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# Week 7: Constants in Embedded Systems: Debugging and Hacking Constants w/ 1602 LCD I2C Basics
## 🎯 What You'll Learn This Week
By the end of this tutorial, you will be able to:
- Understand the difference between `#define` macros and `const` variables
- Know how constants are stored differently in memory (compile-time vs runtime)
- Understand the I²C (Inter-Integrated Circuit) communication protocol
- Configure I²C peripherals and communicate with LCD displays
- Understand C structs and how the Pico SDK uses them for hardware abstraction
- Use GDB to examine constants, structs, and string literals in memory
- Hack constant values and string literals using a hex editor
- Patch LCD display text without access to source code
---
## 📚 Part 1: Understanding Constants in C
### Two Types of Constants
In C, there are two ways to create values that shouldn't change:
| Type | Syntax | Where It Lives | When Resolved |
| ----------- | ----------------------- | ------------------ | ------------- |
| **#define** | `#define FAV_NUM 42` | Nowhere (replaced) | Compile time |
| **const** | `const int NUM = 1337;` | Flash (.rodata) | Runtime |
### Preprocessor Macros (#define)
A **preprocessor macro** is a text replacement that happens BEFORE your code is compiled:
```c
#define FAV_NUM 42
printf("Value: %d", FAV_NUM);
// Becomes: printf("Value: %d", 42);
```
Think of it like a "find and replace" in a text editor. The compiler never sees `FAV_NUM` - it only sees `42`!
```
┌─────────────────────────────────────────────────────────────────┐
│ Preprocessor Macro Flow │
│ │
│ Source Code Preprocessor Compiler │
│ ┌──────────┐ ┌──────────┐ ┌──────────┐ │
│ │ #define │ │ Replace │ │ Compile │ │
│ │ FAV_NUM │ ─────► │ FAV_NUM │ ─────► │ binary │ │
│ │ 42 │ │ with 42 │ │ code │ │
│ └──────────┘ └──────────┘ └──────────┘ │
│ │
│ FAV_NUM doesn't exist in the final binary! │
│ The value 42 is embedded directly in instructions. │
│ │
└─────────────────────────────────────────────────────────────────┘
```
### Const Variables
A **const variable** is an actual variable stored in memory, but marked as read-only:
```c
const int OTHER_FAV_NUM = 1337;
```
Unlike `#define`, this creates a real memory location in the `.rodata` (read-only data) section of flash:
```
┌─────────────────────────────────────────────────────────────────┐
│ Const Variable in Memory │
│ │
│ Flash Memory (.rodata section) │
│ ┌────────────────────────────────────────────────────────────┐ │
│ │ Address: 0x10001234 │ │
│ │ Value: 0x00000539 (1337 in hex) │ │
│ │ Name: OTHER_FAV_NUM (in debug symbols only) │ │
│ └────────────────────────────────────────────────────────────┘ │
│ │
│ The variable EXISTS in memory and can be read at runtime. │
│ │
└─────────────────────────────────────────────────────────────────┘
```
### Comparison: #define vs const
| Feature | #define | const |
| -------------------- | ------------------------ | ---------------------------- |
| **Type checking** | None (just text) | Yes (compiler enforced) |
| **Memory usage** | None (inlined) | Uses flash space |
| **Debugger visible** | No | Yes (with symbols) |
| **Can take address** | No (`&FAV_NUM` fails) | Yes (`&OTHER_FAV_NUM` works) |
| **Scope** | Global (from definition) | Normal C scoping rules |
---
## 📚 Part 2: Understanding I²C Communication
### What is I²C?
**I²C** (pronounced "I-squared-C" or "I-two-C") stands for **Inter-Integrated Circuit**. It's a way for chips to talk to each other using just TWO wires!
