Second Level Support
Trace Table Entries
An SCP that supports lUCY communications functions correctly in a virtual
machine generated by a CP system that supports lUCY. The lUCY macro instruction generates an operation exception in the real
hardware.
When a virtual machine invokes an lUCY function, it must be in a virtual supervi­
sor state.
A virtual machine must invoke the DECLARE BUFFER function before other lUCY functions except the QUERY function. Failure to do so causes an operation
exception to be reflected to the virtual machine.
Thus, an SCP can support lUCY in a virtual machine exactly as it does on real
hardware.
CP system code invokes lUCY functions through a CALL linkage rather than the lUCY macro instruction. lUCY support generates a trace table entry for each lUCY function. There is one
trace table entry type for lUCY entries (X'lS'). Each entry contains a subtype
field to indicate the exact lUCY function a communicator invoked.
Whether invoked from a virtual machine or from CP system code, all uses of lUCY are recorded in the CP trace table. The address portion of the old PSW is recorded
as part of the entry. A bit in the flags byte indicates whether this address is a real
address (when invoked from CP) or a virtual address (when invoked from a virtual
machine). For virtual machine addresses, the address of the associated VMBLOK can be obtained from preceding trace table entries.
The lUCY trace facilities can be suppressed at assembly time by setting
&TRACE(9) to 0 or at execution time by setting the X'80' bit to 0 in TRACFLG3
in PSA. lUCY functions invoked by other functions are also recorded as if they had been
invoked from CPo These secondary functions include:
The RETRIEVE BUFFER function generates a SEVER for all established
paths.
The SEVER function generates a REJECT for each incoming outstanding
message and a PURGE for each outgoing outstanding message.
A CONNECT issued to a CP system service passes control to that service.
The selected CP system service usually invokes the ACCEPT function.
The CP dispatcher invokes the DESCRIBE and TEST COMPLETION func­
tions to dequeue messages intended for the CP system. Inter-User Communications Vehicle 117

Audit Trail Restrictipns Security Considerations IUCV maintains an audit trail for each message. The audit trail is a bit significant
value that records the status of the message. The value is maintained in the MSGBLOK that represents the message. The audit trail is presented to the source
communicator during execution of the PURGE and TEST COMPLETION func­
tions and when the source receives a message-complete IUCV external interrupt.
The audit trail for a message indicates:
If the message caused a protection or addressing exception on the source com­
municator's send or answer buffer
If the message caused a protection or addressing exception on the target's
receive or reply buffer
If a reply was too long for the source's reply buffer
If a message was rejected by the target
If a path was severed
The following areas of IUCV are limited:
The use of IUCV is supported for a second level CP system. The IUCV func­
tions are not simulated, but are reflected to the second level system.
Each virtual machine is limited to less than 65,536 outstanding connections at
one time. lUCY does not recognize anything smaller than a virtual machine. If two
communicators choose to establish mUltiple communication paths, it is the
responsibility of these communicators to manage these paths.
A CP system service cannot establish communication with itself.
CP system services are limited to a total of 4,096 outstanding connections.
Installations control the use of IUCV through the virtual machine directory entries.
If the installation has not authorized a user for IUCV communications in the direc­
tory, all requests for IUCV communications to virtual machines other than his own
are denied. The installation must specifically authorize each virtual machine which
is to communicate with a CP system service. IUCV moves data from one virtual machine address space to another. At no time
does a virtual machine have access to the storage or registers of CP or another vir­
tual machine. When the user invokes the RECEIVE or REPLY functions, the data
to be moved is described by a starting address and a length. The exact length spec­
ified is the maximum amount of data moved. There are no requirements placed on
a virtual machine as to the location of these buffers.
118 VM/SP System Programmer's Guide