# Cross-Compilation Guide for ARM64

This guide explains how to cross-compile the framebuffer demo for ARM64 (aarch64) architecture, which is used by the R36 Ultra device.

## Prerequisites

### Install ARM64 Cross-Compiler

On Ubuntu/Debian:
```bash
sudo apt-get update
sudo apt-get install gcc-aarch64-linux-gnu
sudo apt-get install valac
```

### Verify Installation
```bash
aarch64-linux-gnu-gcc --version
```

## Compilation Methods

### Method 1: Using the Compile Script (Recommended)

```bash
./compile.sh arm64
```

This will:
1. Compile Vala code to C
2. Cross-compile C code to ARM64 using `aarch64-linux-gnu-gcc`
3. Create a statically-linked binary for ARM64

### Method 2: Manual Cross-Compilation

#### Step 1: Compile Vala to C
```bash
valac --pkg posix -C framebuffer_demo.vala
```

#### Step 2: Cross-compile C to ARM64
```bash
aarch64-linux-gnu-gcc framebuffer_demo.c -o framebuffer_demo \
    $(pkg-config --cflags --libs glib-2.0 gobject-2.0) \
    -static
```

### Method 3: Native Compilation (on R36 Ultra)

If you prefer to compile directly on the R36 Ultra device:
```bash
./compile.sh
```

## Verifying the Binary

Check the compiled binary architecture:
```bash
file framebuffer_demo
```

Expected output:
```
framebuffer_demo: ELF 64-bit LSB executable, ARM aarch64, version 1 (GNU/Linux), statically linked, ...
```

## Transferring to R36 Ultra

### Using SCP
```bash
scp framebuffer_demo user@r36ultra:/home/user/
```

### Using USB/SD Card
1. Copy `framebuffer_demo` to USB drive or SD card
2. Insert into R36 Ultra
3. Copy to device storage

## Running on R36 Ultra

```bash
chmod +x framebuffer_demo
sudo ./framebuffer_demo
```

## Troubleshooting

### Cross-compiler not found
```bash
sudo apt-get install gcc-aarch64-linux-gnu
```

### Missing GLib dependencies
```bash
sudo apt-get install libglib2.0-dev
```

### Binary won't run on device
- Ensure you compiled for the correct architecture (ARM64/aarch64)
- Try static linking: add `-static` flag to gcc command
- Check device architecture: `uname -m` (should show `aarch64`)

### Permission denied on /dev/fb0
```bash
sudo ./framebuffer_demo
```

## Static vs Dynamic Linking

### Static Linking (Recommended for R36 Ultra)
- Binary includes all dependencies
- Larger file size
- No dependency issues on target device
- Use `-static` flag

### Dynamic Linking
- Smaller binary
- Requires compatible libraries on target device
- May have compatibility issues
- Remove `-static` flag

## Architecture Information

- **R36 Ultra**: ARM64 (aarch64)
- **Cross-compiler**: `aarch64-linux-gnu-gcc`
- **Target**: Linux ARM 64-bit