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HP OpenVMS MACRO Compiler Porting and User's Guide

HP OpenVMS MACRO Compiler
Porting and User's Guide

Order Number: AA--PV64E--TE

January 2005

This manual describes how to port Macro-32 source code written for the VAX MACRO compiler to the MACRO compiler for OpenVMS running on either an HP OpenVMS Industry Standard 64 or HP OpenVMS Alpha system. It also describes how to use the compiler.

Revision/Update Information: This revised manual supersedes the OpenVMS MACRO-32 Porting and User's Guide for OpenVMS Version 7.3.

Software Version: OpenVMS I64 Version 8.2
OpenVMS Alpha Version 8.2

Hewlett-Packard Company Palo Alto, California

© Copyright 2005 Hewlett-Packard Development Company, L.P.

Confidential computer software. Valid license from HP required for possession, use or copying. Consistent with FAR 12.211 and 12.212, Commercial Computer Software, Computer Software Documentation, and Technical Data for Commercial Items are licensed to the U.S. Government under vendor's standard commercial license.

The information contained herein is subject to change without notice. The only warranties for HP products and services are set forth in the express warranty statements accompanying such products and services. Nothing herein should be construed as constituting an additional warranty. HP shall not be liable for technical or editorial errors or omissions contained herein.

Intel and Itanium are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States and other countries.

Printed in the US


The HP OpenVMS documentation set is available on CD-ROM.

Contents Index


Intended Audience

This manual is for software engineers responsible for porting application code:

You should have programming skills and an understanding of the relevant operating systems.

Document Structure

This manual is divided into two parts.

Part 1, Concepts and Methodology is composed of the the following chapters:

Part 2, Reference is composed of the following appendixes:

Related Documents

This manual refers readers to the following manuals for additional information on certain topics:

For additional information about HP OpenVMS products and services, visit the following World Wide Web address:



1 This manual has been archived. It is no longer maintained and is not part of the OpenVMS documentation set. However, you can view it online at http://www.hp.com/go/openvms/doc.

Reader's Comments

HP welcomes your comments on this manual. Please send comments to either of the following addresses:
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Part 1
Concepts and Methodology

Chapter 1
Preparing to Port Macro-32 Code

This chapter describes a process that software engineers can use when planning to port Macro-32 code from an OpenVMS VAX system to an OpenVMS Alpha or HP OpenVMS Industry Standard 64 system.

This chapter contains the following topics:


See Appendix A for information on how to invoke the compiler.

1.1 Features of the MACRO Compiler

The OpenVMS MACRO compiler compiles Macro-32 source code written for OpenVMS VAX systems (the VAX MACRO assembler) into machine code that runs on OpenVMS Alpha and OpenVMS I64 systems.


This compiler is provided for porting Macro-32 code to OpenVMS Alpha and OpenVMS I64 systems. For any new development, it is recommended that you use mid- and high-level languages.

While some VAX MACRO code can be compiled without any changes, most code modules will require the addition of entry point directives when ported to OpenVMS Alpha or OpenVMS I64. Many code modules will require other changes as well.

The compiler might detect only a few problems with a module at its initial compilation and then, after you have corrected them, the compiler might discover additional problems. In such cases, the resolution of one problem can allow the compiler to further examine the code and discover other problems the initial one concealed.

The compiler includes many features that make this process easier, such as:

The compiler also provides 64-bit addressing support, which is documented in Chapter 5 and in Appendix E. Support for 64-bit addressing was introduced in OpenVMS Alpha Version 7.0. This support is provided for those rare instances when it is preferable to use VAX MACRO to access 64-bit address space instead of using a high-level language that is supported on OpenVMS Alpha or OpenVMS I64.

1.2 Differences Between the Compiler and the Assembler

It is important to remember that the MACRO compiler is a compiler, not an assembler. It does not create output code that exactly matches the input code. In its optimization process, the compiler might move, replicate, or remove code and interleave instructions. Furthermore, the faulting behavior of the ported code might not match that of VAX code. These differences are described in the following sections.

1.2.1 Moving Code

Mispredicted branches are expensive on an OpenVMS Alpha or OpenVMS I64 system. The compiler attempts to determine the most likely code path through the module and then generates code that consolidates that code path. Code paths deemed unlikely are moved out of line to the end of the module. Consider the following example:

        BLBS    R0,10$ 
        JSB     PROCESS ERROR 

In this example, the compiler will treat the HALT as an unlikely code path and detect that the two code streams do not rejoin at 10$. Because of these conditions, it will determine that the branch is likely to be taken. It will then move the intervening instructions out of line to the end of the module, change the BLBS instruction to a BLBC that branches to the moved code, and continue with in-line code generation at the label 10$, as follows:

        BLBC    L1$ 
10$:      . 
        (routine exit) 

You can change the compiler's determination of the likelihood of conditional branches with the compiler directives .BRANCH_LIKELY and .BRANCH_UNLIKELY (see Section 4.2).

1.2.2 Replicating Code

The compiler might replicate small sections of code multiple times to eliminate excessive branching. For example, when compiling branches to the following VAX code, the compiler will replicate the MOVL at each branch to ERROR1 and then branch directly to COMMON_ERROR.

