> For the complete documentation index, see [llms.txt](https://alham-rizvi.gitbook.io/alhamrizvi/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alham-rizvi.gitbook.io/alhamrizvi/picoctf-writeups/picoctf/reverse-engineering/armssembly-0.md).

# ARMssembly 0

We're given an ARM64 assembly file and two numbers. We need to figure out what the program prints, then convert it to a flag.

### Step 1: Understanding the Structure

The program has two functions:

* **`main`** → takes command line arguments, calls `func1`
* **`func1`** → does some comparison logic, returns a value

### Step 2: Reading func1 (The Important Part)

```asm
func1:
    str  w0, [sp, 12]    ; save first argument (a)
    str  w1, [sp, 8]     ; save second argument (b)
    ldr  w1, [sp, 12]    ; load a
    ldr  w0, [sp, 8]     ; load b
    cmp  w1, w0          ; compare a and b
    bls  .L2             ; if a <= b (unsigned), jump to L2
    ldr  w0, [sp, 12]    ; else return a
    b    .L3
.L2:
    ldr  w0, [sp, 8]     ; return b
```

The key instruction is **`bls`** = "Branch if Lower or Same" (unsigned comparison)

In simple terms:

```
if (a > b):
    return a
else:
    return b
```

> 💡 **This is just a MAX function!** It returns whichever number is bigger.

### Step 3: Reading main

```asm
main:
    ; reads argv[1] → converts to int → stored in w19  (this is 'a')
    ; reads argv[2] → converts to int → stored in w1   (this is 'b')
    ; calls func1(a, b)
    ; prints "Result: %ld\n" with the return value
```

So main just:

1. Takes two numbers from command line
2. Passes them to func1
3. Prints the result

### Step 4: Plug in the Numbers

Given:

* **a = 2593949075**
* **b = 2233560849**

Since `func1` returns the maximum:

```
max(2593949075, 2233560849) = 2593949075
```

**a is bigger, so the answer is 2593949075**

### Step 5: Convert to Flag Format

The flag format requires the number in **32-bit hex, lowercase, no 0x**

```
2593949075 in hex = 9A9C8593
lowercase         = 9a9c8593
```

### Final Flag

```
picoCTF{9a9c8593}
```

### Summary in One Line

> The program finds the **maximum of two numbers** using unsigned comparison (`bls`), and since **2593949075 > 2233560849**, it prints **2593949075**, which is **`9a9c8593`** in hex.


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