CS208 Introduction to Computer Systems
Wednesday, 8 April 2026

+ Today
- characters
- character encodings
- string functions

+ Recap: N-bit two's complement integer representation
- how to recognize >= 0 vs. < 0 (negative if leftmost bit = 1)

- negate N: complement, then add 1

- negate N: "what do I have to add to this number to get 0?"

- integer types in C (exact sizes are compiler-dependent)
  char  : 8 bits / 1 byte
  short : 16 bits / 2 bytes
  int   : 32 bits / 4 bytes
  long  : 64 bits / 8 bytes

- Intel machines are little endian, so if you have

    int x = 0x1234ABCD;

then x is stored in memory with the CD byte first and the 12 byte last

+ String functions to know
- strlen
- strcmp
- strcpy, strncpy
- strcat, strncat
- be aware of the mess that is strlcpy, strscpy, strpcpy,...

+ Characters
- https://sandbox.jeffondich.com/encoder
- Standards mapping characters to integers
    ASCII
    ISO-8859-1
    Unicode
- Unicode "codepoint" - the integer that represents one character
    U+0041 <-> A
    U+00E9 <-> é
    U+2191 <-> ↑
    U+1F60A <-> 😊

+ Lab (25 minutes or so, plus debrief)

+ Character encodings
- given a codepoint, how am I going to store it as a byte sequence?
    (in memory or in a file)
- note: "encode" != "encrypt"
- why is an "encoding" different from a "codepoint"
- easy encodings
    - UTF-16LE
        ↑ <-> U+2191 <-> 91 21 [2-byte sequence]
    - UTF-16BE
        ↑ <-> U+2191 <-> 21 91 [2-byte sequence]

+ Friday: UTF-8