ASM1 (RE6_ASM_1)

Platform: CUSTOM | Category: Reversing | Type: Challenge | Difficulty: Easy | OS: N/A | **Author: XESXOR Techniques: manual_register_tracing, x86_div_mul_semantics

Summary

Walkthrough of the CUSTOM Reversing ASM1 (RE6_ASM_1). Task: given a short x86 NASM snippet (asm_1.s), compute the final value of eax. Solution: hand-trace the registers, applying x86 div (EDX:EAX / r32 -> quotient EAX, remainder EDX) and mul (EDX:EAX = E

Description

Given the x86 NASM source asm_1.s. Answer: eax = ? Submitted via POST field flag.

A beginner reverse task: statically trace a 6-instruction x86 snippet and report the final value of register eax.

Analysis

Source (asm_1.s):

 nasm read-only • copy allowed Copy
xor edx, edx     ; edx = 0
mov eax, 0x64    ; eax = 0x64
mov ebx, 30      ; ebx = 30
div ebx          ; unsigned divide EDX:EAX by ebx
mov eax, edx     ; eax = remainder
mul edx          ; EDX:EAX = eax * edx

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The two traps are the x86 div/mul semantics:

• div r/m32 divides the 64-bit dividend EDX:EAX by the operand. Quotient goes to EAX, remainder goes to EDX. EDX must be initialized (here xor edx, edx clears it so the dividend is just 0x64).
• mul r/m32 computes EDX:EAX = EAX * operand (unsigned). The high dword lands in EDX, the low dword (the answer we care about) in EAX.

Solution

Hand-trace the registers:

1. xor edx, edx → edx = 0
2. mov eax, 0x64 → eax = 0x64
3. mov ebx, 30 → ebx = 30
4. div ebx → dividend EDX:EAX = 0x0_00000064, divisor 30:
   • quotient eax = 0x64 / 30 = 3
   • remainder edx = 0x64 % 30 = 10
5. mov eax, edx → eax = 10 (the remainder)
6. mul edx → EDX:EAX = eax * edx = 10 * 10 = 0x64, so eax = 0x64, edx = 0 ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Final eax = REDACTED.

Verify with an actual CPU if desired:

 bash read-only • copy allowed Copy
cat > /tmp/asm1.s <<'EOF'
section .text
global _start
_start:
    xor edx, edx
    mov eax, 0x64
    mov ebx, 30
    div ebx
    mov eax, edx
    mul edx
    ; eax now holds the answer; pass it as exit code
    mov ebx, eax
    mov eax, 1
    int 0x80
EOF
nasm -f elf32 /tmp/asm1.s -o /tmp/asm1.o && ld -m elf_i386 /tmp/asm1.o -o /tmp/asm1
/tmp/asm1; echo "eax = $?"   # -> eax = REDACTED

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Key Takeaways / Lessons

• Mastering manual_register_tracing, x86_div_mul_semantics was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

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Crafted by XESXOR Research • 2026-06-13 • CUSTOM • ASM1 (RE6_ASM_1) • Difficulty: Easy