DOS Boot Sector

This is an x86 boot sector written in C. It is meant to bootstrap a DOS system on a FAT volume. A more detailed explanation of the code is available at https://crimsonglow.ca/~kjiwa/2010/09/dos-boot-sector.html.

View the source. It works like this at a high level:

• get the drive parameters
• load the contents of the root directory into memory
• look for IO.SYS in the root directory
• load the first 3 sectors of IO.SYS into memory and begin executing it

I've successfully compiled and tested it with the following tools:

• gcc 12.2.0
• binutils 2.40
• dosfstools 4.2
• bochs 2.7

The gcc and binutils host and target are both i386-pc-elf.

$ make boot    # compile and link the boot sector
$ make io.sys  # compile and link IO.SYS
$ make c.img   # create a 10 MB disk image with a FAT volume, boot sector, and IO.SYS
$ make run     # runs Bochs

External Links

• SYS.COM Requirements in MS-DOS versions 2.0-6.0
• int 13
• Memory Map (x86)
• File Allocation Table