CS208 Introduction to Computer Systems
Monday, 25 September 2023

+ Today
- More integers: recap, byte order, and printf
- Bitwise operations
- Character sets & character encodings


+ Integer recap
- Base 2 (binary)        111011
- Base 10 (decimal)      32 + 16 + 8 + 0 + 2 + 1 = 59
- Base 16 (hexadecimal)  0011 1011 --> 3B
- Hexadecimal: 4-bit chunks, 0x notation, %x and %X in printf

+ Fixed-length integers
- each int is 4 bytes (with our version of gcc)
- each long is 8 bytes
- byte order
    - the problem
    - Jonathan Swift, Gulliver's Travels (1726), and eggs
    - "little-endian" vs. "big-endian"
    - "On Holy Wars and a Plea for Peace" -- Danny Cohen, 1980

+ Bitwise operations

    &     bitwise AND
    |     bitwise OR
    ^     bitwise XOR

Try it by hand:

    j = 0xB9;
    k = 0xC3;
    j & k
    j | k
    j ^ k
    j >> 4
    j << 4
    ~j

Try printing the results: 

    int j = 0xB9;
    int k = 0xC3;
    printf("0x%X & 0x%X == 0x%X\n", j, k, j & k);
    printf("0x%X | 0x%X == 0x%X\n", j, k, j | k);
    printf("0x%X ^ 0x%X == 0x%X\n", j, k, j ^ k);
    printf("0x%X >> 4 == 0x%X\n", j, j >> 4);
    printf("0x%X << 4 == 0x%X\n", j, j << 4);
    printf("~0x%X == 0x%X\n", j, ~j); // this gets weird

+ Character sets and character encodings
- ASCII (character set)
    - the "ASCII value" of a character
    - see "codepoint" below
- Unicode (character set)
    - unicode.org
    - codepoints vs. characters
- UTF-16BE (character encoding)
- UTF-16LE (character encoding)

+ Strategies
- to_upper & to_lower
- middle_bits
- to_utf8
- from_utf8

+ Printing integers

    int k = 1234;
    printf("%d\n", k);
    printf("0x%X\n", k);

    int k = -1234;
    printf("%d\n", k);
    printf("0x%X\n", k);

+ Printing characters

    char ch = 'A';
    printf("%c\n", ch);
    printf("

+ char, byte, character
- ASCII
- Unicode (much more next week)
- od (Unix command)
- hexdump (Unix command)