无法追踪汇编中rbp值，请求解析[rbp+var_1]的取值逻辑

Let’s break this down step by step, tying every instruction directly to how it affects the value at [rbp+var_1].

First: What’s var_1? #

You noted var_1 = dword ptr -3—this is a symbolic offset relative to the stack base pointer rbp. In plain terms, [rbp+var_1] translates to [rbp - 3] (since var_1 is -3). This points to a 4-byte (dword) location in the current stack frame’s local variable area (below rbp, since we’re subtracting from the base pointer).

Step-by-Step Value Tracking #

Let’s walk through each relevant instruction in order:

1. Stack Frame Setup #

push rbp
mov rbp, rsp

This is the standard x86-64 function prologue:

• push rbp saves the previous stack frame’s base pointer onto the stack (rsp decreases by 8 bytes).
• mov rbp, rsp sets rbp to the current stack pointer, marking the start of the new stack frame. At this point, rbp points directly to the saved old rbp value.

2. Initial Assignment #

mov [rbp+var_1], 1

Since var_1 = -3, this writes the value 1 to the memory address rbp - 3. Right now, [rbp+var_1] = 1.

3. Conditional Reset Based on eax #

test eax, eax
jnz short loc_12C0
mov [rbp+var_1], 0
loc_12C0:

The test eax, eax instruction checks if eax is zero (it sets flags based on whether eax has any non-zero bits):

• If eax ≠ 0: The jnz (jump if not zero) instruction triggers, skipping the mov [rbp+var_1], 0 line. So [rbp+var_1] stays 1 when we reach loc_12C0.
• If eax = 0: jnz doesn’t trigger, so we execute mov [rbp+var_1], 0. Now [rbp+var_1] = 0 when we reach loc_12C0.

4. Final Comparison #

cmp [rbp+var_1], 0
jnz new_location

This checks the current value of [rbp+var_1] against 0:

• If the value is 1 (from the eax ≠ 0 path): cmp sets the "not zero" flag, so jnz jumps to new_location.
• If the value is 0 (from the eax = 0 path): cmp sets the "zero" flag, so jnz does not jump—we stay at the current code location.

Final Value Summary #

The final value of [rbp+var_1] depends entirely on the value of eax before the test eax, eax instruction:

• If eax was non-zero: Final value is 1
• If eax was zero: Final value is 0

内容的提问来源于stack exchange，提问作者artemis