# Week 5: Integers and Floats in Embedded Systems: Debugging and Hacking Integers and Floats w/ Intermediate GPIO Output Assembler Analysis

***
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**IF YOU DO NOT AGREE WITH THESE TERMS, DO NOT USE THIS REPOSITORY AND COURSE.**

***

## What You'll Learn This Week

By the end of this tutorial, you will be able to:

- Understand how integers and floating-point numbers are stored in memory
- Know the difference between signed and unsigned integers (`uint8_t` vs `int8_t`)
- Understand how floats and doubles are represented using IEEE 754 encoding
- Use inline assembly to control GPIO pins directly at the hardware level
- Debug numeric data types using GDB and the OpenOCD debugger
- Hack integer values by modifying registers at runtime
- Hack floating-point values by understanding and manipulating their binary representation
- Reconstruct 64-bit doubles from two 32-bit registers
---

## Part 1: Understanding Integer Data Types

### What is an Integer?

An **integer** is a whole number without any decimal point. Think of it like counting apples: you can have 0 apples, 1 apple, 42 apples, but you can't have 3.5 apples (that would be a fraction!).

In C programming for embedded systems, we have special integer types that tell the compiler exactly how much memory to use:

```
+-----------------------------------------------------------------+
|  Integer Types - Different Sizes for Different Needs            |
|                                                                 |
|  uint8_t:  1 byte  (0 to 255)       - like a small box          |
|  int8_t:   1 byte  (-128 to 127)    - can hold negatives!       |
|  uint16_t: 2 bytes (0 to 65,535)    - medium box                |
|  uint32_t: 4 bytes (0 to 4 billion) - big box                   |
|                                                                 |
+-----------------------------------------------------------------+
```

### Signed vs Unsigned Integers

The difference between `uint8_t` and `int8_t` is whether the number can be **negative**:

| Type      | Prefix | Range       | Use Case                      |
| --------- | ------ | ----------- | ----------------------------- |
| `uint8_t` | `u`    | 0 to 255    | Ages, counts, always positive |
| `int8_t`  | none   | -128 to 127 | Temperature, can be negative  |

#### The Integer Variables

Let's say a program declares two integer variables that demonstrate the difference between **signed** and **unsigned** types:

```c
uint8_t age = 43;
int8_t range = -42;
```

The variable `age` is a `uint8_t` - an **unsigned** 8-bit integer that can only hold values from `0` to `255`. Since age is always a positive number, unsigned is the right choice. The variable `range` is an `int8_t` - a **signed** 8-bit integer that can hold values from `-128` to `127`. The signed type allows it to represent negative numbers like `-42`. Under the hood, negative values are stored using **two's complement** encoding: the CPU flips all the bits of `42` (`0x2A`) and adds `1`, producing `0xD6`, which is how `-42` lives in a single byte of memory.

---

## Part 2: Understanding Floating-Point Data Types

### What is a Float?

A **float** is a number that can have a decimal point. Unlike integers which can only hold whole numbers like `42`, a float can hold values like `42.5`, `3.14`, or `-0.001`. In C, the `float` type uses **32 bits (4 bytes)** to store a number using the **IEEE 754** standard.

```
+-----------------------------------------------------------------+
|  IEEE 754 Single-Precision (32-bit float)                       |
|                                                                 |
|  +------+----------+---------------------------+                |
|  | Sign | Exponent | Mantissa (Fraction)       |                |
|  | 1bit |  8 bits  |      23 bits              |                |
|  +------+----------+---------------------------+                |
|                                                                 |
|  Value = (-1)^sign * 2^(exponent-127) * 1.mantissa              |
|                                                                 |
|  Example: 42.5                                                  |
|  Sign: 0 (positive)                                             |
|  Exponent: 10000100 (132 - 127 = 5)                             |
|  Mantissa: 01010100000000000000000                              |
|  Full: 0 10000100 01010100000000000000000                       |
|  Hex: 0x422A0000                                                |
|                                                                 |
+-----------------------------------------------------------------+
```

### How to Compute This by Hand (42.5 -> IEEE 754)

Use this exact process any time you need to encode a decimal float manually.

1. Determine the sign bit.
  - `42.5` is positive, so `sign = 0`.

2. Convert the number to binary.
  - Integer part: `42 = 101010 (base 2)`
  - Fractional part: use repeated multiply-by-2 on the fraction.
    - Start with `0.5`
    - `0.5 * 2 = 1.0` -> integer part is `1` (this is the first binary fractional bit)
    - Remaining fractional part is now `0.0`, so we stop.
    - Therefore `0.5 = 0.1 (base 2)`.
  - Combined: `42.5 = 101010.1 (base 2)`

3. Normalize to the form `1.xxxxx * 2^n`.
  - `101010.1 (base 2) = 1.010101 (base 2) * 2^5`
  - So the true exponent is `n = 5`.

4. Compute the stored exponent (bias 127 for float).
  - `stored exponent = n + 127 = 5 + 127 = 132`
  - `132` in binary is `10000100` (8 bits).

  > Tip: **Why 127?** The exponent field is 8 bits wide, giving $2^8 = 256$ total values. Half of that range should represent negative exponents and half positive. The midpoint is $(2^8 / 2) - 1 = 127$. So a stored exponent of `127` means a real exponent of **0**, values below `127` are negative exponents, and values above `127` are positive exponents. Doubles use an 11-bit exponent field so their midpoint (bias) is $( 2^{11} / 2) - 1 = 1023$ instead.

5. Build the mantissa (fraction bits).
  - Take bits after the leading `1.` from `1.010101` -> `010101`
  - Pad with zeros to 23 bits:
  - `01010100000000000000000`

6. Assemble all fields.
  - `sign | exponent | mantissa`
  - `0 | 10000100 | 01010100000000000000000`
  - Full 32-bit pattern:
  - `01000010001010100000000000000000`

7. Convert the 32-bit binary to hex (group by 4 bits).
  - `0100 0010 0010 1010 0000 0000 0000 0000`
  - `4    2    2    A    0    0    0    0`
  - Final result: `0x422A0000`

Quick decode check (reverse direction, fully expanded):

Given the 32-bit pattern:

- `0 | 10000100 | 01010100000000000000000`

Decode it field by field:

1. Sign bit
  - Sign bit is `0` -> number is positive.
  - So the sign multiplier is `(+1)`.

2. Exponent field
  - Exponent bits are `10000100`.
  - Convert to decimal: `10000100 (base 2) = 132`.
  - Float bias is `127`, so true exponent is:
  - `132 - 127 = 5`.

