Step 8. Check Missing Interrupt Control

CP automatically checks I/O devices for missing interrupt conditions. The default is for CP not to handle missing interrupts for your virtual machine. If your virtual machine is at this default setting, CP sends a message to you (the virtual machine operator) when it detects a missing interrupt. If CP is handling missing interrupts for your virtual machine, it does the following:

  1. Cancels the active I/O on the real device on which the missing interrupt was detected.
  2. If the I/O operation your virtual machine requested was active on the real device, CP reflects an interface control check to your virtual machine.
  3. If the I/O operation your virtual machine requested was not yet active on the real device, then CP retries the I/O operation up to five times. If the I/O operation is unsuccessful after five attempts, CP reflects an interface control check to that virtual machine.
Normally, your user directory entry specifies whether CP handles missing interrupts for your virtual machine. To determine whether CP is handling missing interrupts for your virtual machine, enter: 
query set
CP's response includes either MIH ON or MIH OFF commands. MIH ON means that CP is handling missing interrupts. If you do not want CP to handle missing interrupts, enter: 
set mih off
CP still sends you a message when it detects a missing interrupt.
If CP's response to the QUERY SET command includes MIH OFF, CP is not handling missing interrupts. If you do want CP to handle missing interrupts, enter: 
set mih on
Note:
  1. You can use the SET MITIME command (class A, B) to control the interval at which CP checks a real I/O device for missing interrupts. For more information, see z/VM: CP Commands and Utilities Reference.
  2. If you are running z/VM in a virtual machine, the missing interruption detector defaults to OFF at IPL, and the system running in the virtual machine does not detect missing interrupts. You can use the SET MITIME command to turn on the missing interrupt detector. (For more information about this command see z/VM: CP Commands and Utilities Reference.) If you are using the same terminal to communicate with CP and the operating system running in your virtual machine, MITIME should remain OFF for that terminal. Otherwise, the system running in the virtual machine can be logged off.
  3. Missing interrupt detection is not provided for the Asynchronous Data Mover Facility.