Editing IBM PS/2 Model 25 (section)

== ROM BIOS and Reference Diskette ==

The PS/2 line uses '''two parallel BIOS layers''':

* '''CBIOS''' (Compatible BIOS): backward-compatible with the PC/XT/AT BIOS, used by PC DOS.
* '''ABIOS''' (Advanced BIOS): the PS/2 protected-mode / multitasking-aware BIOS, used by OS/2.

The Model 25 BIOS is held in '''two 32 KB EPROMs''' (64 KB total) in sockets U13/U17 (or U19/U20 depending on planar revision). Unlike the PC/AT, the Model 25 does '''not''' have IBM ROM BASIC in the BIOS &mdash; instead, a stub in the BIOS prompts for a boot diskette if none is found.

The Model 25 does not have a built-in SETUP utility. All configuration is performed from the '''IBM Personal System/2 Reference Diskette''', a model-specific boot diskette supplied with the machine. The Reference Diskette includes:

* '''SETUP''' &mdash; sets memory size, drive types, date/time (286/SX only), display, mouse, and adapter card definitions.
* '''Advanced Diagnostics''' &mdash; full hardware test suite (entered with Ctrl-A from the main menu).
* '''Backup utilities''' for copying the Reference Diskette to a working diskette.

Reference Diskettes are '''model-specific'''. A 5170 Diagnostics diskette will not work on a Model 25; a Model 30 Reference Diskette is also '''not''' interchangeable with the Model 25 Reference Diskette.

Cold boots of the Reference Diskette are recommended over warm (Ctrl-Alt-Del) boots, because a warm boot may produce false errors or falsely indicate that a power-on password is already set.