3. Mantissa field
  - Stored mantissa bits are `01010100000000000000000`.
  - IEEE 754 normal numbers use an implicit leading `1`, so significand becomes:
  - `1.010101 (base 2)`.

4. Rebuild the value
  - Formula: `value = (+1) * 1.010101 (base 2) * 2^5`.
  - Shift binary point right by 5:
  - `1.010101 * 2^5 = 101010.1 (base 2)`.

5. Convert `101010.1 (base 2)` to decimal
  - Integer part: `101010 = 32 + 8 + 2 = 42`
  - Fraction part: `.1 = 1/2 = 0.5`
  - Total: `42 + 0.5 = 42.5`

So the decoded value is exactly `42.5`.

### Float vs Integer - Key Differences

| Property       | Integer (`uint8_t`)    | Float (`float`)             |
| -------------- | ---------------------- | --------------------------- |
| **Size**       | 1 byte                 | 4 bytes                     |
| **Precision**  | Exact                  | ~7 decimal digits           |
| **Range**      | 0 to 255               | 3.4  10^38                |
| **Encoding**   | Direct binary          | IEEE 754 (sign/exp/mantissa)|
| **printf**     | `%d`                   | `%f`                        |

### Our Floating-Point Program

Let's look at a simple program that uses a `float` variable:

**File: `0x000e_floating-point-data-type.c`**

```c
#include <stdio.h>
#include "pico/stdlib.h"

int main(void) {
    float fav_num = 42.5;

    stdio_init_all();

    while (true)
        printf("fav_num: %f\r\n", fav_num);
}
```

> Note: `fav_num` is declared inside `main`, so by C rules it is an automatic (stack) variable. In optimized embedded builds, the compiler may avoid creating a real stack slot and instead materialize the value from read-only constant storage (typically `.rodata` and/or an ARM literal pool).
>
> An ARM **literal pool** is a small table of constants that the assembler places near code in memory. Instead of encoding a large immediate value directly in an instruction, the CPU executes a load instruction (such as `ldr`) that reads the constant from that nearby table. That is why Ghidra can show constant loads rather than a classic stack local.

**What this code does:**

1. Declares a `float` variable `fav_num` and initializes it to `42.5`
2. Initializes the serial output
3. Prints `fav_num` forever in a loop using the `%f` format specifier

> Tip: **Why `%f` instead of `%d`?** The `%d` format specifier tells `printf` to expect an integer. The `%f` specifier tells it to expect a floating-point number. Using the wrong one would print garbage!

### Step 1: Flash the Binary to Your Pico 2

1. Hold the BOOTSEL button on your Pico 2
2. Plug in the USB cable (while holding BOOTSEL)
3. Release BOOTSEL - a drive called "RPI-RP2" appears
4. Drag and drop `0x000e_floating-point-data-type.uf2` onto the drive
5. The Pico will reboot and start running!

### Step 2: Verify It's Working

Open your serial monitor (PuTTY) and you should see:

**You should see:**

```
fav_num: 42.500000
fav_num: 42.500000
fav_num: 42.500000
...
```

The program is printing `42.500000` because `printf` with `%f` defaults to 6 decimal places.

---

## Part 2.5: Setting Up Ghidra for Float Analysis

### Step 3: Start Ghidra

**Open a terminal and type:**

```cmd
ghidraRun
```

Ghidra will open. Now we need to create a new project.

### Step 4: Create a New Project

1. Click **File** -> **New Project**
2. Select **Non-Shared Project**
3. Click **Next**
4. Enter Project Name: `0x000e_floating-point-data-type`
5. Click **Finish**

### Step 5: Import the Binary

1. Open your file explorer
2. Navigate to the `Embedded-Hacking` folder
3. Find `0x000e_floating-point-data-type.bin`
4. Select Cortex M Little Endian 32
5. Select Options and set up the .text and offset 10000000
6. **Drag and drop** the `.bin` file into Ghidra's project window

### Step 6: Configure the Binary Format

A dialog appears. The file is identified as a "BIN" (raw binary without debug symbols).

**Click the three dots (...) next to "Language" and:**

1. Search for "Cortex"
2. Select **ARM Cortex 32 little endian default**
3. Click **OK**

**Click the "Options..." button and:**

1. Change **Block Name** to `.text`
2. Change **Base Address** to `10000000` (the XIP address!)
3. Click **OK**

### Step 7: Open and Analyze

1. Double-click on the file in the project window
2. A dialog asks "Analyze now?" - Click **Yes**
3. Use default analysis options and click **Analyze**

Wait for analysis to complete (watch the progress bar in the bottom right).

---

## Part 2.6: Navigating and Resolving Functions

### Step 8: Find the Functions

Look at the **Symbol Tree** panel on the left. Expand **Functions**.

You'll see function names like:

- `FUN_1000019a`
- `FUN_10000210`
- `FUN_10000234`

These are auto-generated names because we imported a raw binary without symbols!

### Step 9: Resolve Known Functions

From our previous chapters, we know what some of these functions are:

| Ghidra Name    | Actual Name   | How We Know                |
| -------------- | ------------- | -------------------------- |
| `FUN_1000019a` | `data_cpy`    | From Week 3 boot analysis  |
| `FUN_10000210` | `frame_dummy` | From Week 3 boot analysis  |
| `FUN_10000234` | `main`        | This is where our code is! |

### Step 10: Update Main's Signature

For `main`, let's also fix the return type:

1. Right-click on `main` in the Decompile window
2. Select **Edit Function Signature**
3. Change to: `int main(void)`
4. Click **OK**

---

## Part 2.7: Analyzing the Main Function

### Step 11: Examine Main in Ghidra

Click on `main` (or `FUN_10000234`). Look at the **Decompile** window:

You'll see something like:

```c
int main(void)

{
  undefined4 uVar1;
  undefined4 extraout_r1;
  undefined4 uVar2;
  undefined4 extraout_r1_00;

  FUN_10002f5c();
  uVar1 = DAT_1000024c;
  uVar2 = extraout_r1;
  do {
    FUN_100030ec(DAT_10000250,uVar2,0,uVar1);
    uVar2 = extraout_r1_00;
  } while( true );
}
```

### Step 12: Resolve stdio_init_all

1. Click on `FUN_10002f5c`
2. Right-click -> **Edit Function Signature**
3. Change to: `bool stdio_init_all(void)`
4. Click **OK**

### Step 13: Resolve printf

1. Click on `FUN_100030ec`
2. Right-click -> **Edit Function Signature**
3. Change to: `int printf(char *format,...)`
4. Check the **Varargs** checkbox (printf takes variable arguments!)
5. Click **OK**

### Step 14: Understand the Float Encoding

Look at the decompiled code after resolving functions:

```c
int main(void)

{
  undefined4 uVar1;
  undefined4 extraout_r1;
  undefined4 uVar2;
  undefined4 extraout_r1_00;

  stdio_init_all();
  uVar1 = DAT_1000024c;
  uVar2 = extraout_r1;
  do {
    printf(DAT_10000250,uVar2,0,uVar1);
    uVar2 = extraout_r1_00;
  } while( true );
}
```

**Where's `float fav_num = 42.5`?** It's been optimized into an immediate value!

The compiler replaced our float variable with constants passed directly to `printf`. But wait - we see **two** values: `0x0`, in `r2` and `DAT_1000024c` or `0x40454000`, in `r3`. That's because `printf` with `%f` always receives a **double** (64-bit), not a `float` (32-bit). The C standard requires that `float` arguments to variadic functions like `printf` are **promoted to `double`**.