        BRW     COMMON_ERROR 

1.2.3 Removing Code

The compiler's optimizations might determine that some instructions do not contribute to the code flow. In such instances, the instructions might be removed. An example of this is a CMP or TST instruction with no subsequent conditional branch, such as the following:

        CMPB    (R2),511(R2) 
        JSB     EXE$SENDMSG 

Removal of this CMPB instruction could cause a problem if its purpose was to touch two memory locations to ensure that the memory pages were faulted in before calling the routine. This would likely have to be changed in porting to OpenVMS Alpha or OpenVMS I64 anyway because of the different page sizes of VAX and Alpha or I64 systems. In addition to changing the page size, you should replace the instruction with MOVx instructions, such as the following:

        MOVB    (R2),R1 
        MOVB    8191(R2),R0 
        JSB     EXE$SENDMSG 

Note that the two MOVB instructions operated on two different registers. The compiler does not currently remove instructions that load values into a register which is never subsequently read before being overwritten. However, this optimization might be done in the future.


In general, code requiring that a memory read reference actually touch memory should be examined carefully, as current or future optimizations might move or remove the references.

1.2.4 Interleaving Instructions

Instruction scheduling, which is performed by default (see Section 4.3), will interleave the Alpha or Itanium instructions generated from one VAX instruction with the Alpha or Itanium instructions generated by surrounding VAX instructions.

1.2.5 Reserved Operand Faults

On VAX systems, some VAX MACRO instructions might generate a reserved operand fault if certain operands are out of a required range. For example, on a bit manipulation instruction such as INSV, if the size operand is greater than 32, a VAX system will generate a run-time reserved operand fault.

On OpenVMS Alpha and OpenVMS I64 systems, if the operand that is out of range is a compile-time constant, the compiler will flag this condition with an error message. However, if this operand is variable at run time, the compiler makes no attempt to generate run-time range checks on it. If the operand is out of range, the resulting operation might cause incorrect results yet not create a fault.

1.3 Step-by-Step Porting Process for OpenVMS VAX to OpenVMS Alpha or OpenVMS I64

The following steps have proven to be an efficient means for porting VAX MACRO code to OpenVMS Alpha or OpenVMS I64:

  1. Inspect each module you intend to port, from beginning to end, for coding practices that prohibit its successful porting. Such scrutiny is necessary, because it is impossible for the compiler to account for the myriad imaginative uses of VAX MACRO code that take advantage of a comprehensive knowledge of the VAX architecture. Such uses, if not detected and modified, can undermine an effort to port VAX MACRO code to OpenVMS Alpha or OpenVMS I64.
  2. At each entry point in the module, add the appropriate entry-point directive.
    These directives are:
    You do not need to change the VAX MACRO entry point .ENTRY to .CALL_ENTRY unless you want to use .CALL_ENTRY clauses. Nor do you need to add the register arguments at this time.
    See Section 2.3 for guidelines for the correct placement of these directives.
    See Appendix B for a full syntax description of each.
  3. Invoke the compiler to compile the module. A suggested command procedure for doing this appears in Section 2.12.
    By default, the compiler flags unaligned stack and memory references, routine branches, potentially problematic instructions, and self-modifying code. If you specify /FLAG=HINTS on the command line, the compiler will provide suggestions for the input and output register arguments of the entry point directives you inserted at Step 2.
  4. Take note of the problems reported by the compiler.
    The MACRO compiler generally issues the same messages on OpenVMS Alpha systems and on OpenVMS I64 systems, with the exception of the prefix:
    For example:

    AMAC-I-ARGLISHOME, argument list must be homed in caller 


    IMAC-I-ARGLISHOME, argument list must be homed in caller 

    You can invoke the Help Message utility for an explanation and user action for each compiler message you receive.
    For information about the Help Message utility, refer to DCL help or the OpenVMS System Messages: Companion Guide for Help Message Users. Also, Section 1.5 and Chapter 3 provide specific details about VAX MACRO coding practices that cannot be directly translated to OpenVMS Alpha or OpenVMS I64 code.
    Remember that resolution of the problems detected on this pass might allow the compiler to discover additional problems on a subsequent pass.

  5. Edit the Macro-32 source code. Fix the problems indicated by the compiler and look for others the compiler might have missed.
    However, do not change code just to avoid compiler informational diagnostic messages. Most of the information-level messages are there to point out code that will result in less optimal performance on an OpenVMS Alpha or OpenVMS I64 processor but will compile correctly. If you have examined the offending instructions in the source code and are convinced that all is well, leave the code alone. Remember that you can use the command line qualifiers /FLAG and /WARN to control diagnostic message generation. Also, the .ENABLE and .DISABLE directives can turn off information level messages for segments of code within a module.
  6. Add input, output, preserve, and scratch arguments, as appropriate, to the entry point directives you provided in Step 2 and supply a list of pertinent registers for each specified argument. Section 2.6 can help you determine which registers to list.
  7. For OpenVMS I64 systems only, add linkage directives (.CALL_LINKAGE, .DEFINE_LINKAGE) to tell the compiler about routines that return values in registers other than R0/R1 and about routines written in a languge other than Macro-32 that are called with the JSB instruction. In addition, add the .USE_LINKAGE directive at each indirect CALLS/CALLG instruction if the routine has a non-standard return; add the .USE_LINKAGE directive at each indirect JSB instruction if the routine is written in a language other than Macro-32. See Appendix B for more information.
  8. Repeat Steps 3 through 7 until the compiler generates informational messages only for Macro-32 source code that you have verified produces correct OpenVMS Alpha or OpenVMS I64 object code.
  9. If your module is common to both OpenVMS VAX and OpenVMS Alpha or OpenVMS I64 systems (a coding convention discussed in Section 1.7), your porting effort is not complete until the module is acceptable to the VAX MACRO assembler as well as the compiler.

Once you have some experience in porting VAX MACRO modules, it will be easier to recognize certain problems while inspecting the source and to fix them before your initial invocation of the compiler.

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