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CROSS_COMPILE.md

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+# 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