[1][2] Development can begin from existing resources like a kernel, an operating system, or a bootloader, or it can also be made completely from scratch.
The use of assembly language is common with small systems, especially those based on eight bit microprocessors such as the MOS Technology 6502 family or the Zilog Z80, or in systems with a lack of available resources because of its small output size and low-level efficiency.
For example, AtheOS was a hobby operating system with a graphical interface written entirely by one programmer.
[4] The BIOS also sets up a standard interface for several low-level device drivers at boot time.
These are used because video cards and disk drives vary significantly on different machines and specialised drivers are often difficult to write.