Original PC Hardware Reference

An Introduction to the AT CD: 39
AT BIOS Versions
Three different BIOS versions have been used in the IBM AT. They can be identified by their date
and summarized as follows:
■ January 1, 1984. The first AT BIOS version supported only 1.2MB and 360B floppy drives
directly. Only 14 hard disk types were supported. It came on the Model 068 and 099 systems
with the Type 1 motherboard.
■ June 10, 1985. The second AT BIOS added support for 720KB 3 1/2-inch drives directly (no
drivers were required). Also, more hard disk drive types were added, for a total of 22 usable
types. A new test was added to the POST that caused the POST to fail if the clock speed was
altered from 6MHz. This BIOS was used on the Model 239 with a Type 2 motherboard.
■ November 15, 1985. The third and final AT BIOS added support for 1.44M 3 1/2-inch drives
(no drivers required). Enhanced 101-key keyboard support was added. The POST test
checked for 8MHz operation, and failed if the system was running at any other speed. This
BIOS was used on Model 319 and 339 systems, and came on a Type 2 motherboard.
Refer to Table 2 for a list of the different IBM Family/1 (PC, XT, and AT) BIOS versions. This table
also shows the ID, submodel, and revision bytes that can be determined by a software function
call.
Int 15h, C0 = Return System Configuration Parameters
Some of the systems such as the PC and earlier XT and AT systems only support the ID byte; the
submodel and revision bytes had not been established when those systems were developed. The
table also shows the number of drive types supported in the AT and XT-286 systems BIOS.
The BIOS date is stored in all PC-compatible systems at memory address FFFF5h. To display the
date of your BIOS, a DEBUG command can be used to view this address. DEBUG is a command
program supplied with MS-DOS. At the DOS prompt, execute the following commands to run
DEBUG, display the date stored in your BIOS, and then exit back to DOS:
C:\>DEBUG
-D FFFF:5 L 8
FFFF:0000 30 31 2F-32 32 2F 39 37 01/22/97
-Q
In this example, the system queried shows a BIOS date of 01/22/97.
AT Motherboard BIOS Hard Drive Tables
The AT BIOS contains a special table that is used by the hard disk controller driver to determine
the hard drive parameters. When a hard disk is installed into this type of system, the “type” of
drive is entered into the CMOS RAM by whoever has installed the drive. Then, every time the
system boots, it looks up the parameters by consulting the CMOS RAM for the particular type
that has been selected.
Older systems were therefore limited to what different drives they could support or recognize by
the entries burned into their BIOS table. The table used in IBM AT and PS/2 systems is shown in
this section.