Atari ST series Technical Reference Manual page 19

The lowest-level
are in the section of the operation system known as the BIOS
(Basic Input/Output System). The BIOS contains three basic
types of I/O routines.
The first group of I/O routines contains routines for com­
munication with character-oriented I/O devices like the
printer, the screen, the serial port, and MIDI port. The sec­
ond group contains the basic functions used to communicate
with the disk drive at the hardware level. These allow you
determine how many drives are connected, whether a disk
has been changed in a drive, and where to find the BIOS pa­
rameter block for a drive, which gives information about the
drive configuration. They also let you read or write to the
disk at the sector level, which is a lower level of organization
than the normal disk filing system. Finally, the BIOS con­
tains some miscellaneous routines that perform various sys­
tem functions, such as reading or setting the exception vec­
tors and returning information about the memory
management system and the precision level of the system
clock.
The ST BIOS routines can be called from user mode, and
are reentrant to three levels. They use registers A0-A2 and
D0-D2 as scratch registers, which means if you're program­
ming in machine language and using these registers to store
important information, you must save their contents before
making a BIOS call and restore them after the BIOS call.
Each of the BIOS routines has a command number associated
with it. It may also be associated with command parameters
that specify more precisely what the function should do.
For example, the BIOS function to output a character to
a device is command number 3. It requires two command pa­
rameters: One tells the function which character to print and
the other specifies the output device to use.
ST Input/Output routines
11