Miscellaneous A YS1 virtual .achine with the handshaking feature avoids many of the
instructions or procedures that would duplicate the function that VM/370 provides: For V51 avoids: • ISK (Insert Storage Key) instructions and uses a key table • Seek separation for 2314 direct access devices • ENABLE/DISABLE sequences in the VS1 I/O Supervisor (lOS) • TCB (Test Channel)
instructions
instructions preceding 510 (Start I/O) CP Introduction 1-45

CP Interruption Handling Interruption processing occurs within the CP environaent. Bore than 30 aodules control the process of interrupting events brought about by CP or virtual machine activity. Each aodule handles a particular I/O device or class or a function of CP, (for exaaple: tiaers, paging. SVCs)@ For an overview of interruption handling, see Figure 14. Program Interruption Program interruptions occur in two states. If the CPU is in the
supervisor state, the interruption indicates a systea failure in the CP nucleus and causes a system abnoLaal If the cpo in the problem state, a virtual machine is in execution. If the prograa
interruption indicates that the Dynaaic Address Translation (DAT) feature has an exception, a virtual machine issued a privileged
instruction, or a protection exception occurred for a shared segment
systea, CP takes control and performs any required processing to satisfy
the exception. Usually, the interruption is not apparent to the virtual
aachine. Most other program interruptions result from virtual aachine
processing and. are reflected to the virtual machine for handling. When a prograa interruption occurs, the prograa interruption handler (DMKPRG) is entered. Program interruptions can result froa: • Normal paging requests • A paging request by a virtual aachine in EC mode (virtual relocate
aode) • Privileged instructions • Program errors For information about paging requests, see "Allocation Banagement" in
this section. Privileged Instructions If a program interruption is by the virtual machine issuing a
privileged instruction when it 1S running in supervisor state, DBKPRVLG obtains the address of the privileged instruction and determines the
type of operation requested. If the virtual machine was running in
problem state, the interruption is reflected back to the virtual
lIachine. I/O PRIVILEGED INSTRUCTIONS DMKPRVLG transfers control to the (DMKVS lEX) • virtual I/O executive program 1-46 IBM VM/370 System Logic and Problem Determination--Volume 1