A 64-bit double is passed in two 32-bit registers:

| Register | Value        | Role         |
| -------- | ------------ | ------------ |
| `r2`     | `0x00000000` | Low 32 bits  |
| `r3`     | `0x40454000` | High 32 bits |

Together they form `0x40454000_00000000` - the IEEE 754 **double-precision** encoding of `42.5`.

### Step 15: Verify the Double Encoding

We need to decode `0x4045400000000000` field by field. The two registers give us the full 64-bit value:

```
r3 (high 32 bits): 0x40454000 = 0100 0000 0100 0101 0100 0000 0000 0000
r2 (low  32 bits): 0x00000000 = 0000 0000 0000 0000 0000 0000 0000 0000
```

Laid out as a single 64-bit value with every bit numbered:

```
Bit:    63  62-52 (11 bits)         51-32 (20 bits)                            31-0 (32 bits)
      +---+-----------------------+------------------------------------------+----------------------------------+
      | 0 | 1 0 0 0 0 0 0 0 1 0 0 | 0 1 0 1 0 1 0 0 0 0 0 0 0  0 0 0 0 0 0 0 | 00000000000000000000000000000000 |
      +---+-----------------------+------------------------------------------+----------------------------------+
       Sign     Exponent (11)              Mantissa high 20 bits                  Mantissa low 32 bits
                                           (from r3 bits 19-0)                    (from r2, all zero)
```

**Step-by-step field extraction:**

**1. Sign bit**

In IEEE 754, the **sign bit** is the very first (leftmost) bit of the 64-bit double. In the full 64-bit layout we call it **bit 63**:

```
64-bit double:  [bit 63] [bit 62 ... bit 0]
                    ^
                 sign bit
```

But we don't have a single 64-bit register - we have **two** 32-bit registers. The high register `r3` holds bits 63-32 of the double. So bit 63 of the double is the same physical bit as **bit 31 of r3** (the topmost bit of r3):

```
r3 holds bits 63-32 of the double
r2 holds bits 31-0  of the double
```

Now let's check it. IEEE 754 uses a simple rule for the sign bit:

| Sign bit | Meaning  |
|----------|----------|
| `0`      | Positive |
| `1`      | Negative |

```
r3 = 0x40454000 = 0100 0000 0100 0101 0100 0000 0000 0000
                  ^
                  r3 bit 31 = 0 -> sign = 0 -> Positive number
```

The topmost bit of r3 is `0`, so the number is **positive**. If that bit were `1` instead (e.g. `0xC0454000`), the number would be negative (`-42.5`).

**2. Exponent - bits 62-52 of the 64-bit value = bits 30-20 of r3**

Extract bits 30-20 from `0x40454000`:

```
0x40454000 in binary:  0      10000000100    01010100000000000000
                       sign   exponent       mantissa (top 20 bits)
```

Exponent bits: `10000000100`

Convert to decimal: $2^{10} + 2^{2} = 1024 + 4 = 1028$

But `1028` is **not** the actual power of 2 yet. IEEE 754 stores exponents with a **bias** - a fixed number that gets added during encoding so that the stored value is always positive (no sign bit needed for the exponent). For doubles, the bias is **1023**.

> Tip: **Why 1023?** The exponent field is 11 bits wide, giving $2^{11} = 2048$ total values. Half of that range should represent negative exponents and half positive. The midpoint is $(2^{11} / 2) - 1 = 1023$. So a stored exponent of `1023` means a real exponent of **0**, values below `1023` are negative exponents, and values above `1023` are positive exponents.

To recover the real exponent, we subtract the bias:

$$\text{real exponent} = \text{stored exponent} - \text{bias}$$

$$\text{real exponent} = 1028 - 1023 = \mathbf{5}$$

This means the number is scaled by $2^5 = 32$. In other words, the mantissa gets shifted left by 5 binary places.

**3. Mantissa - bits 51-0 of the 64-bit value**

- **High 20 bits of mantissa** (bits 51-32) = bits 19-0 of r3:

```
r3 bits 19-0: 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0
```

- **Low 32 bits of mantissa** (bits 31-0) = all of r2:

```
r2 = 0x00000000 -> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
```

Full 52-bit mantissa:

```
0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 | 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
  <- top 20 bits from r3 ->                   <- bottom 32 bits from r2 (all zero) ->
```

IEEE 754 always prepends an **implied leading `1`**, so the actual value represented is:

```
1.010101 00000... (the 1. is implicit, not stored)
```

**4. Reconstruct the value**

$$1.010101\text{ (base 2)} \times 2^5$$

Shift the binary point 5 places right:

$$101010.1\text{ (base 2)}$$

Now convert each bit position to decimal:

| Bit position | Power of 2 | Value |
|---|---|---|
| `1` (bit 5) | $2^5$ | 32 |
| `0` (bit 4) | $2^4$ | 0 |
| `1` (bit 3) | $2^3$ | 8 |
| `0` (bit 2) | $2^2$ | 0 |
| `1` (bit 1) | $2^1$ | 2 |
| `0` (bit 0) | $2^0$ | 0 |
| `1` (bit -1) | $2^{-1}$ | 0.5 |

