CS208 Introduction to Computer Systems
Wednesday, 15 October 2025

+ Exam
- Median 33.5/43
  40-43:   3
  30-39.5: 9
  20-29.5: 5
  10-19.5: 5
- Learning checkpoint
- Time
- Asking questions
- Asking for different kinds of help

#9

void remove_last_token(char **tokens, int *tokens_count) {
    // core of this function
    free(tokens[*tokens_count - 1]);
    tokens[*tokens_count - 1] = NULL; // OK, but not necessary
    *tokens_count--;
}


----

+ What's in registers eax, ebx, rax, rbx after this?

    movl $5, %ebx
    movl $4, %eax
    addl %ebx, %eax

+ Mini-history
- AT&T / Bell Labs developed this instruction set
    (Unix tools like gcc use AT&T syntax)
- Intel also developed this instruction set
    Notations are different

+ Homework advice
- Figure out which parameters the function uses
    rdi, rsi, rdx, rcx,...
- Figure out whether there's a return value, and how
    big it is
- Draw a picture and step through the code
    with some value for the parameter(s)

+ puzzle0
- Function parameters
    %edi is present, %esi/%rsi is not
    so this looks like a 1-parameter function
    looks like probably an int, because %edi is 32 bits
    Note: also, %edi is used without being initialized
        which is what happens with parameters
- Return value
    Does eax or rax or ax ... get a value?
    Yes: eax gets a value, and it's 32 bits
- Let's try function1(5)
    testl %edi, %edi -- bitwise AND %edi with %edi and remember result
        (5)
    js - jump if the result of the test/cmp was negative
    execute movl, testl, js, ret
        i.e. label L2 then L2
    ...eax ends up containing -5...

- Let's try function1(-5)   
    execute movl, testl, js, negl, jmp, ret
        i.e. labels: L3, then L2
    ...eax ends up containing 5...


// looks like absolute value
int function1(int x) {

}