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DEC Ada
Technical Overview and Comparison on DIGITAL Platforms
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11.4.2 Porting from DEC Ada for OpenVMS VAX Systems to DIGITAL UNIX Systems

The 64-bit Alpha architecture provides many benefits, particularly in the areas of data representation, data access, and data alignment. However, porting from the 32-bit VAX architecture involves various issues. Table 11-3 highlights the portability issues with pointers to additional information in this guide and notes.

Table 11-3 DEC Ada on OpenVMS VAX and DIGITAL UNIX: Pointers and Notes
Topic Pointers and Notes
Integer types Section 2.1, Table 2-1

Size of LONG_INTEGER differs between platforms.

Floating-point numbers and
representations 		Section 2.2, Table 2-2, Table 2-3

On VAX systems, floating-point numbers are represented as VAX floating-point types. On DIGITAL UNIX systems, floating-point numbers are represented as IEEE floating-point types.

There are no floating-point representations common to both systems.

Pragma FLOAT_REPRESENTATION Section 2.2.1

On VAX systems, the default and allowable value is VAX_FLOAT. On DIGITAL UNIX, the default and allowable value is IEEE_FLOAT.

Pragma LONG_FLOAT Section 2.2.2

This pragma is available on OpenVMS systems only.

For this pragma to have effect on VAX systems, it must have a value of VAX_FLOAT.

Representation attributes for floating-point types Section 2.2.3

The values of the representation attributes for floating-point types are summarized in Table 2-5.

Model numbers The model numbers for each floating-point type can be found in Table 2-6.
Safe numbers The safe numbers for each floating-point type can be found in Table 2-7.
Operations on fixed-point types Section 2.3

Unless specified otherwise, handled as follows:

  • On VAX, results that are powers of 2 are truncated towards 0.0. Results that are not powers of two are rounded.
  • On Alpha, all results are rounded.
Component alignment Section 2.10

Default alignments differ across systems.

Size specification for discrete types Section 2.1, Section 2.3, Section 2.11.1
Record representation clause alignment Section 2.5, Section 2.10
Address clauses Section 2.7
Access types Section 2.11
Pragmas Chapter 3

DIGITAL supports a similar set of pragmas across platforms. For specific details, see Chapter 3.

Pragma INTERFACE Chapter 3

Pragma INTERFACE recognizes the following language names:

  • On VAX, ADA, BLISS, C, FORTRAN, DEFAULT (default)
  • On DIGITAL UNIX, ADA, BLISS, C (default), FORTRAN
Pragma SYSTEM_NAME Chapter 3

Takes an enumeration literal as its single argument as follows:

  • On VAX, VAX_VMS
  • On DIGITAL UNIX, DEC_OSF1_AXP
Delay statements and related issues Section 4.6.2

This section covers platform differences relating to delay statements, type DURATION, SYSTEM.TICK, type TIME, and function CLOCK.

Type AST_HANDLER Provided on OpenVMS systems only.
Tasking Chapter 4

This chapter should be read in its entirety to understand the differences in the implementation of tasking on these two platforms.

Threads/DECthreads Section 4.1

DIGITAL UNIX uses DECthreads to implement tasking. DECthreads routines can be called from DEC Ada programs running on DIGITAL UNIX systems.

On VAX, the Ada RTL has its own threading system. DECthreads routines cannot be called from DEC Ada programs running on VAX systems.

Task ID assignment Section 4.1.1
Disabling time slicing Section 4.3.2
Pragma TIME_SLICE Section 4.3.2
Task scheduling Section 4.3

On DIGITAL UNIX systems, the operating system schedules Ada tasks along with other threads that are currently running. On OpenVMS VAX systems, the operating system issues the process and the Ada RTL divides up tasks within the process.

Scheduling strategies Section 4.3.1

On VAX, the default is FIFO with preemption. On Alpha systems, the default is round-robin with preemption.

Assigning and controlling task priorities Section 4.3.3
Duration of a time slice Section 4.3.2

On OpenVMS systems, the duration of a time slice can be specified.

Subtype PRIORITY Section 4.3.3

On DIGITAL UNIX systems, DEC Ada allows task priorities to be changed dynamically at run time to values of the subtype PRIORITY, as well as to the values of DIGITAL UNIX real-time and system priorities.

