Retro1.org codex

EXTERNAL                                                 CP Privilege Class:  G

Use the EXTERNAL command to simulate an external interrupt to the virtual
machine and return control to that machine.  The format of the ATTN command is:
+----------+------------------------------------------------------------------+
| EXTernal | [40|code]                                                        |
+----------+------------------------------------------------------------------+
where:

40 | code
         is the interrupt code, a hexadecimal number to be associated with the
         external interrupt.  All codes within the range X'01' through X'FF'
         are valid.  If your virtual machine has the ECMODE option specified in
         the VM/370 directory, you can also code X'1004' (clock comparator
         interrupt) or X'1005' (CPU timer interrupt).  The default is the
         external interrupt button on the system console, X'40'.


Usage notes:

This simulates pressing the interrupt key on the real computer console, or
other functions which cause an external interrupt.  Control is given to the
virtual machine immediately.