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WEEK09/WEEK09-02.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 2: Invert the Temperature Reading
+
+#### Objective
+Using GDB to locate the IEEE-754 scaling constant `0.1f` at file offset `0x42C`, patch it to `-0.1f` using a hex editor, and verify on hardware that the serial output now displays negative temperature values.
+
+#### Prerequisites
+- Completed Week 9 tutorial (GDB, Ghidra, 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 DHT11 driver uses a scaling constant of `0.1f` stored at address `0x1000042C` (file offset `0x42C`) to convert raw sensor data into human-readable values. By changing this constant to `-0.1f`, you will invert the decimal component of the temperature calculation, causing the reported temperature to drop. This exercise teaches IEEE-754 float encoding and how a single 4-byte patch can dramatically change sensor behavior.
+
+#### 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: Verify the Current Scaling Constant
+
+Examine the float constant at the known address:
+
+```gdb
+(gdb) x/wx 0x1000042c
+```
+
+Output:
+```
+0x1000042c: 0x3dcccccd
+```
+
+This is `0.1f` in IEEE-754 encoding (approximately — the repeating binary fraction makes it `0x3dcccccd`).
+
+##### Step 3: Understand the IEEE-754 Encoding
+
+**Current value (0.1f):**
+
+| Field    | Bits       | Value |
+| -------- | ---------- | ----- |
+| Sign     | `0`        | Positive |
+| Exponent | `01111011` | 123 (biased) |
+| Mantissa | `10011001100110011001101` | ~1.6 |
+
+**New value (-0.1f):**
+- Flip only the sign bit (bit 31) from `0` to `1`
+- `0x3dcccccd` → `0xbdcccccd`
+
+| Value  | Hex          | Little-Endian Bytes |
+| ------ | ------------ | ------------------- |
+| 0.1f   | `0x3dcccccd` | `cd cc cc 3d`       |
+| -0.1f  | `0xbdcccccd` | `cd cc cc bd`       |
+
+> 💡 **Key insight:** To negate an IEEE-754 float, you only need to flip the most significant bit. In little-endian, this is the **last** byte — change `3d` to `bd`.
+
+##### Step 4: 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 offset: `42C`
+3. You should see: `cd cc cc 3d` (or `cc cc cc 3d`)
+4. Replace with: `cd cc cc bd` (or `cc cc cc bd`)
+
+> ⚠️ **Note:** The exact bytes may be `cc cc cc 3d` or `cd cc cc 3d` depending on compiler rounding. Just change the last byte from `3d` to `bd`.
+
+##### Step 5: 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 6: 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:**
+- All operator values remain unchanged (50, 5, 0, 0, 12, 11)
+- Temperature should now display with an inverted decimal component
+- Humidity will also be affected (same constant is shared)
+
+#### Expected Output
+
+After completing this exercise, you should be able to:
+- Decode and encode IEEE-754 floating-point values
+- Understand that flipping one bit (sign bit) negates a float
+- Patch floating-point constants in compiled binaries
+- Predict how a constant change propagates through calculations
+
+#### Questions for Reflection
+
+###### Question 1: Why does changing one byte (`3d` → `bd`) negate the entire float value? What does the sign bit (bit 31) control in IEEE-754?
+
+###### Question 2: The scaling constant `0.1f` is used by BOTH the humidity and temperature `vfma.f32` instructions. Why does patching this single constant affect both readings?
+
+###### Question 3: If you wanted to change the constant to `0.5f` (`0x3f000000`, little-endian `00 00 00 3f`) instead of `-0.1f`, how would the temperature reading change? If the raw decimal part is 8, what would the new output be?
+
+###### Question 4: Could you achieve negative temperature by patching the `vfma.f32` instruction itself instead of the constant? What instruction might you replace it with?
+
+#### Tips and Hints
+- IEEE-754 sign bit is the MSB (bit 31) — `0` = positive, `1` = negative
+- In little-endian, the sign bit is in the **last** (highest address) byte
+- Use an online IEEE-754 converter to verify your understanding
+- If the output looks completely wrong (NaN, inf), you may have changed the wrong byte — start over with a fresh copy of the `.bin` file