$$32 + 8 + 2 + 0.5 = \mathbf{42.5}$$

### Step 16: Examine the Assembly

Look at the **Listing** window (assembly view). Find the main function:

```
                             *************************************************************
                             *                           FUNCTION
                             *************************************************************
                             int  __stdcall  main (void )
             int               r0:4           <RETURN>
                             main+1                                          XREF[1,1]:   1000018c (c) , 1000018a (*)
                             main
        10000234 38 b5           push       {r3,r4,r5,lr}
        10000236 02 f0 91 fe     bl         stdio_init_all                                   bool stdio_init_all(void)
        1000023a 00 24           movs       r4,#0x0
        1000023c 03 4d           ldr        r5,[DAT_1000024c ]                               = 40454000h
                             LAB_1000023e                                    XREF[1]:     10000248 (j)
        1000023e 22 46           mov        r2,r4
        10000240 2b 46           mov        r3,r5
        10000242 03 48           ldr        r0=>s_fav_num:_%f_100034a8 ,[DAT_10000250 ]      = "fav_num: %f\r\n"
                                                                                             = 100034A8h
        10000244 02 f0 52 ff     bl         printf                                           int printf(char * format, ...)
        10000248 f9 e7           b          LAB_1000023e
        1000024a 00              ??         00h
        1000024b bf              ??         BFh
                             DAT_1000024c                                    XREF[1]:     main:1000023c (R)
        1000024c 00 40 45 40     undefine   40454000h
                             DAT_10000250                                    XREF[1]:     main:10000242 (R)
        10000250 a8 34 00 10     undefine   100034A8h                                        *  ->  100034a8
```

>  **Key Insight:** The `mov.w r2, #0x0` loads the low 32 bits (all zeros) and `ldr r3, [DAT_...]` loads the high 32 bits (`0x40454000`) of the double. Together, `r2:r3` = `0x40454000_00000000` = `42.5` as a double.

### Step 17: Find the Format String

In the Listing view, click on the data reference to find the format string:

```
                             s_fav_num:_%f_100034a8                          XREF[1]:     main:10000242 (*)
        100034a8 66 61 76        ds         "fav_num: %f\r\n"
                 5f 6e 75
                 6d 3a 20
```

This confirms `printf` is called with the format string `"fav_num: %f\r\n"` and the double-precision value of `42.5`.

---

## Part 2.8: Patching the Float - Changing 42.5 to 99.0

### Step 18: Calculate the New IEEE 754 Encoding

We want to change `42.5` to `99.0`. First, we need to figure out the double-precision encoding of `99.0`:

**Step A - Convert the integer part (99) to binary:**

| Division      | Quotient | Remainder |
|---------------|----------|-----------|
| 99  2       | 49       | **1**     |
| 49  2       | 24       | **1**     |
| 24  2       | 12       | **0**     |
| 12  2       | 6        | **0**     |
| 6  2        | 3        | **0**     |
| 3  2        | 1        | **1**     |
| 1  2        | 0        | **1**     |

Read remainders bottom-to-top: 99 (base 10) = 1100011 (base 2)

**Step B - Convert the fractional part (.0) to binary:**

There is no fractional part - `.0` is exactly zero, so the fractional binary is just `0`.

**Step C - Combine:**

$$99.0\text{ (base 10)} = 1100011.0\text{ (base 2)}$$

**Step D - Normalize to IEEE 754 form** (move the binary point so there's exactly one `1` before it):

$$1100011.0\text{ (base 2)} = 1.100011\text{ (base 2)} \times 2^6$$

We shifted the binary point 6 places left, so the exponent is **6**.

**Step E - Extract the IEEE 754 fields:**

1. **Sign:** `0` (positive)
2. **Exponent:** $6 + 1023 = 1029 = 10000000101\text{ (base 2)}$
3. **Mantissa:** `1000110000000000...` (everything after the `1.`, padded with zeros to 52 bits)
4. **Full double:** `0x4058C00000000000`

| Register | Old Value    | New Value    |
| -------- | ------------ | ------------ |
| `r2`     | `0x00000000` | `0x00000000` |
| `r3`     | `0x40454000` | `0x4058C000` |

Since `r2` stays `0x00000000`, we only need to patch the high word loaded into `r3`.

### Step 19: Find the Value to Patch

Look in the Listing view for the data that loads the high word of the double:

```
        1000024c 00 40 45 40   undefined4  40454000h
```

This is the 32-bit constant that gets loaded into `r3` - the high word of our double `42.5`.

### Step 20: Patch the Constant

1. Click on Window -> Bytes
2. Click on the Pencil icon to enable byte editing
3. At address `1000024c`, overwrite `00 40 45 40` with `00 C0 58 40` (little-endian for `0x40454000 -> 0x4058C000`)
4. Press Enter

This changes the high word from `0x40454000` (42.5 as double) to `0x4058C000` (99.0 as double).

---

## Part 2.9: Export and Test the Hacked Binary

### Step 21: Export the Patched Binary

1. Click **File** -> **Export Program**
2. Set **Format** to **Raw Bytes**
3. Navigate to your build directory
4. Name the file `0x000e_floating-point-data-type-h.bin`
5. Click **OK**

### Step 22: Convert to UF2 Format

**Open a terminal and navigate to your project directory:**

```cmd
cd C:\Users\flare-vm\Desktop\Embedded-Hacking-main\0x000e_floating-point-data-type
```

**Run the conversion command:**

```cmd
python ..\uf2conv.py build\0x000e_floating-point-data-type-h.bin --base 0x10000000 --family 0xe48bff59 --output build\hacked.uf2
```

### Step 23: Flash the Hacked Binary

1. Hold BOOTSEL and plug in your Pico 2
2. Drag and drop `hacked.uf2` onto the RPI-RP2 drive
3. Open your serial monitor

**You should see:**

```
fav_num: 99.000000
fav_num: 99.000000
fav_num: 99.000000
...
```

 **BOOM! We hacked the float!** The value changed from `42.5` to `99.0`!

---

## Part 3: Understanding Double-Precision Floating-Point Data Types

### What is a Double?

A **double** (short for "double-precision floating-point") is like a `float` but with **twice the precision**. While a `float` uses 32 bits, a `double` uses **64 bits (8 bytes)**, giving it roughly **15-16 significant decimal digits** of precision compared to a float's ~7.

```
+-----------------------------------------------------------------+
|  IEEE 754 Double-Precision (64-bit double)                      |
|                                                                 |
|  +------+-----------+--------------------------------------+    |
|  | Sign | Exponent  | Mantissa (Fraction)                  |    |
|  | 1bit |  11 bits  |      52 bits                         |    |
|  +------+-----------+--------------------------------------+    |
|                                                                 |
|  Value = (-1)^sign * 2^(exponent-1023) * 1.mantissa             |
|                                                                 |
|  Example: 42.52525                                              |
|  Sign: 0 (positive)                                             |
|  Exponent: 10000000100 (1028 - 1023 = 5)                        |
|  Mantissa: 0101010000110011101101100100010110100001110010101100 |
|  Hex: 0x4045433B645A1CAC                                        |
|                                                                 |
+-----------------------------------------------------------------+
```

### Float vs Double - Key Differences

| Property        | Float (`float`)        | Double (`double`)           |
| --------------- | ---------------------- | --------------------------- |
| **Size**        | 4 bytes (32 bits)      | 8 bytes (64 bits)           |
| **Precision**   | ~7 decimal digits      | ~15 decimal digits          |
| **Exponent**    | 8 bits (bias 127)      | 11 bits (bias 1023)         |
| **Mantissa**    | 23 bits                | 52 bits                     |
| **Range**       | 3.4  10^38          | 1.8  10^308               |
| **printf**      | `%f`                   | `%lf`                       |
| **ARM passing** | Promoted to double     | Native in `r2:r3`           |

> Tip: **Why does precision matter?** With a `float`, the value `42.52525` might be stored as `42.525249` due to rounding. A `double` can represent it as `42.525250` with much higher fidelity. For scientific or financial applications, that extra precision is critical!

