CS208 Introduction to Computer Systems
Wednesday, 12 November 2025

+ Exam 1 corrections
- problems 7 and 8: you can try the code
- problem 9
    - error-checking, short-circuited || and &&
    - * vs. []

void remove_last_token(char **tokens, int *tokens_count) {
    //if (tokens == NULL || tokens_count == NULL || *tokens_count <= 0) {
    if (tokens == NULL || *tokens_count <= 0 || tokens_count == NULL) {
        //the second one crashes of tokens_count == NULL
        return;
    }

    ...
    free(tokens[*tokens_count - 1]);
    //free(*tokens[*tokens_count - 1]);  <-- WRONG (arg here is type char)
    ...
}

+ Shell questions

+ Exam prep (see below)

+ Networking
- clients
- servers
- protocols
- TCP connections
- the Unix command nc (also known as netcat)

+ Playing with tcp connections
- nc
- telnet
- curl
- python3 -m http.server PORT

===== Exam 2 prep =====
+ Question types
- Short answers: vocabulary, simple computations, etc.
- Here's some C code; what happens?
- Here are some register values
  Here's some asm code
  What are the register values now?
- Write a little code
- Explain why something is the way it is, or does what it does

+ Memory
- writing bytes as hexadecimal integers
- byte order
- what happens if you interpret a pointer to an array of
    char as a pointer to an int or an array of ints?
- how do * and & work in their various contexts in C?
- what's the difference between these two?
      char *p = "moose";
      char p[6] = "moose";
- what does malloc do? what's the meaning of its return value?
- what does free do? what's its return value? [trick question]
- under what conditions would you malloc a chunk of memory instead
  of just declaring a big enough array?

+ i86-64 assembly language
- What do these instructions do?
    (Check out Dive Into Systems)
    - mov, test, cmp, jmp, add, sub, mul, lea
    - call, ret
    - je (same as jz) - jump if ZF=1
    - jne (same as jnz) - jump if ZF=0
    - jg - jump if >
    - jge - jump if >=
    - jl - jump if <
    - jle - jump if <=

- what are the purposes of the various special registers?
    - eflags (especially the ZF and SF bits)
    - rip
    - rsp
    - rax
    - rdi, rsi, rdx, rcx, ...

- sizes
  - rax vs. eax vs. ax vs. al
  - movq vs. movl vs. movw vs. movb

- how do the addressing modes work?
    movl $1, %eax             # immediate ($1) and register (%eax)
    movl $1, 0x604892         # direct (address is constant value)
    movl $1, (%rax)           # indirect (address is in register %rax)
    movl $1, -24(%rbp)        # indirect with displacement
    movl $1, 8(%rsp, %rdi, 4) # indirect with displacement and scaled-index
    movl $1, (%rax, %rcx, 8)  # (special case scaled-index, displ assumed 0)
    movl $1, 0x8(, %rdx, 4)   # (special case scaled-index, base assumed 0)
    movl $1, 0x4(%rax, %rcx)  # (special case scaled-index, scale assumed 1)

+ C structures --> i86-64
- What does an if-else look like in i86-64?
- What does a loop look like in i86-64?
- How does function-calling work in i86-64?
- How to interpret the output of "objdump -d compiled-code"

+ gdb
- interpreting an "examine memory" printout
- si vs. ni
- x/s vs. x/10wx vs. x/10bx vs. x/10gx

+ processes in C and Unix
- what is a process?
- what does the Unix command ps do?
- what's a file descriptor?
- what file descriptors are pre-initialized when you create a new process?
- what do these C functions do?
    - fork
    - execlp/execvp
    - wait
    - dup2
    - open, read, write
- what is the point of dup2?
- what is a pipe (on the command line)?
- what is a pipe (in C code)?