Comments from MUSIC/SP source module $sys:iucv.s :

*
*     IUCV - MUSIC IUCV INTERFACE AND TCP/IP (SOCKETS) SUPPORT
*
*   This module handles the IUCV requests from applications, keeps
*   track of what application owns a conversation and passes the
*   external interrupts to the appropriate application.
*
*   During system initialization, a DECLARE_BUFFER is issued to
*   define MUSIC's IUCV external interrupt buffer. There is only
*   one of these for the MUSIC virtual machine. When a task issues
*   a DECLARE_BUFFER to initiate IUCV, it is basically treated as a
*   NOP operation. When an external interrupt arrives that applies
*   to that task, the interrupt handler passes the information to
*   task via the inter-task communications queue. This information
*   can be retrieved using the QRD routine. This handling of the
*   external interrupt buffer data makes the MUSIC implimentation
*   a bit different from CMS.
*
*   When a task issues a CONNECT function the assigned PATHID is
*   saved with the TCB number the PATHID_TABLE. When an external
*   interrupt occurs, this table is used to find the TCB number of
*   MUSIC task that is to receive the interrupt information.
*
*   If an outside task is to initiate a connection with a MUSIC task
*   it must know the resource name associated with the MUSIC task.
*   The MUSIC task sets the resource name in the ENQ table. So when
*   an interrupt comes in requesting a connection, the TCB number of
*   MUSIC task is retrieved from the ENQ_TABLE.
*
*   In addition to executing the IUCV instructions and passing on
*   interrupts this module must also deal with MUSIC's virtual
*   storage. The request parameters are copied from the user region
*   to system storage before the request is made. The results are
*   copied back afterwards. If a request involves data buffers,
*   these are allocated in the system area and the virtual addresses
*   replaced by real addresses. Once the request is complete, the
*   data is copied to the user's virtual storage.
*
*
* Note on addresses passed to IUCV:
*   All addresses passed to VM IUCV (registers, in parm list,
*   in address lists, etc.) must be REAL addresses (i.e. real to
*   MUSIC).
*
*
*     Assumptions and Limitations of this Support
*     -------------------------------------------
*
*  This module does not support general IUCV.  It supports
*  what is needed for communication between MUSIC and the TCPIP
*  virtual machine, for support of TCP/IP socket routines.
*  This is mainly IUCV Connect, Send, and Sever requests.
*  For TCP/IP, MUSIC acts as the "source communicator".
*  Some other IUCV requests may work, but have not been tested.
*