```
┌─────────────────────────────────────────────────────────────────┐
│ I²C Bus - Two Wires, Many Devices │
│ │
│ 3.3V │
│ │ │
│ ┴ Pull-up ┴ Pull-up │
│ │ │ │
│ SDA ─┼────────────┼─────────────────────────────────────── │
│ │ │ │
│ SCL ─┼────────────┼─────────────────────────────────────── │
│ │ │ │ │ │
│ ┌───┴────┐ ┌──┴───┐ ┌────┴──┐ ┌─────┴───┐ │
│ │ Pico │ │ LCD │ │Sensor │ │ EEPROM │ │
│ │(Master)│ │ 0x27 │ │ 0x48 │ │ 0x50 │ │
│ └────────┘ └──────┘ └───────┘ └─────────┘ │
│ │
│ Each device has a unique address (0x27, 0x48, 0x50...) │
│ │
└─────────────────────────────────────────────────────────────────┘
```
### The Two I²C Wires
| Wire | Name | Purpose |
| ------- | ------------ | ------------------------------------ |
| **SDA** | Serial Data | Carries the actual data bits |
| **SCL** | Serial Clock | Timing signal that synchronizes data |
### Why Pull-Up Resistors?
I²C uses **open-drain** signals, meaning devices can only pull the line LOW. They can't drive it HIGH! Pull-up resistors are needed to bring the lines back to HIGH when no device is pulling them down.
The Pico 2 has internal pull-ups that we can enable with `gpio_pull_up()`.
### I²C Addresses
Every I²C device has a unique **7-bit address**. Common addresses:
| Device Type | Typical Address |
| --------------------- | ---------------- |
| 1602 LCD with PCF8574 | `0x27` or `0x3F` |
| Temperature sensor | `0x48` |
| EEPROM | `0x50` |
| Real-time clock | `0x68` |
### I²C Communication Flow
```
┌─────────────────────────────────────────────────────────────────┐
│ I²C Transaction │
│ │
│ 1. Master sends START condition │
│ 2. Master sends device address (7 bits) + R/W bit │
│ 3. Addressed device sends ACK (acknowledge) │
│ 4. Data is transferred (8 bits at a time) │
│ 5. Receiver sends ACK after each byte │
│ 6. Master sends STOP condition │
│ │
│ START ──► Address ──► ACK ──► Data ──► ACK ──► STOP │
│ │
└─────────────────────────────────────────────────────────────────┘
```
---
## 📚 Part 3: Understanding C Structs
### What is a Struct?
A **struct** (short for "structure") is a way to group related variables together under one name. Think of it like a form with multiple fields:
```c
// A struct definition - like a template
struct student {
char name[50];
int age;
float gpa;
};
// Creating a variable of this struct type
struct student alice = {"Alice", 16, 3.8};
```
### Why Use Structs?
Instead of passing many separate variables:
```c
void print_student(char *name, int age, float gpa); // Messy!
```
You pass one struct:
```c
void print_student(struct student s); // Clean!
```
### The typedef Keyword
Writing `struct student` everywhere is tedious. The `typedef` keyword creates an alias:
```c
typedef struct student student_t;
// Now you can write:
student_t alice; // Instead of: struct student alice;
```
### Forward Declaration
Sometimes you need to tell the compiler "this struct exists" before defining it:
```c
typedef struct i2c_inst i2c_inst_t; // Forward declaration + alias
// Later, the full definition:
struct i2c_inst {
i2c_hw_t *hw;
bool restart_on_next;
};
```
---
## 📚 Part 4: Understanding the Pico SDK's I²C Structs
### The i2c_inst_t Struct
The Pico SDK uses a struct to represent each I²C controller:
```c
struct i2c_inst {
i2c_hw_t *hw; // Pointer to hardware registers
bool restart_on_next; // SDK internal flag
};
```
**What each member means:**
| Member | Type | Purpose |
| ----------------- | ------------ | ---------------------------------------- |
| `hw` | `i2c_hw_t *` | Pointer to the actual hardware registers |
| `restart_on_next` | `bool` | Tracks if next transfer needs a restart |
### The Macro Chain
When you write `I2C_PORT` in your code, here's what happens:
```
┌─────────────────────────────────────────────────────────────────┐
│ Macro Expansion Chain │
│ │
│ In your code: #define I2C_PORT i2c1 │
│ │ │
│ ▼ │
│ In i2c.h: #define i2c1 (&i2c1_inst) │
│ │ │
│ ▼ │
│ In i2c.c: i2c_inst_t i2c1_inst = {i2c1_hw, false}; │
│ │ │
│ ▼ │
│ In i2c.h: #define i2c1_hw ((i2c_hw_t *)I2C1_BASE) │
│ │ │
│ ▼ │
│ In addressmap.h: #define I2C1_BASE 0x40098000 │
│ │
└─────────────────────────────────────────────────────────────────┘
```
So `I2C_PORT` eventually becomes a pointer to a struct that contains a pointer to hardware registers at address `0x40098000`!
