IBM AT 5170 Technical Reference page 445

(625 ms-read)
OFEH yes (18 second)
printer
The asynchronous support has been omitted. The Serial/Parallel
Adapter will generate interrupts, but BIOS does not support it in
the interrupt mode. Therefore, the support should be included in
the multitasking system code if that device is to be supported.
Time-Outs
To support time-outs properly, the multitasking dispatcher must
be aware of time. If a device enters a busy loop, it generally
should remain there for a specific amount of time before
indicating an error. The dispatcher should return to the BIOS
wait loop with the carry bit set if a time-out occurrs.
Machine-Sensitive Code
~
Programs may select machine specific features, but they must test
for specific machine type. Location of the specific machine
identification codes can be found through interrupt 15 function
code AH (See
I
Configuration Parameters
I
in BIOS Listing). The
code is two bytes. The first byte shows the machine type and the
second byte shows the series type. They are as follows:
First Second
Byte Byte Machine Identification
FF
00
IBM Personal Computer
FE
00
IBM Personal Computer XT
FE
00
IBM Portable Personal Computer
FD
00
IBM PCJr
FC
00
IBM Personal Computer
AT
Machine Identification Code
IBM will define methods for uniquely determining the specific
~
machine type or I/O feature for any new device.
Compatibility 9-19