CS208 Introduction to Computer Systems
Friday, 19 May 2023

+ How would you answer these questions?
- Suppose you do this
    - fork()
    - parent calls fgets
    - child calls fgets
    - user types something
  What happens?

- What if you fork() once and call wait() twice?

- Suppose you capture SIGINT. How can your program be killed?

- ...other questions you wonder about?...

+ Next up
- Back to assembly language: buffer overflow assignment (posted today)
- Takehome final (handed out next Friday)
- Network programming
- Assembling all these pieces into a story of how computers work

+ Machine language thought experiment

    MOV $0x7, %eax
    RETQ
    CALLQ some_function_address
    ADD 0x4(%rsp,%rbx,8), %r8
    ...

Pretend
    - every instruction takes up 8 bytes
Goal
    - come up with a scheme for representing the instructions above (and others)
        as 8-byte sequences

Try this
    - one byte says "this is MOV" or "this is TEST"
    - MOV $0x7, %eax -- indicate that the first operand is immediate, value 7
        so...second byte indicates addressing modes for both operands (4 bits
        apiece)
      later bytes, meaning depends on addressing mode
        ...

+ Machine language in real life

    gcc -o something -g -O1 -Wall -Werror something.c
    objdump -d something > something.d

+ Reminders on how functions get called (callq, retq, rsp, rip)