Classes - IBM AT 5170 Technical Reference

Wait (Busy)
Whenever the BIOS is about to enter a busy loop, it first issues an
interrupt hex 15 with a function code of hex 90 in AH. This
signals a wait condition. At this point, the dispatcher should save
the task status and dispatch another task. This allows overlapped
execution of tasks when the hardware is busy. The following is an
outline of the code that has been added to the BIOS to perform
this function.
MOV AX, 90XXH wait code in AH and
type code in AL
INT ISH issue call
JC TIMEOUT optional: for time-out or
if carry is set, time-out
occurred
NORMAL TIMEOUT LOGIC normal time-out
Post (Interrupt)
Whenever the BIOS has set an interrupt flag for a corresponding
busy loop, an interrupt 15 occurs with a function code of hex 91
in AH. This signals a post condition. At this point, the
dispatcher should set the task status to "ready to run" and return
to the interrupt routine. The following is an outline of the code
added to BIOS that performs this function.
MOV AX, 9IXXH post code AH and
type code AL
INT ISH issue call
Classes
~
The following types of wait loops are supported:
• The class for hex 0 to 7F 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.
Compatibility 9-17