### Our Double-Precision Program

Let's look at a program that uses a `double` variable:

**File: `0x0011_double-floating-point-data-type.c`**

```c
#include <stdio.h>
#include "pico/stdlib.h"

int main(void) {
    double fav_num = 42.52525;

    stdio_init_all();

    while (true)
        printf("fav_num: %lf\r\n", fav_num);
}
```

**What this code does:**

1. Declares a `double` variable `fav_num` and initializes it to `42.52525`
2. Initializes the serial output
3. Prints `fav_num` forever in a loop using the `%lf` format specifier

> Tip: **`%lf` vs `%f`:** While `printf` actually treats `%f` and `%lf` identically (both expect a `double`), using `%lf` makes your intent clear - you're explicitly working with a `double`, not a `float`. It's good practice to match the format specifier to your variable type.

### Step 1: Flash the Binary to Your Pico 2

1. Hold the BOOTSEL button on your Pico 2
2. Plug in the USB cable (while holding BOOTSEL)
3. Release BOOTSEL - a drive called "RPI-RP2" appears
4. Drag and drop `0x000A_intro-to-doubles.uf2` onto the drive
5. The Pico will reboot and start running!

### Step 2: Verify It's Working

Open your serial monitor (PuTTY) and you should see:

**You should see:**

```
fav_num: 42.525250
fav_num: 42.525250
fav_num: 42.525250
...
```

The program is printing `42.525250` because `printf` with `%lf` defaults to 6 decimal places.

---

## Part 3.5: Setting Up Ghidra for Double Analysis

### Step 3: Start Ghidra

**Open a terminal and type:**

```cmd
ghidraRun
```

Ghidra will open. Now we need to create a new project.

### Step 4: Create a New Project

1. Click **File** -> **New Project**
2. Select **Non-Shared Project**
3. Click **Next**
4. Enter Project Name: `0x000A_intro-to-doubles`
5. Click **Finish**

### Step 5: Import the Binary

1. Open your file explorer
2. Navigate to the `Embedded-Hacking` folder
3. Find `0x0011_double-floating-point-data-type.bin`
4. Select Cortex M Little Endian 32
5. Select Options and set up the .text and offset 10000000
6. **Drag and drop** the `.bin` file into Ghidra's project window

### Step 6: Configure the Binary Format

A dialog appears. The file is identified as a "BIN" (raw binary without debug symbols).

**Click the three dots (...) next to "Language" and:**

1. Search for "Cortex"
2. Select **ARM Cortex 32 little endian default**
3. Click **OK**

**Click the "Options..." button and:**

1. Change **Block Name** to `.text`
2. Change **Base Address** to `10000000` (the XIP address!)
3. Click **OK**

### Step 7: Open and Analyze

1. Double-click on the file in the project window
2. A dialog asks "Analyze now?" - Click **Yes**
3. Use default analysis options and click **Analyze**

Wait for analysis to complete (watch the progress bar in the bottom right).

---

## Part 3.6: Navigating and Resolving Functions

### Step 8: Find the Functions

Look at the **Symbol Tree** panel on the left. Expand **Functions**.

You'll see function names like:

- `FUN_1000019a`
- `FUN_10000210`
- `FUN_10000238`

These are auto-generated names because we imported a raw binary without symbols!

### Step 9: Resolve Known Functions

From our previous chapters, we know what some of these functions are:

| Ghidra Name    | Actual Name   | How We Know                |
| -------------- | ------------- | -------------------------- |
| `FUN_1000019a` | `data_cpy`    | From Week 3 boot analysis  |
| `FUN_10000210` | `frame_dummy` | From Week 3 boot analysis  |
| `FUN_10000238` | `main`        | This is where our code is! |

### Step 10: Update Main's Signature

For `main`, let's also fix the return type:

1. Right-click on `main` in the Decompile window
2. Select **Edit Function Signature**
3. Change to: `int main(void)`
4. Click **OK**

---

## Part 3.7: Analyzing the Main Function

### Step 11: Examine Main in Ghidra

Click on `main` (or `FUN_10000234`). Look at the **Decompile** window:

You'll see something like:

```c
int main(void)

{
  undefined4 uVar1;
  undefined4 uVar2;
  undefined4 extraout_r1;
  undefined4 uVar3;
  undefined4 extraout_r1_00;

  uVar2 = DAT_10000258;
  uVar1 = DAT_10000254;
  FUN_10002f64();
  uVar3 = extraout_r1;
  do {
    FUN_100030f4(DAT_10000250,uVar3,uVar1,uVar2);
    uVar3 = extraout_r1_00;
  } while( true );
}
```

### Step 12: Resolve stdio_init_all

1. Click on `FUN_10002f64`
2. Right-click -> **Edit Function Signature**
3. Change to: `bool stdio_init_all(void)`
4. Click **OK**

### Step 13: Resolve printf

1. Click on `FUN_100030f4`
2. Right-click -> **Edit Function Signature**
3. Change to: `int printf(char *format,...)`
4. Check the **Varargs** checkbox (printf takes variable arguments!)
5. Click **OK**

### Step 14: Understand the Double Encoding

Look at the decompiled code after resolving functions:

```c
int main(void)

{
  undefined4 uVar1;
  undefined4 uVar2;
  undefined4 extraout_r1;
  undefined4 uVar3;
  undefined4 extraout_r1_00;

  uVar2 = DAT_10000258;
  uVar1 = DAT_10000254;
  stdio_init_all();
  uVar3 = extraout_r1;
  do {
    printf(DAT_10000250,uVar3,uVar1,uVar2);
    uVar3 = extraout_r1_00;
  } while( true );
}
```

**Where's `double fav_num = 42.52525`?** It's been optimized into immediate values!

This time we see **two** non-zero values: `0x645a1cac` and `0x4045433b`. Unlike the float example where the low word was `0x0`, a double with a fractional part like `42.52525` needs **all 52 mantissa bits** - so both halves carry data.

