Type ASSEMBLE
Source Lines : 292
Fixes Applied : 3
Most Recent Version : Tuesday, December 12, 1978
Last Fix ID : [HRC004DS]
Module
DMSITI P3031
Function
TO HANDLE ALL INPUT/OUTPUT INTERRUPTIONS, PASS
CONTROL TO THE INTERRUPT PROCESSING ROUTINE, AND
RETURN CONTROL TO THE INTERRUPTED PROGRAM.
Attributes
NUCLEUS RESIDENT, RE-ENTRANT
Operation
DMSITI SAVES THE I/O OLD PSW AND THE CHANNEL STATUS
WORD (CSW). IT THEN DETERMINES THE NATURE OF THE
DEVICE CAUSING THE INTERRUPTION AND PASSES CONTROL TO
THE PROGRAM THAT PROCESSES INTERRUPTIONS FROM THAT
DEVICE. IT DOES THIS BY SCANNING THE ENTRIES IN THE
DEVICE TABLE (DEVTAB) UNTIL IT FINDS THE ONE
CONTAINING THE DEVICE ADDRESS THAT IS THE SAME AS
THAT OF THE INTERRUPTING DEVICE.
WHEN THE INTERRUPT HANDLING PROGRAM CORRESPONDING TO
THE INTERRUPTING DEVICE COMPLETES ITS PROCESSING, IT
RETURNS CONTROL TO DMSITI. AT THIS POINT, DMSITI
TESTS THE WAIT BIT IN THE SAVED I/O OLD PSW. IF THIS
BIT IS OFF, IT USUALLY INDICATES THAT THE
INTERRUPTION WAS CAUSED BY A TERMINAL (ASYNCHRONOUS)
I/O OPERATION. IN THIS CASE, DMSITI RETURNS CONTROL
TO THE INTERRUPTED PROGRAM BY LOADING THE I/O OLD
PSW.
IF THE WAIT BIT IS ONE, THIS USUALLY INDICATES THAT
THE INTERRUPTION WAS CAUSED BY A NON-TERMINAL
(SYNCHRONOUS) I/O OPERATION AND THAT THE PROGRAM THAT
INITIATED THE OPERATION CALLED THE WAIT FUNCTION
PROGRAM TO WAIT FOR A PARTICULAR TYPE OF INTERRUPTION
(USUALLY A DEVICE END, SIGNALING THE COMPLETION OF AN
I/O OPERATION). IN THIS CASE, DMSITI DETERMINES
WHETHER OR NOT AN INTERRUPTION FROM THE INTERRUPTING
DEVICE IS BEING WAITED FOR. IT DOES THIS BY CHECKING
THE PSEUDO-WAIT BIT IN THE DEVICE TABLE ENTRY FOR THE
INTERRUPTING DEVICE. IF THIS BIT IS OFF, THE SYSTEM
IS WAITING FOR SOME EVENT OTHER THAN THE INTERRUPTION
FROM THE INTERRUPTING DEVICE; DMSITI RETURNS TO THE
WAIT STATE BY LOADING THE SAVED I/O OLD PSW. (THIS
PSW HAS THE WAIT BIT ON.)
IF THE PSEUDO-WAIT BIT IS ON, AN INTERRUPTION FROM A
PARTICULAR DEVICE IS BEING WAITED FOR. (THE WAIT
FUNCTION PROGRAM SETS THIS BIT WHEN CALLED BY A
PROGRAM THAT IS WAITING FOR AN I/O INTERRUPTION FROM
A PARTICULAR DEVICE.) IN THIS CASE, DMSITI
DETERMINES WHETHER OR NOT THE INTERRUPTION WAS THE
ONE BEING WAITED FOR. THE PROGRAM THAT PROCESSES THE
INTERRUPTION FROM THE INTERRUPTING DEVICE WILL INFORM
DMSITI OF THIS. IF THE INTERRUPTION IS NOT THE ONE
BEING WAITED FOR, IOINT LOADS THE SAVED I/O OLD PSW.
THIS WILL AGAIN PLACE THE MACHINE IN THE WAIT STATE.
IF THE INTERRUPTION IS THE ONE BEING WAITED FOR,
DMSITI RESETS BOTH THE PSEUDO-WAIT BIT IN THE DEVICE
TABLE ENTRY AND THE WAIT BIT IN THE I/O OLD PSW. IT
THEN LOADS THAT PSW. THIS CAUSES CONTROL TO BE
RETURNED TO THE WAIT FUNCTION PROGRAM, WHICH, IN
TURN, RETURNS CONTROL TO THE PROGRAM THAT CALLED IT
TO WAIT FOR THE INTERRUPTION.
Calls to Other Routines
CALLS VARIOUS INTERRUPT PROCESSING ROUTINES.