Calling Director Procedures

 This chapter describes how a subsystem acquires access to the Director
 procedures by forming System Call descriptors from the pairs of values
 (i,j) associated with the identifiers of the procedures forming the
 Director interface.  Knowledge of the 2900 Series System Call mechanism
 (Ref. 5) is assumed.  The fields SC IDENS AD, SC IDENS and SCT DATE in
 the record format UINFF are relevant.  SC IDENS AD is the address of a
 record array whose elements have the following format:

      %recordformat SC IDF (%string(31)IDEN, %integer I, J)

 SC IDENS is the number of elements in the record array.

 In the Edinburgh standard object module format, calls to external
 routines in modules compiled separately involve a machine instruction
 CALL with a 64-bit operand which is a descriptor descriptor.  If the
 operand instead is a System Call descriptor then the System Call
 mechanism is invoked, providing controlled access to higher-privilege
 procedures (such as the Director procedures).

 When a version of Director is created, a list of identifiers specifying
 permitted procedure entry points is referenced, a new System Call Table
 entry is created, and the identifier and (i,j) values are placed in the
 record array described above.  The task of a subsystem requiring access
 to a Director procedure is thus that of selecting the correct (i,j)
 pair from the record array, creating a System Call descriptor
 containing the (i,j) values, and making that descriptor the operand of
 a CALL instruction.  Subsystem writers using the Edinburgh standard
 object module format should consult Ref. 6.

 The field SCT DATE has been provided to remove the need for a
 subsystem to satisfy its references to Director procedures dynamically,
 and to enable them to be fixed up when the subsystem is created.
 SCT DATE is a unique identifier associated with a given version of
 Director.  If the SCT DATE current at the time a subsystem is entered
 is identical to that current when the subsystem was created, then
 clearly the (i,j) values and entry identifiers obtaining when the
 subsystem was created will serve for invocations of the new subsystem.
 The new subsystem will of course have to retain code for fixing up
 Director references dynamically should it find that SCT DATE is
 different from that current when it was created.


DNEWINWARDCALL

%externalintegerfn DNEW INWARD CALL(%integer KEY, DR0, DR1,
      %integername I, J)

 A new System call table entry is created for an inward call to the
 current ACR (ie the ACR of the program calling this procedure).  ACCESS
 KEY specifies the ACR at and below which access is allowed to the
 specified procedure, and DR0, DR1 is a descriptor descriptor to the
 entry descriptor for the procedure.  The i and j values specifying the
 System call table entry are returned in I and J.

 Entries created by this procedure are cumulative and cannot
 subsequently be removed from the System call table.


DNEWOUTWARDCALL

%externalintegerfn DNEW OUTWARD CALL(%integer NEWACR, EMAS,
   NEW STACK SEG, DR0, DR1, %integername I, J)

 A System call table entry is created for an outward call, according
 to Ref. 8.  In the entry created, KEY is set equal to the ACR of the
 program calling this function, E is set to 0 or 1 according to the
 value of EMAS. Target SSN is set with the value of NEW STACK SEG, which
 must be even and in the range 34 to HISEG.  DR0 and DR1 should supply the
 descriptor descriptor to the required entry descriptor for the
 procedure to be outward-called.  I and J are set on return to the i and
 j values to be used in making the outward call.

 Only one of these System call table entries will be extant at any time
 a further call of DNEW OUTWARD CALL will cause the previous outward
 call entry to be lost.

 The descriptor supplied is not checked.


DNOMINATESTACK

%externalintegerfn DNOMINATE STACK(%integer SEG)

 This function is provided to satisfy requirement number 4 of  Ref. 8,
 by calling the OUT12 (defined in Ref. 9) to notify the Local Controller
 to enable it to supply the locked-down SSN+1.


DPLUGINJVECTOR

%externalintegerfn DPLUGIN JVECTOR(%integername IVALUE,
      %integer ADDRESS, TOP ENTRY)

 ADDRESS points to the zeroth element of a record array (0:TOP ENTRY),
 format (integer TYPE, ACR, DR0, DR1), which is to be a new ("vertical")
 J-vector in the System Call table.  If the call is successful, IVALUE
 is set to the I-value to be used in subsequent System Calls accessing
 the vector.  The procedure allows a subsystem to cause outward calls
 and to allow users' programs to do inward calls to the subsystem.  The
 address must lie in a non-write-access file, readable at Director's
 ACR.    No target ACR in any of the entries may be less than the
 caller's ACR, nor may the target PRIV bit be set.  After a successful
 call, the file may not be disconnected (except by calling DSTOP), nor
 may its size or access mode be changed (error result 84 is returned if
 attempt is made).  The vector may contain a maximum of 512 System Call
 entries. If the vector exceeds this limit the error result 8 is
 returned.  Subsequent calls of DPLUGIN JVECTOR cause the previously
 plugged-in vector to be forgotten (though the file still may not be
 disconnected).