Multiboot Specification

Node: Convenience to operating systems, Next: Boot modules, Previous: Boot-time configuration, Up: Overview

### How to make OS development easier

OS images should be easy to generate. Ideally, an OS image should simply be an ordinary 32-bit executable file in whatever file format the operating system normally uses. It should be possible to `nm` or disassemble OS images just like normal executables. Specialized tools should not be required to create OS images in a special file format. If this means shifting some work from the operating system to a boot loader, that is probably appropriate, because all the memory consumed by the boot loader will typically be made available again after the boot process is created, whereas every bit of code in the OS image typically has to remain in memory forever. The operating system should not have to worry about getting into 32-bit mode initially, because mode switching code generally needs to be in the boot loader anyway in order to load operating system data above the 1MB boundary, and forcing the operating system to do this makes creation of OS images much more difficult.

Unfortunately, there is a horrendous variety of executable file formats even among free Unix-like PC-based operating systems -- generally a different format for each operating system. Most of the relevant free operating systems use some variant of a.out format, but some are moving to ELF. It is highly desirable for boot loaders not to have to be able to interpret all the different types of executable file formats in existence in order to load the OS image -- otherwise the boot loader effectively becomes operating system specific again.

This specification adopts a compromise solution to this problem. Multiboot-compliant OS images always contain a magic Multiboot header (see OS image format), which allows the boot loader to load the image without having to understand numerous a.out variants or other executable formats. This magic header does not need to be at the very beginning of the executable file, so kernel images can still conform to the local a.out format variant in addition to being Multiboot-compliant.