Bios Services; Device 1/0 Services - Tandy 1000 SL Technical Reference Manual

BIOS Services
Device I/O Services
Introduction
The BIOS (Basic Input/Output System) is the lowest-level interface
between other software (application programs and the operating sys-
tem itself) and the hardware. The BIOS routines provide various
device input/output services as well as bootstrap and print screen and
other services. Some of the services that BIOS provides are not avail-
able 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 conventions are, for the
most part, identical.
The following pages describe the entry and exit requirements for each
BIOS routine. To execute a BIOS call, load the registers as indicated
under the "Entry Conditions" banner. (Register AH will contain the
function number in cases where a single interrupt can perform more
than one operation.) Then issue the interrupt given for the call. The
following example can be used to read a character from the keyboard:
MOV AH,O
INT 16H
1.