HYDROGEN

A bare-bones kernel that prints “Hello, World!” on the screen.

REQUIREMENTS

1. QEMU:
QEMU is a machine emulator and virtualizer.
Install it using:

    1. Linux: `$ sudo apt-get install qemu-system`  
    2. Mac OS: `brew install qemu`

2. NASM:
NASM is an assembler and disassembler for the Intel x86 architecture.
Install it using: $ sudo apt-get install nasm

BUILD INSTRUCTIONS

Method 1 (is automated, try Method 2 to get a more geeky feeling):

1. Build kernel executable: ./buildkernel.sh
2. Place the kernel executable in the grub image: ./buildimage.sh
3. Run the kernel: ./run.sh

Method 2:

Build the kernel executable:

1. Use NASM to produce an object file of entry.asm.

   • nasm -f elf32 entry.asm -o entry.o
   • elf32 flag is used to produce a 32-bit ELF object file.

2. Use cc to produce an object file of c_start.c.

   • cc -m32 -c -Wall c_start.c
   • From the man page of gcc:
     -m32
Generate code for a 32-bit or 64-bit environment.
The -m32 option sets "int", "long", and pointer
types to 32 bits, and generates code that runs on
any i386 system.

3. Use ld (the GNU Linker) to link the above object files into a single executable (hydrogen).

   • ld -m elf_i386 -T link.ld -o hydrogen c_start.o entry.o
   • elf_i386 flag is used to emulate the elf_i386 linker.
   • link.ld is a linker template file.

Modify the grub image:

1. Setup grub.img as a loop device:

sudo losetup /dev/loop0 references/grub.img  

2. Create a directory in the mnt folder:

sudo mkdir /mnt/sos

3. Mount the sos directory at /dev/loop0:

sudo mount /dev/loop0 /mnt/sos

4. Copy kernel executable to /mnt/sos/SOS:

sudo cp hydrogen /mnt/sos/SOS

5. Synchronize data on disk with memory:

sudo sync

6. Unmount loop device:

sudo umount /dev/loop0

7. Delete loop:

sudo losetup -d /dev/loop0

Run:

qemu-system-i386 -fda references/grub.img

• -fda option tells qemu to use grub.img as a floppy disk image.