### The Hardware Register Pointer
The `i2c_hw_t *hw` member points to the actual silicon:
```
┌─────────────────────────────────────────────────────────────────┐
│ Memory Map │
│ │
│ Address 0x40098000: I²C1 Hardware Registers │
│ ┌────────────────────────────────────────────────────────────┐ │
│ │ Offset 0x00: IC_CON (Control register) │ │
│ │ Offset 0x04: IC_TAR (Target address register) │ │
│ │ Offset 0x10: IC_DATA_CMD (Data command register) │ │
│ │ ... │ │
│ └────────────────────────────────────────────────────────────┘ │
│ │
│ The i2c_hw_t struct maps directly to these registers! │
│ │
└─────────────────────────────────────────────────────────────────┘
```
---
## 📚 Part 5: The ARM Calling Convention (AAPCS)
### How Arguments Are Passed
On ARM Cortex-M, the **ARM Architecture Procedure Call Standard (AAPCS)** defines how functions receive arguments:
| Register | Purpose |
| -------- | ---------------- |
| `r0` | First argument |
| `r1` | Second argument |
| `r2` | Third argument |
| `r3` | Fourth argument |
| Stack | Fifth+ arguments |
| `r0` | Return value |
### Example: i2c_init(i2c1, 100000)
```c
i2c_init(I2C_PORT, 100000);
```
In assembly:
```assembly
ldr r0, [address of i2c1_inst] ; r0 = pointer to struct (first arg)
ldr r1, =0x186A0 ; r1 = 100000 (second arg)
bl i2c_init ; Call the function
```
---
## 📚 Part 6: Setting Up Your Environment
### Prerequisites
Before we start, make sure you have:
1. A Raspberry Pi Pico 2 board
2. A Raspberry Pi Pico Debug Probe
3. OpenOCD installed and configured
4. GDB (`arm-none-eabi-gdb`) installed
5. Python installed (for UF2 conversion)
6. A serial monitor (PuTTY, minicom, or screen)
7. A 1602 LCD display with I²C backpack (PCF8574)
8. A hex editor (HxD, ImHex, or similar)
9. The sample project: `0x0017_constants`
### Hardware Setup
Connect your LCD like this:
| LCD Pin | Pico 2 Pin |
| ------- | ---------- |
| VCC | 3.3V or 5V |
| GND | GND |
| SDA | GPIO 2 |
| SCL | GPIO 3 |
```
┌─────────────────────────────────────────────────────────────────┐
│ I²C LCD Wiring │
│ │
│ Pico 2 1602 LCD + I²C Backpack │
│ ┌──────────┐ ┌──────────────────────┐ │
│ │ │ │ │ │
│ │ GPIO 2 │─────── SDA ─────►│ SDA │ │
│ │ (SDA) │ │ │ │
│ │ │ │ ┌────────────┐ │ │
│ │ GPIO 3 │─────── SCL ─────►│ SCL│ Reverse │ │ │
│ │ (SCL) │ │ │Engineering │ │ │
│ │ │ │ └────────────┘ │ │
│ │ 3.3V │─────── VCC ─────►│ VCC │ │
│ │ │ │ │ │
│ │ GND │─────── GND ─────►│ GND │ │
│ │ │ │ │ │
│ └──────────┘ └──────────────────────┘ │
│ │
└─────────────────────────────────────────────────────────────────┘
```
### Project Structure
```
Embedded-Hacking/
├── 0x0017_constants/
│ ├── build/
│ │ ├── 0x0017_constants.uf2
│ │ └── 0x0017_constants.bin
│ ├── 0x0017_constants.c
│ └── lcd_1602.h
└── uf2conv.py
```
---
