Eit 23 of the virtual interval timer is decremented like the real
interval timer. If a virtual machine is executing (the real PSi is in
problem state) and the real interval tiaer needs updating, both the real
interval timer and the virtual interval timer are updated. The virtual
interval timer is decremented only if the following conditions are true: • The real PSi is in the problem state (bit 15 = 1) • Virtual machine assist is enabled (control register 6 bit 0 = 1) • Virtual interval timer assist is enabled (control register 6 bit 7 =
1)
A virtual interval timer interrupt request is recognized when the
virtual interval timer is decremented from a positive value, including
zero, to a negative value. When this condition occurs, an interval timer interrupt is presented to the virtual aachine.
If the virtual interval tiaer interrupt cannot be presented to the
virtual machine, the virtual interval timer interrupt is presented to
the real machine. When presented to the real machine, the virtual
interval timer interrupt is distinguished from the real interval timer interrupt by a unique external interrupt code (bit 7 of the halfword
interrupt code set to 1 and bits 0-6 set to 0). Bits 8-15 of the
interrupt code are zero unless set to one for another condition that may be concurrently indicated. (Locations X'84' and X'85' are set to zero.) 1-422 IBM VM/370 System Logic and Problem Determination--Voluae 1

Appendix B: VM/370 MSS Support VM/370 MSS Support Following are annotated flow diagrams for the logic to support the IBM 3850 Mass Storage system. LOGON A USER BAVING A MINIDISK ON AN UNMOUNTED SYSTEM VOLUME A required system volume is not mounted, try to get a 3330V mounted
if the minidisk is a 3330. DMKSSSLN to mount an MSS system volume. DMKSSS, FINDRDEV real 3330V device. This may involve demounting a
volume which is mounted but not in use. If there are none such
volumes available, issue message DMKSSS080E and return with return
code 8. DMKSSS, ---Construct an MSSCOM, filling in the volume serial, device address
selected, type of request (mount), and userid. DMKSSS, SETMNTFG ---auIld-i-cPEIBLOK for the return to DMKLNK after the MSC has processed
the request. Chain it from field MSSTASK2. Build a CPEXBLOK for the
return to DMKLNK after the mount is complete (pack change
interruption received on the 3330V). Chain it fro. field MSSTASK1. DMKSSS, SCBMSSC ---Put the-ftSSCOM in the queue, generate an attention interruption for
the communication device if necessary. and exit to DMKDSP. BVCQ!!!% Entry when DIAGNOSE code X'18', subcode 4 is received. OS/VS is
ready to process an MSC request. Place the next 8SSCOM in the
virtual machine, and return to DMKBVC. DMKSSS, BVC08ENT -- Entry-froi--D8KBVC when DIAGNOSE code X'18', subcode 8 is received.
The MSC has processed the mount request. If there was an MSS error, write message DMKSSS083E and return to DMKLNK with return code 8. If there was not an MSS error, indicate
waiting for the pack change interruption.
Return to DMKLINK with return code 4. that the MSSC08 is now Write message D8KSSS088I. Return from DMKSSS. Save the current workarea and control
information. Return to caller.
Appendix B: VM/370 MSS Support 1-423