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WEEK09/WEEK09-01.md

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+# Embedded Systems Reverse Engineering
+[Repository](https://github.com/mytechnotalent/Embedded-Hacking)
+
+## Week 9
+Operators in Embedded Systems: Debugging and Hacking Operators w/ DHT11 Temperature & Humidity Sensor Single-Wire Protocol Basics
+
+### Exercise 1: Change the Sleep Duration
+
+#### Objective
+Find the `sleep_ms(2000)` call in the `0x001a_operators` binary using GDB, identify the immediate value `0x7d0` (2000) being loaded into `r0`, calculate the file offset, patch it to `0x1388` (5000) using a hex editor, and verify on hardware that the serial output now prints every 5 seconds instead of every 2 seconds.
+
+#### Prerequisites
+- Completed Week 9 tutorial (GDB and hex editor sections)
+- `0x001a_operators.elf` and `0x001a_operators.bin` available in your build directory
+- GDB (`arm-none-eabi-gdb`) and OpenOCD installed
+- A hex editor (HxD, ImHex, or similar)
+- Python installed (for UF2 conversion)
+- Raspberry Pi Pico 2 with DHT11 sensor connected
+
+#### Task Description
+The program calls `sleep_ms(2000)` at the end of its main loop, causing a 2-second delay between each set of serial output. The value `2000` (`0x7D0`) is loaded into register `r0` before the `bl sleep_ms` call. You will locate this value in the disassembly, find the corresponding bytes in the `.bin` file, and patch them to `5000` (`0x1388`) so the loop runs every 5 seconds instead.
+
+#### Step-by-Step Instructions
+
+##### Step 1: Start the Debug Session
+
+**Terminal 1 - Start OpenOCD:**
+
+```powershell
+openocd ^
+  -s "C:\Users\flare-vm\.pico-sdk\openocd\0.12.0+dev\scripts" ^
+  -f interface/cmsis-dap.cfg ^
+  -f target/rp2350.cfg ^
+  -c "adapter speed 5000"
+```
+
+**Terminal 2 - Start GDB:**
+
+```powershell
+arm-none-eabi-gdb build\0x001a_operators.elf
+```
+
+**Connect to target:**
+
+```gdb
+(gdb) target remote :3333
+(gdb) monitor reset halt
+```
+
+##### Step 2: Find the sleep_ms Call
+
+Disassemble main and look for the `sleep_ms` call:
+
+```gdb
+(gdb) x/60i 0x10000234
+```
+
+Look for an instruction pattern like:
+
+```
+ldr  r0, =0x7d0     ; 2000 milliseconds
+bl   sleep_ms
+```
+
+The value `0x7d0` will be loaded from the literal pool.
+
+##### Step 3: Examine the Literal Pool Value
+
+Once you find the `ldr r0, [pc, #offset]` instruction, examine the literal pool entry it references:
+
+```gdb
+(gdb) x/wx <literal_pool_address>
+```
+
+You should see `0x000007d0` (2000 in hex).
+
+##### Step 4: Calculate the File Offset
+
+```
+file_offset = literal_pool_address - 0x10000000
+```
+
+Note the file offset of the 4-byte word containing `0x7d0`.
+
+##### Step 5: Encode the New Value
+
+The new value `5000` in hex is `0x1388`. In little-endian byte order:
+
+| Value | Hex        | Little-Endian Bytes |
+| ----- | ---------- | ------------------- |
+| 2000  | `0x000007D0` | `D0 07 00 00`       |
+| 5000  | `0x00001388` | `88 13 00 00`       |
+
+##### Step 6: Patch with HxD
+
+1. In HxD, open `C:\Users\flare-vm\Desktop\Embedded-Hacking-main\0x001a_operators\build\0x001a_operators.bin`
+2. Press **Ctrl+G** and enter the file offset you calculated
+3. You should see: `D0 07 00 00`
+4. Replace with: `88 13 00 00`
+
+##### Step 7: Save and Convert
+
+1. Click **File** → **Save As** → `0x001a_operators-h.bin`
+
+```powershell
+cd C:\Users\flare-vm\Desktop\Embedded-Hacking-main\0x001a_operators
+python ..\uf2conv.py build\0x001a_operators-h.bin --base 0x10000000 --family 0xe48bff59 --output build\hacked.uf2
+```
+
+##### Step 8: Flash and Verify
+
+1. Hold BOOTSEL and plug in your Pico 2
+2. Drag and drop `hacked.uf2` onto the RPI-RP2 drive
+
+**Check the serial monitor:**
+- Output should now appear every **5 seconds** instead of every 2 seconds
+- All operator values should remain unchanged (50, 5, 0, 0, 12, 11)
+
+#### Expected Output
+
+After completing this exercise, you should be able to:
+- Locate literal pool values referenced by `ldr` instructions
+- Understand little-endian byte ordering for 32-bit values
+- Patch timing constants in embedded firmware
+- Calculate file offsets from memory addresses
+
+#### Questions for Reflection
+
+###### Question 1: The value `2000` is stored in the literal pool as a 32-bit word, not as an immediate in the instruction. Why can't `2000` be encoded as an immediate in a single `movs` instruction?
+
+###### Question 2: If you wanted to change the delay to exactly 1 second (1000ms = `0x3E8`), what bytes would you write in little-endian order?
+
+###### Question 3: The literal pool value is shared — could other code in the binary also reference this same `0x7D0` value? How would you check?
+
+###### Question 4: What would happen if you changed the sleep value to `0` (`00 00 00 00`)? Would the program crash or just run extremely fast?
+
+#### Tips and Hints
+- `movs` can only encode immediates 0-255; values larger than 255 must be loaded from the literal pool via `ldr`
+- Always verify the bytes BEFORE patching — make sure you see `D0 07 00 00` at your calculated offset
+- The literal pool is typically right after the function's `b.n` (loop branch) instruction
+- Use a stopwatch or count "one-one-thousand" to verify the timing change