CS208 Introduction to Computer Systems
Monday, 22 January 2024

+ Questions

+ Topics for the week
- Bit operations
- Byte order
- Character encodings (including UTF-16LE, UTF-16BE, UTF-8)
- Prep for the exam

+ Bit operations
- "bitwise"
- OR, AND, NOT, XOR
- |, &, ~, ^  ... also << and >>

    0010 1110 | 1010 0000 = 1010 1110       (note: a | 0 == a)

    0010 1110 & 1010 1111 = 0010 1110       (note a & ~0 == a & all-ones == a)

    0010 1110 ^ 1010 0000 = 1000 1110        (a ^ 0 == a)
    0010 1110 ^ 0000 1111 = 0010 0001        (a ^ all-ones == ~a)

    ~ 0010 1110 = 1101 0001

    0010 1110 >> 2  = 0000 1011              (equiv. of integer division by power of 2)
    0010 1110 << 2  = 1011 1000              (mult. by power of 2; 2^2 here)

- Turning individual bits on and off

    0110 0101 = x
  & 1101 1111 = mask     --> turns off the 3rd bit from the left in x
                        build the mask: mask = ~(0x20)

    0100 0101 = x
  | 0010 0000 = mask      --> turns on the 3rd bit from the left

Interpret those examples as x == an ASCII character

    0110 0101 == 'e'
  & 1101 1111
    ---------
    0100 0101 == 'E'


- Masking

    0110 0101
  & 0000 1111

+ Byte order
- What's the problem?
- "Endianness"
- "Gulliver's Travels" and "On Holy Wars & a Plea For Peace"
- Bits are bits; context determines meaning

    Intel x86-64 architecture uses little-endian
        therefore, so will we

+ Character encodings
- What's a character encoding?
- UTF-16, UTF-16LE, UTF-16BE
- UTF-8
- BOM