A 64-bit double is passed in two 32-bit registers:

| Register | Value        | Role         |
| -------- | ------------ | ------------ |
| `r2`     | `0x645A1CAC` | Low 32 bits  |
| `r3`     | `0x4045433B` | High 32 bits |

Together they form `0x4045433B645A1CAC` - the IEEE 754 **double-precision** encoding of `42.52525`.

>  **Key Difference from Float:** In the float example, `r2` was `0x00000000` because `42.5` has a clean fractional part. But `42.52525` has a repeating binary fraction, so the low 32 bits are non-zero (`0x645A1CAC`). This means **both** registers matter when patching doubles with complex fractional values!

### Step 15: Verify the Double Encoding

We need to decode `0x4045433B645A1CAC` field by field. The two registers give us the full 64-bit value:

```
r3 (high 32 bits): 0x4045433B = 0100 0000 0100 0101 0100 0011 0011 1011
r2 (low  32 bits): 0x645A1CAC = 0110 0100 0101 1010 0001 1100 1010 1100
```

Laid out as a single 64-bit value with every bit numbered:

```
Bit:    63  62-52 (11 bits)         51-32 (20 bits)                            31-0 (32 bits)
      +---+-----------------------+------------------------------------------+------------------------------------------+
      | 0 | 1 0 0 0 0 0 0 0 1 0 0 | 0 1 0 1 0 1 0 0 0 0 1 1 0 0 1 1 1 0 1 1  | 01100100010110100001110010101100         |
      +---+-----------------------+------------------------------------------+------------------------------------------+
       Sign     Exponent (11)              Mantissa high 20 bits               Mantissa low 32 bits
                                           (from r3 bits 19-0)                 (from r2)
```

**Step-by-step field extraction:**

**1. Sign bit**

The sign bit is bit 63 of the 64-bit double, which is bit 31 of r3 (the high register holds bits 63-32):

```
r3 = 0x4045433B = 0100 0000 0100 0101 0100 0011 0011 1011
                  ^
                  r3 bit 31 = 0 -> sign = 0 -> Positive number ✓
```

**2. Exponent - bits 62-52 = bits 30-20 of r3**

Extract bits 30-20 from `0x4045433B`:

```
0x4045433B in binary:  0      10000000100    01010100001100111011
                       sign   exponent       mantissa (top 20 bits)
```

Exponent bits: `10000000100`

Convert to decimal: $2^{10} + 2^{2} = 1024 + 4 = 1028$

Subtract the bias (same formula as Part 2 - the bias is 1023 for all doubles):

$$\text{real exponent} = 1028 - 1023 = \mathbf{5}$$

This means the mantissa gets shifted left by 5 binary places (i.e. multiplied by $2^5 = 32$).

**3. Mantissa - bits 51-0**

Unlike the `42.5` example where r2 was all zeros, **both registers contribute non-zero bits** here:

- **High 20 bits of mantissa** (bits 51-32) = bits 19-0 of r3:

```
r3 bits 19-0:  0 1 0 1 0 1 0 0 0 0 1 1 0 0 1 1 1 0 1 1
```

- **Low 32 bits of mantissa** (bits 31-0) = all of r2:

```
r2 = 0x645A1CAC -> 0 1 1 0 0 1 0 0 0 1 0 1 1 0 1 0 0 0 0 1 1 1 0 0 1 0 1 0 1 1 0 0
```

Full 52-bit mantissa:

```
0 1 0 1 0 1 0 0 0 0 1 1 0 0 1 1 1 0 1 1 | 0 1 1 0 0 1 0 0 0 1 0 1 1 0 1 0 0 0 0 1 1 1 0 0 1 0 1 0 1 1 0 0
       <- top 20 bits from r3 ->                        <- bottom 32 bits from r2 ->
```

IEEE 754 always prepends an **implied leading `1`**, so the actual value represented is:

```
1.0101010000110011101101100100010110100001110010101100 (the 1. is implicit, not stored)
```

**4. Reconstruct the value**

$$1.0101010000110011101101100100...\text{ (base 2)} \times 2^5$$

Shift the binary point 5 places right:

$$101010.10000110011101101100100010110100001110010101100\text{ (base 2)}$$

**Integer part** (`101010`):

| Bit position | Power of 2 | Value |
|---|---|---|
| `1` (bit 5) | $2^5$ | 32 |
| `0` (bit 4) | $2^4$ | 0 |
| `1` (bit 3) | $2^3$ | 8 |
| `0` (bit 2) | $2^2$ | 0 |
| `1` (bit 1) | $2^1$ | 2 |
| `0` (bit 0) | $2^0$ | 0 |

$$32 + 8 + 2 = \mathbf{42}$$

**Fractional part** (`.10000110011101101...`):

| Bit position | Power of 2 | Decimal value |
|---|---|---|
| `1` (bit -1) | $2^{-1}$ | 0.5 |
| `0` (bit -2) | $2^{-2}$ | 0 |
| `0` (bit -3) | $2^{-3}$ | 0 |
| `0` (bit -4) | $2^{-4}$ | 0 |
| `0` (bit -5) | $2^{-5}$ | 0 |
| `1` (bit -6) | $2^{-6}$ | 0.015625 |
| `1` (bit -7) | $2^{-7}$ | 0.0078125 |
| `0` (bit -8) | $2^{-8}$ | 0 |
| `0` (bit -9) | $2^{-9}$ | 0 |
| `1` (bit -10) | $2^{-10}$ | 0.0009765625 |
| `1` (bit -11) | $2^{-11}$ | 0.00048828125 |
| `1` (bit -12) | $2^{-12}$ | 0.000244140625 |
| ... | ... | *(remaining 35 bits add smaller and smaller fractions)* |

First 12 fractional bits sum: $0.5 + 0.015625 + 0.0078125 + 0.0009765625 + 0.00048828125 + 0.000244140625 \approx 0.5251$

The remaining 35 fractional bits refine this to $\approx 0.52525$. This is because `0.52525` is a **repeating fraction** in binary - it can never be represented with a finite number of bits, so double precision stores the closest possible 52-bit approximation.