External interrupts Section 4.4, Section 4.5

OpenVMS systems use ASTs to respond to certain events detected by the operating system and the pragma AST_ENTRY to identify an entry that will handle an AST. On DIGITAL UNIX systems, external interrupts can be associated with task entries through operating system signals.

Shared variables Section 4.6.3

See this section for differences in the implementation of the pragma SHARED on VAX and Alpha systems.

Package SYSTEM differences Section titled Package SYSTEM Platform Differences in Chapter 6
Input-output Chapter 5

On VAX systems, DEC Ada uses OpenVMS RMS to perform operations on external files. On DIGITAL UNIX systems, DEC Ada uses system calls. The attributes of the external file, the storage medium, and the input-output package determine which operations can be used to manipulate data.

Library of predefined units Chapter 6
Implementation-defined packages Section 6.2

See this section for lists of packages common to all platforms, available on DIGITAL UNIX only, and available on ULTRIX only.

Bindings Section 6.4
Program library units Section 7.1.1
Compilation/program library management Section 7.1.2

Commands used in compilation and program library management differ between these two platforms. See Table 7-2 for a summary of the commands on both platforms.

Working with multiple libraries Section 7.1.3

Commands used in working with multiple libraries differ between these two platforms.

Exceptions Section 7.4

On OpenVMS systems, exceptions are handled using the OpenVMS condition-handling mechanism. On DIGITAL UNIX, exception handling is implemented using the DIGITAL UNIX calling standard.

Mixed-language programming Section 10.3

In general, all systems support the same interface. However, when you mix code from different languages, some details differ, for example:

  • On VAX systems, C expects data to be byte-aligned.
  • On Alpha systems, C expects data to be naturally aligned.
Package SYSTEM-defined constants Table 10-1
Debugging facilities Chapter 9, Section 9.1, Section 9.2

On VAX systems, the OpenVMS Debugger provides extensive facilities to aid in debugging. On DIGITAL UNIX systems, DEC Ada programs can be debugged using one of several debuggers, including DECladebug for DIGITAL UNIX systems.

Software development tools Table 1-1

11.4.3 Porting from DEC Ada for ULTRIX Systems to DIGITAL UNIX Systems

Main areas of difference are as follows:

  • Differences between the ULTRIX and the DIGITAL UNIX operating system
  • Differences between the RISC architecture (ULTRIX) and the Alpha architecture (DIGITAL UNIX)
  • Differences between Version 1.1 and Version 3.2 of DEC Ada

Table 11-4 highlights these topics and others of concern when considering a port between these two platforms. It also provides pointers to additional information in this guide and, where appropriate, notes on the individual topics.

Table 11-4 DEC Ada on ULTRIX and DIGITAL UNIX: Pointers and Notes
Topic Pointers and Notes
Character set Section 10.1

In DEC Ada on ULTRIX, each graphic character corresponds to a unique code of ISO 646. In DEC Ada on DIGITAL UNIX each graphic character corresponds to a unique code of ISO 8859/1 (Latin-1).

In DEC Ada on ULTRIX, the internal codes for the enumeration literal STANDARD.CHARACTER are in the range 0 .. 127. In DEC Ada on DIGITAL UNIX the internal codes are in the range 0 .. 255.

Enumeration types In DEC Ada on ULTRIX and DIGITAL UNIX, for simple enumeration objects and enumeration components of unpacked arrays and records, the compiler chooses 1 byte (8 bits), 2 bytes (16 bits), or 4 bytes (32 bits)--whichever is smallest but adequate--to represent an object of an enumeration type. On DIGITAL UNIX, 8 bytes (64 bits)is also available.
Integer types Section 2.1, Table 2-1
Floating-point numbers and
representations 		Section 2.2, Table 2-2 Table 2-3

IEEE format is used by both DIGITAL UNIX and ULTRIX.

Pragma FLOAT_REPRESENTATION Section 2.2.1
Operations on fixed-point types Section 2.3

Unless specified otherwise, on Alpha, all results are rounded.

Record representation clause alignment Section 2.5
Component alignment Section 2.10

Default alignments differ across systems. By convention on Digital UNIX and ULTRIX systems, all noncomposite components are aligned on natural boundaries.

The Alpha hardware runs more efficiently with naturally aligned data.

