CS208 Introduction to Computer Systems
Monday, 27 October 2025

+ Today
- zoo and gdb questions
- zoo and gdb work time

+ Questions

rax points to Jeff's input string
--
before call phase_2
    rsp = 0x7fffffffe980
    rip = 0x400da5
    address of the instruction after "call phase_2" is
        0x400daa
    address of phase_2
        0x400e8e




+ My zoo workflow
- list and break
- run: "r passcodes.txt"
- step into phase
- watch the stack
- think about each instruction before stepping over it
- check the results after each instruction
- watch for the cmp/jx and test/jx behavior
    before jumping to "trigger_alarm"

+ gdb techniques summary
- run the program
    - with no command-line arguments ("r")
    - with command-line arguments    ("r passcodes.txt")
    - start over                     ("r" or "r passcodes.txt" and say yes)
- view assembly language             ("layout asm")
- fix the display after output       (Ctrl-L)
- set a breakpoint                   ("br 57", "br *0x401c94")
- step through the code        
    - next instruction               ("nexti" or "ni")
    - step into                      ("stepi" or "si")
    - continue                       ("c")
- look at all registers              ("info reg" or "i r")
- look at specific registers         ("i r rsp,rip,eax")
- look at memory contents
    - as a null-terminated string    ("x/s 0x41238c")
    - as a sequence of bytes         ("x/32bx 0x41238c" -- b=byte, x=hex)
    - as a sequence of ints          ("x/20wx 0x41238c" -- w=word=4-byte chunks)
    - as a sequence of addresses     ("x/20gx 0x41238c" -- g=giant=8-byte chunks)
    - ...and interpreting what you see
- look at the memory pointed to by a register  ("x/20wx $rsp")
- go slowly!

+ Work time