$$42 + 0.52525 = \mathbf{42.52525} \checkmark$$

### Step 16: Examine the Assembly

Look at the **Listing** window (assembly view). Find the main function:

```
                             *************************************************************
                             *                           FUNCTION
                             *************************************************************
                             int  __stdcall  main (void )
             int               r0:4           <RETURN>
                             main+1                                          XREF[1,1]:   1000018c (c) , 1000018a (*)
                             main
        10000238 38 b5           push       {r3,r4,r5,lr}
        1000023a 06 a5           adr        r5,[0x10000254 ]
        1000023c d5 e9 00 45     ldrd       r4,r5,[r5,#0x0 ]=>DAT_10000254                   = 645A1CACh
                                                                                             = 4045433Bh
        10000240 02 f0 90 fe     bl         stdio_init_all                                   bool stdio_init_all(void)
                             LAB_10000244                                    XREF[1]:     1000024e (j)
        10000244 22 46           mov        r2,r4
        10000246 2b 46           mov        r3,r5
        10000248 01 48           ldr        r0=>s_fav_num:_%lf_100034b0 ,[DAT_10000250 ]      = "fav_num: %lf\r\n"
                                                                                             = 100034B0h
        1000024a 02 f0 53 ff     bl         printf                                           int printf(char * format, ...)
        1000024e f9 e7           b          LAB_10000244
                             DAT_10000250                                    XREF[1]:     main:10000248 (R)
        10000250 b0 34 00 10     undefine   100034B0h                                        ?  ->  100034b0
                             DAT_10000254                                    XREF[1]:     main:1000023c (R)
        10000254 ac 1c 5a 64     undefine   645A1CACh
                             DAT_10000258                                    XREF[1]:     main:1000023c (R)
        10000258 3b 43 45 40     undefine   4045433Bh
```

>  **Key Insight:** Notice that **both** `r2` and `r3` are loaded from data constants using `ldr`. Compare this to the float example where `r2` was loaded with `mov.w r2, #0x0`. Because `42.52525` requires all 52 mantissa bits, neither word can be zero - the compiler must store both halves as separate data constants.

### Step 17: Find the Format String

In the Listing view, click on the data reference to find the format string:

```
                             s_fav_num:_%lf_100034b0                         XREF[1]:     main:10000248 (*)
        100034b0 66 61 76        ds         "fav_num: %lf\r\n"
                 5f 6e 75
                 6d 3a 20
```

This confirms `printf` is called with the format string `"fav_num: %lf\r\n"` and the double-precision value of `42.52525`.

---

## Part 3.8: Patching the Double - Changing 42.52525 to 99.99

### Step 18: Calculate the New IEEE 754 Encoding

We want to change `42.52525` to `99.99`. First, we need to figure out the double-precision encoding of `99.99`:

1. $99.99 = 1.5623... \times 2^6 = 1.100011111111...\text{ (base 2)} \times 2^6$
2. **Sign:** `0` (positive)
3. **Exponent:** $6 + 1023 = 1029 = 10000000101\text{ (base 2)}$
4. **Mantissa:** `1000111111010111000010100011110101110000101000111... (base 2)`
5. **Full double:** `0x4058FF5C28F5C28F`

| Register | Old Value    | New Value    |
| -------- | ------------ | ------------ |
| `r2`     | `0x645A1CAC` | `0x28F5C28F` |
| `r3`     | `0x4045433B` | `0x4058FF5C` |

Unlike the float example, **both** registers change! The value `99.99` has a repeating binary fraction, so both the high and low words are different.

### Step 19: Find the Values to Patch

Look in the Listing view for the two data constants:

**Low word (loaded into `r2`):**
```
        10000254 ac 1c 5a 64   undefined4  645A1CACh
```

**High word (loaded into `r3`):**
```
        10000258 3b 43 45 40   undefined4  4045433Bh
```

### Step 20: Patch Both Constants

**Patch the low word:**

1. Click on the data at address `10000254` containing `645A1CAC`
2. Open the Bytes window and enable byte editing (Pencil icon)
3. Overwrite bytes `ac 1c 5a 64` with `8f c2 f5 28` (little-endian for `0x645A1CAC -> 0x28F5C28F`)
4. Press Enter

**Patch the high word:**

1. Click on the data at address `10000258` containing `4045433B`
2. Keep byte editing enabled in the Bytes window
3. Overwrite bytes `3b 43 45 40` with `5c ff 58 40` (little-endian for `0x4045433B -> 0x4058FF5C`)
4. Press Enter

This changes the full 64-bit double from `0x4045433B645A1CAC` (42.52525) to `0x4058FF5C28F5C28F` (99.99).

>  **Key Difference from Float Patching:** When we patched the float `42.5`, we only needed to change one word (the high word in `r3`) because the low word was all zeros. With `42.52525 -> 99.99`, **both** words change. Always check whether the low word is non-zero before patching!

---

## Part 3.9: Export and Test the Hacked Binary

### Step 21: Export the Patched Binary

1. Click **File** -> **Export Program**
2. Set **Format** to **Raw Bytes**
3. Navigate to your build directory
4. Name the file `0x0011_double-floating-point-data-type-h.bin`
5. Click **OK**

### Step 22: Convert to UF2 Format

**Open a terminal and navigate to your project directory:**

```cmd
cd C:\Users\flare-vm\Desktop\Embedded-Hacking-main\0x0011_double-floating-point-data-type
```

**Run the conversion command:**

```cmd
python ..\uf2conv.py build\0x0011_double-floating-point-data-type-h.bin --base 0x10000000 --family 0xe48bff59 --output build\hacked.uf2
```

### Step 23: Flash the Hacked Binary

1. Hold BOOTSEL and plug in your Pico 2
2. Drag and drop `hacked.uf2` onto the RPI-RP2 drive
3. Open your serial monitor

**You should see:**

```
fav_num: 99.990000
fav_num: 99.990000
fav_num: 99.990000
...
```

 **BOOM! We hacked the double!** The value changed from `42.52525` to `99.99`!

---

## Part 3.95: Summary - Float and Double Analysis

### What We Accomplished

1. **Learned about IEEE 754** - How floating-point numbers are encoded in 32-bit (float) and 64-bit (double) formats
2. **Discovered float-to-double promotion** - `printf` with `%f` always receives a `double`, even when you pass a `float`
3. **Decoded register pairs** - 64-bit doubles are split across `r2` (low) and `r3` (high)
4. **Patched a float value** - Changed `42.5` to `99.0` by modifying only the high word
5. **Patched a double value** - Changed `42.52525` to `99.99` by modifying **both** words
6. **Understood the key difference** - Clean fractions (like `42.5`) have a zero low word; complex fractions (like `42.52525`) require patching both words

