Simple bootloader

Brief

This is a simple bootloader I wrote for self-interest during my Bachelors, that loads, relocates, and runs a 16 bit C++ executable into memory from a bootable CD. You can run it on an Intel machine, or use VirtualBox.

Details

I started getting interested in Operating Systems after reading parts of the MINIX book. Later, I read the Intel Software Developer's Manual, which explains 32-protected mode, paging, and how to switch between 16 bit to 32 bit mode during boot. To understand the boot process better, I decided to write a simple bootloader myself - a tiny step forward to understanding Operating Systems.

I first tried two experiments - booting from a Floppy and booting from a CD. In the floppy experiment, I wrote a simple assembly code (a COM file) that starts execution from 07c00h of the RAM - it simply prints "Hello" on the screen using BIOS routines. After going through the "El-Torito Bootable CD Specification", I wrote a program that generates a bootable iso image file from a binary image file, to load the COM file, but from a CD instead. After this, I started going through Linux Internals to understand better about virtual memory & paging (MINIX doesn't support paging) from the book Understanding the Linux Kernel. At the same time, I started reading another excellent book - The Indispensable PC Hardware Book - this book explains the old PC AT XT architectures, and lots more.

So far, I was using a real machine for testing. I came across a emulator called Bochs (and later VirtualBox, which is much faster since it runs directly on the hardware). I wrote a Makefile sequence that would compile an image, convert it into a bootable ISO CD image using MagicISO, and run it in Bochs.

With the build system in place, I wrote a simple bootloader to load, relocate, and run a C++ program into memory from the bootable CD image. Later, I switched to a GNU Makefile setup and the GNU assembler. The GNU linker script makes it much easier to control the structure of the output image.