OTHER OPERATING SYSTEMS are availible for the IBM PC. These include:
The Disk Operating System (DOS) and the ROM BIOS serve as an insulating layer between the application program and the machine, and as a source of services to the application program. The system heirarchy may be thought of as a tree, with the lowest level being the actual hardware. The INTEL\80x86 or NEC\Vx processor sees the computer's address space as a ladder two bytes wide and one million bytes long. Parts of this ladder are in ROM, parts in RAM, and parts are not assigned. There are also 256 ports that the processor can use to control devices. The hardware is normally addressed by the ROM BIOS, which will always know where everything is in its particular system. The chips may usually also be written to directly, by telling the processor to write to a specific address or port. This sometimes does not work as the chips may not always be at the same addresses or have the same functions from machine to machine.