## 🔬 Part 7: Hands-On Tutorial - Constants and I²C LCD
### Step 1: Review the Source Code
Let's examine the constants code:
**File: `0x0017_constants.c`**
```c
#include <stdio.h>
#include <string.h>
#include "pico/stdlib.h"
#include "hardware/i2c.h"
#include "lcd_1602.h"
#define FAV_NUM 42
#define I2C_PORT i2c1
#define I2C_SDA_PIN 2
#define I2C_SCL_PIN 3
const int OTHER_FAV_NUM = 1337;
int main(void) {
stdio_init_all();
i2c_init(I2C_PORT, 100000);
gpio_set_function(I2C_SDA_PIN, GPIO_FUNC_I2C);
gpio_set_function(I2C_SCL_PIN, GPIO_FUNC_I2C);
gpio_pull_up(I2C_SDA_PIN);
gpio_pull_up(I2C_SCL_PIN);
lcd_i2c_init(I2C_PORT, 0x27, 4, 0x08);
lcd_set_cursor(0, 0);
lcd_puts("Reverse");
lcd_set_cursor(1, 0);
lcd_puts("Engineering");
while (true) {
printf("FAV_NUM: %d\r\n", FAV_NUM);
printf("OTHER_FAV_NUM: %d\r\n", OTHER_FAV_NUM);
}
}
```
**What this code does:**
1. **Lines 7-10:** Define preprocessor macros for constants and I²C configuration
2. **Line 12:** Define a `const` variable stored in flash
3. **Line 15:** Initialize UART for serial output
4. **Lines 17-21:** Initialize I²C1 at 100kHz, configure GPIO pins, enable pull-ups
5. **Lines 23-27:** Initialize LCD and display "Reverse" on line 0, "Engineering" on line 1
6. **Lines 29-32:** Infinite loop printing both constant values to serial terminal
### Step 2: 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 `0x0017_constants.uf2` onto the drive
5. The Pico will reboot and start running!
### Step 3: Verify It's Working
**Check the LCD:**
- Line 1 should show: `Reverse`
- Line 2 should show: `Engineering`
**Check the serial monitor (PuTTY/screen):**
```
FAV_NUM: 42
OTHER_FAV_NUM: 1337
FAV_NUM: 42
OTHER_FAV_NUM: 1337
...
```
---
## 🔬 Part 8: Debugging with GDB (Dynamic Analysis)
> 🔄 **REVIEW:** This setup is identical to previous weeks. If you need a refresher on OpenOCD and GDB connection, refer back to Week 3 Part 6.
### Starting the Debug Session
**Terminal 1 - Start OpenOCD:**
```bash
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:**
```bash
arm-none-eabi-gdb build/0x0017_constants.elf
```
**Connect to target:**
```gdb
(gdb) target remote :3333
(gdb) monitor reset halt
```
### Step 4: Examine Main Function
Let's examine the main function. Disassemble from the entry point:
```
x/60i 0x10000234
```
You should see output like:
```
0x10000234: push {r4, r5, r6, r7, lr}
0x10000236: sub sp, #12
0x10000238: bl 0x10003014 ; stdio_init_all
0x1000023c: ldr r0, [pc, #108] ; Load i2c1_inst pointer
0x1000023e: ldr r1, =0x186A0 ; 100000 (baud rate)
0x10000240: bl 0x100002b4 ; i2c_init
0x10000244: movs r0, #2 ; GPIO 2 (SDA)
0x10000246: movs r1, #3 ; GPIO_FUNC_I2C
0x10000248: bl 0x100002c8 ; gpio_set_function
0x1000024c: movs r0, #3 ; GPIO 3 (SCL)
0x1000024e: movs r1, #3 ; GPIO_FUNC_I2C
0x10000250: bl 0x100002c8 ; gpio_set_function
...
