IBM 5170 Technical Reference page 392

• The class for 0- > 7Fh is serially reusable. This means that for
the devices that use these codes, access to the BIOS must be
restricted to only one task at a time.
• The class for 80h->BFh is reentrant. There is no restriction
on the number of tasks which may access the device.
• The class for COh->FFh is non-interrupt. There is no
corresponding interrupt for the wait loop. Therefore, it is the
responsibility of the dispatcher to determine what satisfies this
condition to exit the loop.
Function Code Classes
type code (AL)
Description
OOh->7Fh
serially reusable devices; operating system
must serialize access
80h->OBFh reentrant devices; ES:BX is used to
distinguish different calls (multiple I/O
calls are allowed simultaneously)
OCOh->OFFh
wait only calls; there is no complementary
"POST" for these waits--these are timeout
only. Times are function number
dependent.
Function Code Assignments
The following are specific assignments for the IBM Personal
Computer AT BIOS. They are grouped according to the classes
described under "Function Code Classes".
Type Code (AL)
Timeout Description
OOH yes (6 sec) IBM Personal
Computer AT fixed
disk
9-14 Compatibility