IBM 5150 PC Technical Reference (6025005, August, 1981) (PDF)

ROM BIOS
The ROM resident Basic I/O System (BIOS) provides the device
level control of the major I/O devices in the System Unit. The
BIOS routines allow the assembly language programmer to
perform block (diskette and cassette) or character (Video, ~
communications, keyboard and printer) level I/O operations
without any concern for device address and operating characteristics.
Additionally, system services such as time of day and
memory size determination are provided. The goal is to provide
an operational interface to the system and relieve the programmer from
concern over hardware device characteristics.
Finally the BIOS interface· insulates the user from the hardware
allowing new devices to be added to the System Unit, yet
retaining the BIOS level interface to the device. In this manner,
user programs become transparent to hardware modifications
and enhancements. A complete listing of the BIOS is provided
in Appendix "A".
Use of BIOS
Access to the BIOS function is through the 8088 software
interrupts. Each BIOS entry point is available through its own
interrupt, which can be found in the interrupt vector listing.
The software interrupts lOH through lAH each access a
different BIOS routine. For example, to determine the amount
of memory available in the system,
INT 12H
will invoke the memory size determination routine in BIOS
and return the value to the caller.
Parameter Passing
All parameters passed to and from the BIOS routines go through
the 8088 registers. The prologue of each BIOS function indicate
the registers used on the call and the return. For the memory
size example above, no parameters are passed, and the result,
memory size in 1 K Byte increments is returned in the
AX register.
Where a BIOS function has several possible operations, the AH
register is used on input to indicate the desired operation. For
example, to set the time of day, the following code is required.
3-2