```
### Step 5: Set a Breakpoint at Main
```
b *0x10000234
c
```
GDB responds:
```
Breakpoint 1 at 0x10000234
Continuing.
Breakpoint 1, 0x10000234 in ?? ()
```
### Step 6: Find the #define Constant (FAV_NUM)
Step through to the printf call and examine the registers:
```
x/20i 0x1000028e
```
Look for:
```
0x1000028e: movs r1,#0x2a ; 0x2a = 42 decimal (FAV_NUM)
```
The `#define` constant is embedded directly as an immediate value in the instruction!
### Step 7: Find the const Variable (OTHER_FAV_NUM)
Continue examining the code:
```
x/10i 0x10000296
```
Look for a load instruction:
```
0x10000296: ldr r1, [pc, #offset] ; Load OTHER_FAV_NUM from memory
```
The `const` variable is loaded from a memory address because it actually exists in flash!
### Step 8: Examine the const Value in Memory
Find where OTHER_FAV_NUM is stored:
```
x/1wx 0x100002ac
```
This should show the address pointing to the value. Then examine that address:
```
x/1wd 0x10003xxx
```
You should see `1337` (or `0x539` in hex).
### Step 9: Examine the I²C Struct
Find the i2c1_inst struct address loaded into r0 before i2c_init:
```
x/2wx 0x2000062c
```
You should see:
```
0x2000062c: 0x40098000 ; hw pointer (I2C1 hardware base)
0x20000630: 0x00000000 ; restart_on_next = false
```
### Step 10: Examine the LCD String Literals
Find the strings passed to lcd_puts:
```
x/s 0x10003ee8
```
Output:
```
0x10003ee8: "Reverse"
```
```
x/s 0x10003ef0
```
Output:
```
0x10003ef0: "Engineering"
```
### Step 11: Step Through I²C Initialization
Use `si` to step through instructions and watch the I²C setup:
```
si
i r r0 r1
```
Observe how:
- `r0` gets the i2c_inst pointer
- `r1` gets the baud rate (100000)
- The function call happens via `bl`
---
## 🔬 Part 9: Understanding the Assembly
Now that we've explored the binary in GDB, let's make sense of the key patterns we found.
### Step 12: Analyze #define vs const in Assembly
From GDB, we discovered a critical difference:
**For FAV_NUM (42) — a `#define` macro:**
```
0x1000028e: movs r1, #0x2a ; 0x2a = 42 decimal
```
The value is embedded **directly as an immediate** in the instruction. There is no memory location — the preprocessor replaced `FAV_NUM` with `42` before compilation.
**For OTHER_FAV_NUM (1337) — a `const` variable:**
```
0x10000296: ldr r1, [pc, #offset] ; Load value from flash
```
The value is **loaded from a memory address** because `const` creates a real variable stored in the `.rodata` section of flash.
### Step 13: Analyze the I²C Struct Layout
In GDB, we examined the `i2c1_inst` struct at `0x2000062c`:
```gdb
(gdb) x/2wx 0x2000062c
0x2000062c: 0x40098000 0x00000000
```
This maps to the `i2c_inst_t` struct:
```
┌─────────────────────────────────────────────────────────────────┐
│ i2c_inst_t at 0x2000062c │
│ │
│ ┌────────────────────────────────────────────────────────────┐ │
│ │ Offset Type Name Value │ │
│ │ 0x00 i2c_hw_t * hw 0x40098000 │ │
│ │ 0x04 bool restart_on_next 0x00 (false) │ │
│ └────────────────────────────────────────────────────────────┘ │
│ │
└─────────────────────────────────────────────────────────────────┘
```
The first member (`hw`) points to `0x40098000` — the I²C1 hardware register base. This is the end of the macro chain: `I2C_PORT` → `i2c1` → `&i2c1_inst` → `hw` → `0x40098000`.
