HP 3000 Manuals

Switch Programming User's Guide

Switch Subsystem 

The Switch subsystem is an integral part of the MPE XL operating system.
Switch consists of a set of intrinsics that provide the following
services:

 *  Ability to call subsystem, intrinsic, or user-written CM SL
    procedures from NM system or user code

 *  Ability to call subsystem, intrinsic, or user-written NM XL
    procedures from CM system or user code


NOTE  MPE V/E intrinsics (except for those no longer supported) are
      directly callable from both Compatibility Mode and Native Mode.
      You do not need to code any Switch calls to access these
      intrinsics.


Figure 1.8 and Figure 1.9 demonstrate the role of the Switch intrinsics
in the process of carrying out mixed-mode procedure calls:


          Figure 1.8.  Role of Switch Intrinsics in CM--> NM Switch


          Figure 1.9.  Role of Switch Intrinsics in NM--> CM Switch

Purpose 

Due to architectural differences between MPE V/E- and MPE XL-based
systems, programs that switch between CM and NM execution may encounter
differences in data representation.  The primary purpose of Switch is to
resolve this situation by providing mixed-mode execution access and
parameter translation between Compatibility Mode and Native Mode.

Invoking Switch 

One way to invoke Switch is to use an intermediate, user-written
procedure that sets up variables and calls Switch to translate these data
references whenever the program changes execution mode.  This
intermediate procedure is called a Switch stub.  The primary purpose of a
stub is to provide transparency to the calling procedure/program and
thereby avoid source changes in the original code.  To do this, the stub
must have the same name as the original procedure.

You can use the SWitch Assist Tool (SWAT) to automatically generate
NM-to-CM stub procedures, or you can write your own Switch stubs.  Refer
to Chapter 2 for information on SWAT and to Chapters 3-5 for information
on the Switch subsystem and the process of writing your own stubs.

Another way to invoke Switch is to code the Switch call, and its
attendant setting up of variables and error checking, directly in line in
the calling procedure.  Although this involves modification of your
program, there are factors that may lead you to consider this
alternative.  For example, every time you call a procedure or function,
the call and the entry and exit code add to execution overhead.  This
overhead could increase execution time if it occurred in a critical
section of your program.

In-line switch code may be appropriate for other-mode procedures that
should execute quickly and/or are executed often (for example, in a
loop).  If an other-mode procedure is referenced in many places, then the
convenience of the stub can justify the added overhead of a procedure
call.  Another factor influencing the choice between stubs and in-line
switches is whether data setups can be reused.  If not, the in-line
switch is not justified.  Finally, if backward source compatibility is
important to you, you should use Switch stubs.  To compile an application
or module on an MPE V/E system, removing the stub from the compile
procedures is all that is required.

Switch to CM Overview 

Figure 1.10 represents the process of mixed-mode procedure calling in the
NM--> CM direction.


          Figure 1.10.  Mixed-mode Procedure Call:  NM--> CM

A mixed-mode procedure call in the direction NM--> CM involves the
following steps, as indicated in Figure 1.10:

1.               The NM code needing to access a CM routine calls a
                 Switch intrinsic, either by means of in-line code or a
                 Switch stub.

2.               The in-line code or NM Switch stub sets up the data
                 structures and parameters required by Switch and makes a
                 call to the HPSWITCHTOCM intrinsic (call by name), or
                 the HPLOADCMPROCEDURE and HPSWITCHTOCM intrinsics (call
                 by plabel).

3.               HPSWITCHTOCM calls the CM routine and passes the
                 parameters, translating addresses and/or copying data to
                 Compatibility Mode as required by their type, size, and
                 location.

4.               The CM routine executes using the passed parameters and
                 returns its data values to HPSWITCHTOCM.

5.               HPSWITCHTOCM receives the data values returned by the CM
                 routine, back-translating addresses and recopying data
                 as needed.

6.               HPSWITCHTOCM returns the translated values to the
                 caller.  The stub or in-line code checks whether the
                 Switch operation was successful and then control returns
                 to the NM routine.


NOTE  HPSWITCHTOCM is the system intrinsic that makes NM parameters
      addressable to Compatibility Mode, changes the execution mode, and
      invokes the CM routine.  HPSWITCHTOCM and HPLOADCMPROCEDURE reside
      in the NM system library, NL.PUB.SYS.


Switch to NM Overview 

Figure 1.11 represents the process of mixed-mode procedure calling in the
CM--> NM direction.


          Figure 1.11.  Mixed-mode Procedure Call:  CM--> NM

A mixed-mode procedure call in the direction CM--> NM involves the
following steps, as indicated in Figure 1.11:

1.               The CM code needing to access an NM routine calls
                 Switch, either by means of in-line code or a Switch
                 stub.

2.               The in-line code or CM Switch stub sets up the data
                 structures and parameters required by Switch and makes a
                 call to either the HPSWTONMNAME intrinsic or the
                 HPLOADNMPROC and HPSWTONMPLABEL intrinsics.

3.               HPSWTONMNAME or HPSWTONMPLABEL calls the NM routine,
                 after preparing the NM registers as though the call was
                 being made from Native Mode.

4.               The NM routine executes as though called from Native
                 Mode and returns the functional return value (if any) to
                 the calling Switch intrinsic.  Reference parameters are
                 modified by the NM routine.

5.               If the NM routine has a functional return value,
                 HPSWTONMNAME or HPSWTONMPLABEL copies the value back to
                 the CM stack.

6.               The stub or in-line code checks whether the Switch
                 operation was successful and then control returns to the
                 CM routine.


NOTE  HPSWTONMNAME or a combination of HPLOADNMPROC and HPSWTONMPLABEL
      are the system intrinsics called to pass CM values and parameters
      to Native Mode, to change the execution mode, and to invoke an NM
      routine.  HPSWTONMNAME, HPLOADNMPROC, and HPSWTONMPLABEL reside in
      the CM system library, SL.PUB.SYS. The only difference between
      using HPSWTONMNAME and HPLOADNMPROC/HPSWTONMPLABEL is the manner in
      which the target routine is identified.  For details on when and
      why to use names versus plabels, see Chapter 4.

MPE/iX 5.0 Documentation