CS208 Introduction to Computer Systems
Wednesday, 24 January 2024

+ Exam prep
- In-class exam, Monday, January 29

- Closed-everything-except-your-brain (no calculator, no notes)

- General topics
    - data representation
    - C

- 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 via classroom topics,
        homework assignments, sample programs, and the quiz

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

- Types of questions
    - Short answers
    - Here's some code; what's the output?
    - Write this tiny amount of code to do X (never more than 3-4 lines)
    - 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
        (octal, though important, will not be on the exam)
    - two's complement representation
    - byte order (little-endian vs. big-endian)

- Characters
    - difference between characters and C char
    - codepoints vs. encodings
    - ASCII (chart will be provided)
    - UTF-8 encoding (chart will be provided)
    - 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 (32-bit vs. 64-bit)
    - 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
    - strlen
    - strcpy vs. strncpy (including what if source is longer than buffer in strncpy)
    - strcmp
    - printf %s vs. %p

- 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, also known as xor)

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

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+ Questions about program or quiz

+ That quiz memory grid
- How big are things?
- Where are they?
- When are we talking about the address as itself
    and when about the thing it points to?
- What does printf do and expect in various contexts?

+ Misc: copying bits, sign extension, etc.

- What happens when you do this?

    char ch = 0xC3;
    printf("0x%x\n", ch);

- Converting from argv[x] or other char * to int

    strtoul
    atoi
    atol

- What happens when you do this?

    int x = -1;
    unsigned int y = x;

- What happens when you do this?

    char a = 0xC3;
    char b = 0x41;
    int x = a;
    int y = b;

- What happens when you do this?

    unsigned char a = 0xC3;
    unsigned char b = 0x41;
    unsigned int x = a;
    unsigned int y = b;

- What happens when you do this?

    char a = 0xC3;
    char b = 0x41;
    unsigned int x = a;
    unsigned int y = b;

+ Byte order in practice
- integers1.c

    int k = 123456
    printf("%d", k);

versus

    fwrite(&k, sizeof(int), 1, stdout);