### Step 14: Locate the String Literals
We found the LCD strings in flash memory:
```gdb
(gdb) x/s 0x10003ee8
0x10003ee8: "Reverse"
(gdb) x/s 0x10003ef0
0x10003ef0: "Engineering"
```
These are stored consecutively in the `.rodata` section. Note the addresses — we'll need them for patching.
---
## 🔬 Part 10: Hacking the Binary with a Hex Editor
Now for the fun part — we'll patch the `.bin` file directly using a hex editor!
> 💡 **Why a hex editor?** GDB can modify values in RAM at runtime, but those changes are lost when the device reboots. To make **permanent** changes, we edit the `.bin` file on disk and re-flash it.
### Step 15: Open the Binary in a Hex Editor
1. Open **HxD** (or your preferred hex editor: ImHex, 010 Editor, etc.)
2. Click **File** → **Open**
3. Navigate to `0x0017_constants/build/`
4. Open `0x0017_constants.bin`
### Step 16: Calculate the File Offset
The binary is loaded at base address `0x10000000`. To find the file offset of any address:
```
file_offset = address - 0x10000000
```
For example:
- Address `0x1000028e` → file offset `0x28E` (654 in decimal)
- Address `0x10003ee8` → file offset `0x3EE8` (16104 in decimal)
### Step 17: Hack #1 — Change FAV_NUM from 42 to 43
From GDB, we know the instruction at `0x1000028e` is:
```
movs r1, #0x2a → bytes: 2a 21
```
To change the value from 42 (`0x2a`) to 43 (`0x2b`):
1. In HxD, press **Ctrl+G** (Go to offset)
2. Enter offset: `28E`
3. You should see the byte `2A` at this position
4. Change `2A` to `2B`
5. The instruction is now `movs r1, #0x2b` (43 in decimal)
> 🔍 **How Thumb encoding works:** In `movs r1, #imm8`, the immediate value is the first byte, and the opcode `21` is the second byte. So the bytes `2a 21` encode `movs r1, #0x2a`.
### Step 18: Hack #2 — Change OTHER_FAV_NUM from 1337 to 1344
The `const` value 1337 (`0x539`) is stored in the data section, not as an instruction immediate. From GDB, we found its memory address.
Examine the literal pool to find where `0x539` is stored:
```gdb
(gdb) x/4wx 0x100002a8
```
Look for the word `0x00000539`. Calculate its file offset and patch it:
1. In HxD, go to the offset where `0x539` is stored
2. The bytes will be `39 05 00 00` (little-endian)
3. Change to `40 05 00 00` (`0x540` = 1344 in decimal)
> 🔍 **Little-endian byte order:** The RP2350 stores multi-byte values with the least significant byte first. So `0x00000539` appears as `39 05 00 00` in memory.
### Step 19: Hack #3 — Change LCD Text from "Reverse" to "Exploit"
**IMPORTANT:** The new string must be the **same length** as the original! "Reverse" and "Exploit" are both 7 characters — perfect!
From GDB, we know "Reverse" is at address `0x10003ee8`:
1. In HxD, press **Ctrl+G** and enter offset: `3EE8`
2. You should see the bytes for "Reverse": `52 65 76 65 72 73 65 00`
3. Change the bytes to spell "Exploit": `45 78 70 6c 6f 69 74 00`
**ASCII Reference:**
| Character | Hex |
| --------- | ------ |
| E | `0x45` |
| x | `0x78` |
| p | `0x70` |
| l | `0x6c` |
| o | `0x6f` |
| i | `0x69` |
| t | `0x74` |
### Step 20: Save the Patched Binary
1. Click **File** → **Save As**
2. Save as `0x0017_constants-h.bin` in the build directory
3. Close the hex editor
---
## 🔬 Part 11: Converting and Flashing the Hacked Binary
### Step 21: Convert to UF2 Format
Open a terminal and navigate to your project directory:
```bash
cd Embedded-Hacking/0x0017_constants
```
Run the conversion command:
```bash
python ../uf2conv.py build/0x0017_constants-h.bin --base 0x10000000 --family 0xe48bff59 --output build/hacked.uf2
```
### Step 22: 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. Check your LCD and serial monitor
### Step 23: Verify the Hacks
**Check the LCD:**
- Line 1 should now show: `Exploit` (instead of "Reverse")
- Line 2 should still show: `Engineering`
**Check the serial monitor:**
```
FAV_NUM: 43
OTHER_FAV_NUM: 1344
FAV_NUM: 43
OTHER_FAV_NUM: 1344
...