### IEEE 754 Quick Reference for Common Values

| Value    | Double Hex               | High Word (r3)  | Low Word (r2)  |
| -------- | ------------------------ | --------------- | -------------- |
| 42.0     | `0x4045000000000000`     | `0x40450000`    | `0x00000000`   |
| 42.5     | `0x4045400000000000`     | `0x40454000`    | `0x00000000`   |
| 42.52525 | `0x4045433B645A1CAC`     | `0x4045433B`    | `0x645A1CAC`   |
| 43.0     | `0x4045800000000000`     | `0x40458000`    | `0x00000000`   |
| 99.0     | `0x4058C00000000000`     | `0x4058C000`    | `0x00000000`   |
| 99.99    | `0x4058FF5C28F5C28F`     | `0x4058FF5C`    | `0x28F5C28F`   |
| 100.0    | `0x4059000000000000`     | `0x40590000`    | `0x00000000`   |
| 3.14     | `0x40091EB851EB851F`     | `0x40091EB8`    | `0x51EB851F`   |

### The Float/Double Patching Workflow

```
+-----------------------------------------------------------------+
|  1. Identify the float/double value in the decompiled view      |
|     - Look for hex constants like 0x40454000 or 0x4045433B      |
+-----------------------------------------------------------------+
|  2. Determine if it's float (32-bit) or double (64-bit)         |
|     - printf promotes floats to doubles!                        |
|     - Check if value spans r2:r3 (double) or just r0 (float)    |
+-----------------------------------------------------------------+
|  3. Check if the low word (r2) is zero or non-zero              |
|     - Zero low word = only patch the high word                  |
|     - Non-zero low word = patch BOTH words                      |
+-----------------------------------------------------------------+
|  4. Calculate the new IEEE 754 encoding                         |
|     - Convert your desired value to IEEE 754                    |
|     - Split into high/low words                                 |
+-----------------------------------------------------------------+
|  5. Patch the constant(s) in Ghidra                             |
|     - Edit bytes in the Bytes window (Pencil mode)             |
|     - Replace the old encoding with the new one                 |
+-----------------------------------------------------------------+
|  6. Export -> Convert to UF2 -> Flash -> Verify                    |
|     - Same workflow as integer patching                         |
+-----------------------------------------------------------------+
```

> Tip: **Key takeaway:** Hacking doubles is the same process as hacking floats - find the IEEE 754 constant, calculate the new encoding, patch it. The only extra step is checking whether the **low word** (`r2`) is also non-zero. Clean values like `42.5` only need one patch; messy fractions like `42.52525` need two!

---

---

## Key Takeaways

1. **Integers have fixed sizes** - `uint8_t` is 1 byte (0-255), `int8_t` is 1 byte (-128 to 127). The `u` prefix means unsigned.

2. **IEEE 754 encodes floats in binary** - Sign bit, exponent (with bias), and mantissa form the encoding for both 32-bit floats and 64-bit doubles.

3. **printf promotes floats to doubles** - Even when you pass a `float`, `printf` receives a 64-bit `double` due to C's variadic function rules.

4. **64-bit values span two registers** - On ARM Cortex-M33, doubles use `r2` (low 32 bits) and `r3` (high 32 bits).

5. **Clean fractions have zero low words** - Values like `42.5` have `0x00000000` in the low word; complex fractions like `42.52525` have non-zero low words.

6. **Inline assembly controls hardware directly** - The `mcrr` coprocessor instruction talks to the GPIO block without any SDK overhead.

7. **Binary patching works on any data type** - Integers, floats, and doubles can all be patched in Ghidra using the same workflow.

---

## Glossary

| Term                    | Definition                                                                     |
| ----------------------- | ------------------------------------------------------------------------------ |
| **Bias**                | Constant added to the exponent in IEEE 754 (127 for float, 1023 for double)    |
| **Double**              | 64-bit floating-point type following IEEE 754 double-precision format          |
| **Exponent**            | Part of IEEE 754 encoding that determines the magnitude of the number          |
| **Float**               | 32-bit floating-point type following IEEE 754 single-precision format          |
| **FUNCSEL**             | Function Select - register field that assigns a GPIO pin's function (e.g., SIO)|
| **GPIO**                | General Purpose Input/Output - controllable pins on a microcontroller          |
| **IEEE 754**            | International standard for floating-point arithmetic and binary encoding       |
| **Inline Assembly**     | Assembly code embedded directly within C source using `__asm volatile`         |
| **int8_t**              | Signed 8-bit integer type (-128 to 127)                                        |
| **IO_BANK0**            | Register block at `0x40028000` that controls GPIO pin function selection       |
| **Mantissa**            | Fractional part of IEEE 754 encoding (23 bits for float, 52 bits for double)   |
| **mcrr**                | ARM coprocessor register transfer instruction used for GPIO control            |
| **PADS_BANK0**          | Register block at `0x40038000` that controls GPIO pad electrical properties    |
| **Promotion**           | Automatic conversion of a smaller type to a larger type (float -> double)       |
| **Register Pair**       | Two 32-bit registers (r2:r3) used together to hold a 64-bit value              |
| **UF2**                 | USB Flashing Format - file format for Pico 2 firmware                          |
| **uint8_t**             | Unsigned 8-bit integer type (0 to 255)                                         |

---

## Additional Resources

### GPIO Coprocessor Reference

The RP2350 GPIO coprocessor instructions:

| Instruction                 | Description                  |
| --------------------------- | ---------------------------- |
| `mcrr p0, #4, Rt, Rt2, c0`  | Set/clear GPIO output        |
| `mcrr p0, #4, Rt, Rt2, c4`  | Set/clear GPIO output enable |

### RP2350 Memory Map Quick Reference

| Address      | Description              |
| ------------ | ------------------------ |
| `0x10000000` | XIP Flash (code)         |
| `0x20000000` | SRAM (data)              |
| `0x40028000` | IO_BANK0 (GPIO control)  |
| `0x40038000` | PADS_BANK0 (pad control) |
| `0xd0000000` | SIO (single-cycle I/O)   |

### IEEE 754 Encoding Formula

```
+-----------------------------------------------------------------+
|  Float (32-bit):   [1 sign] [8 exponent] [23 mantissa]          |
|  Double (64-bit):  [1 sign] [11 exponent] [52 mantissa]         |
|                                                                 |
|  Value = (-1)^sign * 2^(exponent - bias) * (1 + mantissa)       |
|                                                                 |
|  Float bias:  127                                               |
|  Double bias: 1023                                              |
+-----------------------------------------------------------------+
```

---

**Remember:** Every binary you encounter in the real world can be analyzed and understood using these same techniques. Whether it's an integer, a float, or a double - it's all just bits waiting to be decoded. Practice makes perfect!

Happy hacking!

