CS208 Introduction to Computer Systems
Wednesday, 29 April 2026

+ questions

+ advice: step through the code manually

+ cmp, test, and jxx
- EFLAGS: ZF="was the result 0?", SF="was the result negative?"
- e.g., what does ADDL do to ZF and SF?
- cmp R1, R2 --> R2 - R1 and set flags and some JXX's
- test A, B --> A & B and set flags
- test A, A --> A & A and set flags
- je, jne, jg, jge, jl, jle,...

+ stack frames, call, and ret

+ addressing modes
- %rcx
- $0x2C
- (%rsp)
- (%rbx, %rcx, 4)
- (%rbx, %rcx, 1) vs. (%rbx, %rcx)
- 0x80(%rsp, %rcx, 4)
- ...

+ what the heck is LEA for?

+ things to try with Compiler Explorer
- https://godbolt.org/
- Let's do some experiments
    - what does simple arithmetic (+, -, *, /, %, |, ^, &, ~) look like?
    - what does an if-else look like?
    - what does a loop look like?

+ what's going on with...
- puzzle2: pushq rbx, popq rbx
    (similar in puzzle4)
- puzzle3: testq, sete, testq, sete, orb, je
- puzzle4: subq $8, %rsp ... addq $8, %rsp
- puzzle5: leal (%rcx,%rax), %esi
- puzzle6:
    - movslq %edx, %rax
      salq $2, %rax
      movl -4(%rdi,%rax), %ecx
    - all those jumps
    - movl %r8d, (%rdi,%rdx,4)


==
void f(int *a, int n, int filler) {
    if (a != 0) {
        for (int k = 0; k < n; k++) {
            a[k] = filler;
        }
    }
}

int g(int b, int n) {
    int result = 1;
    for (int k = 1; k < n; k++) {
        result = result * b;
    }
    return result;
}