Interfaces - IBM 5170 Technical Reference

Interfaces
There are four interfaces to be used by the multi-tasking
dispatcher:
Startup
The fIrst thing to be done is for the startup code to hook interrupt
hex 15. The dispatcher is responsible to check for function codes
AH
=
hex 90 and 91. The "Wait" and "Post" sections describe
these codes. The dispatcher must pass all other functions through
to the previous user of interrupt hex 15. This can be done via a
JMP or a CALL. If the function code is hex 90 or 91, then the
dispatcher should do the appropriate processing and return via the
IRET instruction.
Serialization
It is up to the multi-tasking system to insure that the device driver
code is used in a serial fashion. Multiple entries into the code can
result in very serious errors.
Wait (Busy)
Whenever the BIOS is about to enter a busy loop, it first issues an
interrupt 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 is
an outline of the code which has been added to the BIOS to
implement this function.
EXAMPLE DEVICE BUSY LOOP
DO UNTIL
/
MOV AX, hex 90XX ; WAIT code in AH and
9-12 Compatibility