- Duration: 2008 - 2010
- Language used: Intel assembly, C++
- Platform: Windows Vista
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.
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.
- The bootloader is loaded into RAM by the computer, from the bootable CD image
- The bootloader loads the C++ image into a suitable location in memory
- The bootloader relocates the addresses in the C++ image, using the information in the relocation table
- The bootloader calls all the global C++ object constructors, using information from the exe file
- The bootloader jumps into the C++ image and starts execution
This is a screenshot in Bochs after boot. The C++ program simply prints out the letter 'B' multiple times on screen:
Boot procedure, writing linker scripts, relocation, EXE file format, C++ object initialization
- Operating Systems Design and Implementation (The MINIX Book): For MINIX OS internals
- Operating System Principles (The Dinosaur Book): For OS concepts
- Intel Software Developer's Manuals: For understanding the Intel 32-bit protected mode
- El-Torito Bootable CD Specification: Structure of the bootable ISO CD image
- Understanding the Linux Kernel: For Linux OS internals
- Indispensable PC Hardware Book: For bus system operation