IO

You cannot use any libraries, so how to do IO? Some ways that this can be done:

BIOS functions: http://wiki.osdev.org/BIOS. Not well standardized like it's successor UEFI. Called through interrupts.
https://en.wikipedia.org/wiki/VGA-compatible_text_mode
VBE https://en.wikipedia.org/wiki/VESA_BIOS_Extensions

Showdown and restart can be managed with either:

APM
ACPI https://en.wikipedia.org/wiki/Advanced_Configuration_and_Power_Interface

Newer and better.

Now managed by the same group that manages UEFI.

Spec:
- current: http://uefi.org/specifications
- old: http://www.uefi.org/acpi/specs

See also: http://wiki.osdev.org/Shutdown