```
🎉 **BOOM! We successfully:**
1. Changed FAV_NUM from 42 to 43
2. Changed OTHER_FAV_NUM from 1337 to 1344
3. Changed the LCD text from "Reverse" to "Exploit"
---
## 📊 Part 12: Summary and Review
### What We Accomplished
1. **Learned about constants** - `#define` macros vs `const` variables
2. **Understood I²C communication** - Two-wire protocol for peripheral communication
3. **Explored C structs** - How the Pico SDK abstracts hardware
4. **Mastered the macro chain** - From `I2C_PORT` to `0x40098000`
5. **Examined structs in GDB** - Inspected memory layout of `i2c_inst_t`
6. **Hacked constant values** - Both immediate and memory-stored using a hex editor
7. **Patched string literals** - Changed LCD display text
### #define vs const Summary
```
┌─────────────────────────────────────────────────────────────────┐
│ #define FAV_NUM 42 │
│ ─────────────────── │
│ • Text replacement at compile time │
│ • No memory allocated │
│ • Cannot take address (&FAV_NUM is invalid) │
│ • In binary: value appears as immediate (movs r1, #0x2a) │
│ • To hack: patch the instruction operand │
├─────────────────────────────────────────────────────────────────┤
│ const int OTHER_FAV_NUM = 1337 │
│ ────────────────────────────── │
│ • Real variable in .rodata section │
│ • Takes memory (4 bytes for int) │
│ • Can take address (&OTHER_FAV_NUM is valid) │
│ • In binary: value loaded from memory (ldr r1, [address]) │
│ • To hack: patch the value in the data section │
└─────────────────────────────────────────────────────────────────┘
```
### I²C Configuration Summary
```
┌─────────────────────────────────────────────────────────────────┐
│ I²C Setup Steps │
│ │
│ 1. i2c_init(i2c1, 100000) - Initialize at 100kHz │
│ 2. gpio_set_function(pin, I2C) - Assign pins to I²C │
│ 3. gpio_pull_up(sda_pin) - Enable SDA pull-up │
│ 4. gpio_pull_up(scl_pin) - Enable SCL pull-up │
│ 5. lcd_i2c_init(...) - Initialize the device │
│ │
└─────────────────────────────────────────────────────────────────┘
```
### The Struct Chain
```
┌─────────────────────────────────────────────────────────────────┐
│ I2C_PORT → i2c1 → &i2c1_inst → i2c_inst_t │
│ │ │
│ ├── hw → i2c_hw_t * │
│ │ └── 0x40098000 │
│ │ │
│ └── restart_on_next (bool) │
│ │
└─────────────────────────────────────────────────────────────────┘
```
### Key Memory Addresses
| Address | Description |
| ------------ | ---------------------------------- |
| `0x10000234` | main() entry point |
| `0x1000028e` | FAV_NUM value in instruction |
| `0x10000296` | OTHER_FAV_NUM load instruction |
| `0x10003ee8` | "Reverse" string literal (example) |
| `0x40098000` | I²C1 hardware registers base |
| `0x2000062C` | i2c1_inst struct in SRAM |
---
## ✅ Practice Exercises
### Exercise 1: Change Both LCD Lines
Change "Engineering" to "Hacking!!!" (same number of characters).
