S2-HyperCalc

A minimal 64-bit bootable Linux ISO that runs a custom C program as init. Built using a precompiled Linux kernel, GRUB bootloader, and custom initramfs without a full Linux distro. Designed to run standalone in QEMU or VirtualBox.

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# 🐧 Custom Linux Bootable ISO

This project demonstrates how to build a **minimal 64-bit bootable Linux ISO** that runs a **custom C program as `init`**. It uses:
- A **precompiled Linux kernel (`bzImage`)**
- A **custom `init` program** written in C
- A minimal **initramfs**
- The **GRUB bootloader**
- Works in **QEMU** and **VirtualBox**

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## 📁 Project Structure

. ├── init.c # Your main C program (runs as PID 1) ├── init # Statically compiled binary ├── rootfs/ # Root filesystem for initramfs │ ├── dev/ │ ├── proc/ │ ├── sys/ │ └── init ├── initrd.gz # Compressed initramfs ├── iso/ │ └── boot/ │ ├── grub/ │ │ └── grub.cfg │ ├── vmlinuz # Precompiled Linux kernel (bzImage) │ └── initrd.gz └── mylinux.iso # Final bootable ISO

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## 🛠️ Requirements

- Linux host (Ubuntu recommended)
- `gcc`, `cpio`, `grub-pc-bin`, `xorriso`
- Precompiled Linux kernel (`bzImage`)
- `fakeroot` (for safe initramfs creation)

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## 🧪 Step-by-Step Instructions

### 1. Write Your Init Program (`init.c`)

```c
#include <unistd.h>
#include <fcntl.h>

int main() {
    int fd = open("/dev/console", O_WRONLY);
    write(fd, "✅ Hello from custom init!\n", 27);
    while (1); // Infinite loop to keep the kernel alive
    return 0;
}

2. Compile Statically

gcc -static -o init init.c
chmod +x init

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3. Create Initramfs (initrd.gz)

mkdir -p rootfs/{dev,proc,sys}
cp init rootfs/init

# Create device nodes
sudo mknod rootfs/dev/console c 5 1
sudo mknod rootfs/dev/null c 1 3
chmod 600 rootfs/dev/console
chmod 666 rootfs/dev/null

# Create initramfs
cd rootfs
find . | cpio -o --format=newc | gzip > ../initrd.gz
cd ..

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4. Create GRUB Boot Config

mkdir -p iso/boot/grub
nano iso/boot/grub/grub.cfg

Paste this:

set timeout=0
set default=0

menuentry "Boot Custom Linux" {
    linux /boot/vmlinuz console=tty0 video=vesafb:mtrr:3,ywrap,1024x768-32
    initrd /boot/initrd.gz
}

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5. Add Kernel & Initrd to ISO Structure

cp initrd.gz iso/boot/initrd.gz
cp bzImage iso/boot/vmlinuz

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6. Build ISO

grub-mkrescue -o S2 HyperCalc.iso iso

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▶️ Run in QEMU

qemu-system-x86_64 -cdrom S2 HyperCalc.iso -boot d -m 512

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▶️ Run in VirtualBox

1. Create a new VM:

   • Type: Linux
   • Version: Other Linux (64-bit)
   • RAM: 512 MB
   • No hard disk needed

2. Mount S2 HyperCalc.iso as optical drive.

3. Boot and watch the console for your custom init output.

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📺 Set Default Resolution (e.g. 480)

Edit grub.cfg:

linux /boot/vmlinuz console=tty0 video=vesafb:mtrr:3,ywrap,480 - 80 25

Optional for EFI VMs:

VBoxManage modifyvm "YourVMName" --firmware efi
VBoxManage setextradata "YourVMName" "VBoxInternal2/EfiGraphicsResolution" "1920x1080"

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📦 Credits

• Linux kernel: https://kernel.org
• GRUB bootloader
• cpio for archive creation
• fakeroot for safe permissions during build

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