CS208 Introduction to Computer Systems
Wednesday, 1 October 2025

+ Questions

+ When does the leftmost bit matter?
- "sign extension"
- integer types
    - [1 byte]  char,  unsigned char
    - [2 bytes] short, unsigned short
    - [4 bytes] int,   unsigned int
    - [8 bytes] long,  unsigned long
    - [16? bytes] long long,  unsigned long long
- typecasting

    char c = 0x41; // A or 65
    int n = (int)c; // legal, and makes n's value 65
    int n = c;      //  legal, and makes n's value 65

    c                                0100 0001
    n  0000 0000 0000 0000 0000 0000 0100 0001

If you induce a typecast into a signed type, then the smaller
type (char in this case) gets "sign-extended". That is, if the
smaller thing (c) is a positive integer (leftmost bit 0), then
the int fills in with 0's. If the smaller is negative (leftmost bit 1),
the int fills in with 1's.

    char c = 0xC3; // -61
    int n = (int)c; // legal, and makes n's value -61
    int n = c;      //  legal, and makes n's value -61

    c                                1100 0011
    n  1111 1111 1111 1111 1111 1111 1100 0011

- automatic typecasting
- right shift >>

    int n = -61;
    int y = n >> 4;
    n  1111 1111 1111 1111 1111 1111 1100 0011
    y  1111 1111 1111 1111 1111 1111 1111 1100

+ What's next
- overall memory layout
- malloc, free, and why
- buffer overflow--first glimpse
- structs
    - .
    - typedef
    - syntactical weirdness of typedef
- pointers
    - syntax: &, *, **, [], ->, ++, --
    - looping with pointers
    - arrays, pointer variables, literals, etc.
    - string library functions

+ memory.c
- see today's LAB


https://wiki.sei.cmu.edu/confluence/display/c/INT02-C.+Understand+integer+conversion+rules