CS208 Introduction to Computer Systems
Monday, 10 November 2025

+ Coming up
- Exam Monday, Nov 17
    - see study notes below
    - we'll look closer at those notes Wednesday
- Today: redirection, dup2, etc.
- Wednesday/Friday
    - exam prep
    - simple networking in C

+ Questions so far?

+ files.c

+ redirect.c

+ pipe.c

+ Questions to think about
- why are fork and exec separate?
- why do you open a file instead of just reading
    and writing the file by name?
- what is a file?
- ...

===== some exam study notes =====

- Yet more 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?

- How do the basic instructions work?
  - mov, test, cmp, jx, jmp, add, sub, mul, lea
  - call, ret
- what are the purposes of the various special registers?
  - eflags
  - 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)

- What does an if-else look like in asm?
- What does a loop look like in asm?
- How does function-calling work in asm?
- 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
- C and Unix and processes
  - what does the Unix command ps do?
  - what do these C functions do?
    - fork
    - execlp/execvp
    - wait
    - dup2
  - what's a file descriptor?
  - what file descriptors are pre-initialized when you create a new process?
  - what is the point of dup2?
  - what is a pipe (on the command line)?
  - what is a pipe (in C code)?

- Simple networking
  - what is a client? list some examples
  - what is a server? list some examples
  - what is a protocol? list some examples
  - what do socket, bind, listen, and accept do?