jhall@wpi.WPI.EDU (John Clinton Hall) (11/20/90)
I forgot who it was, but someone here was looking for information regarding
the system ID byte.
From the Dos Programmer's Reference, Second Edition:
A model-identification byte (located at F000:FFFEh) can be used to
differentiate between models. The PS/2 family continues to support this
model identification byte. For non-IBM machines, however, this byte cannot
be relied on; no standard set of values has ever been established.
Interrupt 15h/C0h returns a pointer to system descriptor information:
Calling Registers: AH C0h
Return Registers: PC, PCjr:
Carry flag set
AH 80h
PC XT BIOS (11/08/82), Personal Computer AT BIOS
(01/10/84):
Carry flag set
AH 86h
All Others:
ES:BX Pointer to system descriptor table in
ROM
Comments: The ROM system descriptor table contains useful information
about the system. The following table shows the meaning of the entries:
Offset Meaning
------ -------
00h Byte count of data that follows (minimum 8)
02h Model byte
03h Submodel byte
04h BIOS revision level (00 = first release)
05h Feature information
06-09h Reserved
The model byte contained at offset 02h of the system descriptor table
should be the same as the system ID byte (stored at F000:FFFEh). The
submodel byte (offset 03h) can be used for additional system
identification. From the information shown in the following table, you can
determine what type of IBM computer is being used.
Computer
Type Model Submodel BIOS Revision
-------- ----- -------- -------------
PC FFh
PC XT FEh
PC XT FBh 00h 01h
PC XT FBh 00h 02h
PCjr FDh
AT FCh
AT FCh 00h 01h
AT, COMPAQ 286 FCh 02h 00h
PC XT 286 FCh 02h 00h
PC Convertible F9h 00h 00h
PS/2 Model 30 FAh 00h 00h
PS/2 Model 50 FCh 04h 00h
PS/2 Model 60 FCh 05h 00h
PS/2 Model 80 F8h 00h 00h
The feature information byte is interpreted as shown in the following
table:
Bit
76543210 Meaning
-------- -------
.......x Reserved
......0. PC bus I/O channel
......1. Micro channel architecture
.....1.. Extended BIOS data area (EBDA) allocated
....1... Wait for external event is support
...1.... Keyboard intercept is called by INT 09h
..1..... Real-time clock present
.1...... Second interrupt chip present
1....... DMA channel 3 used by hard disk BIOS
--
----------------------------------------------------------------------------
The highest sounds are hardest to hear. ## ## ## ##### ##
Going forward is a way to retreat. ## ## ## ## ## ## jhall
Great talent shows itself late in life. ## ## ## ##### ## @wpi.wpi.edu