.1224/guide/x86-64.html)
- addressing modes
    (same link)
- instruction subtypes by operand size *(q, l, w, b)
    mov vs. movq, movl, movw, movb
- the FLAGS / EFLAGS register
- reading documentation
    https://www.felixcloutier.com/x86/
- machine language's role
    *operand order in asm
- C to asm
    gcc -g -Og -c source.c [--> source.o]
    objdump -d source.o
    [there are other ways to do this...]
- today and Friday
    - MOV
    - ADD, SUB
    - CMP, TEST
    - JMP, Jcc
    - PUSH, POP
    - *LEA
    - Using gdb to look closer