**Hint:** Find the second string after "Reverse" in memory.
### Exercise 2: Change the I²C Address
The LCD is at address `0x27`. Find where this is passed to `lcd_i2c_init` and change it.
**Warning:** If you change to an invalid address, the LCD won't work!
### Exercise 3: Find All String Literals
Search the binary for all readable strings. How many can you find? What do they reveal about the program?
**Hint:** In GDB, use `x/s` to search for strings in the binary, or scan through the `.bin` file in your hex editor.
### Exercise 4: Trace the Struct Pointer
Follow the `i2c1_inst` pointer from the code to SRAM. What values are stored in the struct?
**Hint:** The first member should point to `0x40098000`.
### Exercise 5: Add Your Own Message
Can you make the LCD display your name? Remember the character limit!
**Hint:** Line 1 and Line 2 each have 16 characters maximum on a 1602 LCD.
---
## 🎓 Key Takeaways
1. **#define is text replacement** - It happens before compilation, no memory used.
2. **const creates real variables** - Stored in .rodata, takes memory, has an address.
3. **I²C uses two wires** - SDA for data, SCL for clock, pull-ups required.
4. **Structs group related data** - The SDK uses them to abstract hardware.
5. **Macros can chain** - `I2C_PORT` → `i2c1` → `&i2c1_inst` → hardware pointer.
6. **ARM passes args in registers** - r0-r3 for first four arguments.
7. **GDB reveals struct layouts** - Examine memory to understand data organization.
8. **String hacking requires same length** - Or you'll corrupt adjacent data!
9. **Constants aren't constant** - With binary patching, everything can change!
10. **Compiler optimization changes code** - `gpio_pull_up` becomes `gpio_set_pulls`.
---
## 📖 Glossary
| Term | Definition |
| ----------------------- | --------------------------------------------------- |
| **#define** | Preprocessor directive for text replacement |
| **AAPCS** | ARM Architecture Procedure Call Standard |
| **const** | Keyword marking a variable as read-only |
| **Forward Declaration** | Telling compiler a type exists before defining it |
| **I²C** | Inter-Integrated Circuit - two-wire serial protocol |
| **Immediate Value** | A constant embedded directly in an instruction |
| **Open-Drain** | Output that can only pull low, not drive high |
| **PCF8574** | Common I²C I/O expander chip used in LCD backpacks |
| **Preprocessor** | Tool that processes code before compilation |
| **Pull-Up Resistor** | Resistor that holds a line HIGH by default |
| **SCL** | Serial Clock - I²C timing signal |
| **SDA** | Serial Data - I²C data line |
| **Struct** | User-defined type grouping related variables |
| **typedef** | Creates an alias for a type |
---
## 🔗 Additional Resources
### I²C Timing Reference
| Speed Mode | Maximum Frequency |
| ---------- | ----------------- |
| Standard | 100 kHz |
| Fast | 400 kHz |
| Fast Plus | 1 MHz |
### Common I²C Addresses
| Device | Address |
| --------------------- | ------------- |
| PCF8574 LCD (default) | `0x27` |
| PCF8574A LCD | `0x3F` |
| DS3231 RTC | `0x68` |
| BMP280 Sensor | `0x76`/`0x77` |
| SSD1306 OLED | `0x3C`/`0x3D` |
### Key ARM Instructions for Constants
| Instruction | Description |
| -------------------- | ------------------------------------------- |
| `movs rN, #imm` | Load small immediate (0-255) directly |
| `ldr rN, [pc, #off]` | Load larger value from literal pool |
| `ldr rN, =value` | Pseudo-instruction for loading any constant |
### RP2350 I²C Memory Map
| Address | Description |
| ------------ | ----------------------- |
| `0x40090000` | I²C0 hardware registers |
| `0x40098000` | I²C1 hardware registers |
---
**Remember:** When you see complex nested structures in a binary, take your time to understand the hierarchy. Use GDB to examine struct layouts in memory and trace pointer chains. And always remember — even "constants" can be hacked!
Happy hacking! 🔧
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