BIOS Services
Device I/O Services
Introduction
The BIOS (Basic Input/Output System) is the lowest-level inter-
face between other software (application programs and the oper-
ating system itself) and the hardware. The BIOS routines
provide various device input/output services, as well as boot strap
and print screen and other services. Some of the services that
BIOS provides are not available through the operating system,
such as the graphics routines.
All calls to the BIOS are made through software interrupts (that
is,
by means of assembly language "INT x" instructions). Each
I/O device is provided with a software interrupt, which transfers
execution to the routine.
Entry parameters to BIOS routines are normally passed in CPU
registers. Similarly, exit parameters are generally returned from
these routines to the caller in CPU registers. To insure BIOS
compatibility with other machines, the register usage and con-
ventions are, for the most part, identical.
The following pages describe the entry and exit requirements for
each BIOS rutine. To execute a BIOS call, load the registers as
indicated under the "Entry Conditions." (Register AH will con-
tain the function number in cases where a single interrupt can
perform more than one operation.) Then issue the interrupt
given for the call. The example, the following can be used to read
a character from the keyboard:
MOV AH,0
INT 16H
Upon return, AL contains the ASCII character and AH the key-
board scan code.
Note: All registers except those used to return parameters to
the caller are saved and restored by the BIOS routines.
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