Size specification for discrete types Section 2.1, Section 2.3, Section 2.11.1

On DIGITAL UNIX and ULTRIX systems, the maximum size is 64 bits for integer and enumeration types and 32 bits for fixed-point types.

Address clauses Section 2.11
Delay statements and related issues Section 4.6.2

This section covers platform differences relating to delay statements, type DURATION, SYSTEM.TICK, type TIME, and function CLOCK.

SYSTEM.ADDRESS size Section 2.7
Access type size Section 2.7
Type AST_HANDLER Provided on OpenVMS systems only.
Pragmas Chapter 3

See especially EXPORT_OBJECT, IMPORT_OBJECT,
FLOAT_REPRESENTATION,
PASSIVE, TASK_STORAGE,
and TIME_SLICE.

Pragma INTERFACE Chapter 3
Pragma SYSTEM_NAME Chapter 3

Takes an enumeration literal as its single argument as follows:

DEC_OSF1_AXP (on DIGITAL UNIX systems)

Tasking Chapter 4

This chapter should be read in its entirety to understand the differences in the implementation of tasking on these two platforms.

Threads/DECthreads Section 4.1
Task storage allocation Section 4.2
Task stack Section 4.2.2
Task scheduling Section 4.3
Scheduling strategies Section 4.3.1

On Alpha systems, the default is round-robin with preemption.

Pragma TIME_SLICE Section 4.3.2
Subtype PRIORITY Section 4.3.3

On DIGITAL UNIX systems, DEC Ada allows task priorities to be changed dynamically at run time to values of the subtype PRIORITY, as well as to the values of DIGITAL UNIX real-time and system priorities.

Handling signals from tasks Section 4.5
Shared variables Section 4.6.3

See this section for implementation details for the pragmas SHARED and VOLATILE on Alpha systems.

UNSIGNED_LONGWORD UNSIGNED_LONGWORD is 32-bits. Addresses on DIGITAL UNIX systems are 64-bits; on ULTRIX, 32-bits. Therefore, DIGITAL recommends using the generic package ADDRESS_OPERATIONS.
Conversions between types INTEGER and ADDRESS Type INTEGER is 32-bits and type ADDRESS is 64-bits. Therefore, DIGITAL recommends using the generic package ADDRESS_OPERATIONS when converting between these types.
The FORM parameter Section 5.2.2 for DIGITAL UNIX information and Section 5.2.3 for ULTRIX information.
Relative and indexed files Available on OpenVMS systems only.

DEC Ada defines "relative access" and "indexed access" on OpenVMS systems only.

TEXT_IO TEXT_IO is provided for OpenVMS systems only.
Predefined instantiations of TEXT_IO packages Table 5-1
Package STANDARD On DIGITAL UNIX systems, the package STANDARD is named _STANDARD.
Library of predefined units Chapter 6

On DIGITAL UNIX systems, when a new program library is created, a default program library context that includes all the units from the predefined library is associated with the newly created library. The predefined library is identified in the default context as /usr/lib/adalib .

Bindings Section 6.4
Platform differences reflected in Package SYSTEM Section titled Package SYSTEM Platform Differences in Chapter 6

See this section for lists of packages common to all platforms, available on DIGITAL UNIX only, and available on ULTRIX only.

Interface Chapter 7

Information can be communicated to a DEC Ada program using command-line arguments and file or interactive input-output facilities. On DIGITAL UNIX systems, information can also be communicated using system-specific features such as environment variables, shared memory segments, semaphore, pipes, fifos, and sockets.

Program library units Section 7.1.1
Pragma EXPORT_VALUED_PROCEDURE Section 7.1.1

When a main function or main procedure delared with this pragma returns a discrete value whose size is less than 64 bits on DIGITAL UNIX systems, the value is zero- or sign-extended as appropriate.

Exception handling Section 7.4

On DIGITAL UNIX, exception handling is implemented using the DIGITAL UNIX calling standard. On ULTRIX systems, exception handling is implemented using the routines and data structures provided in the ULTRIX header file exception.h .

Handling DIGITAL UNIX signals Exceptions can handle those DIGITAL UNIX signals that indicate an error. Other DIGITAL UNIX signals cannot be handled using exception handlers.

If an Ada subprogram handled a DIGITAL UNIX signal that is signaled from an external routine, the handler causes the signal to behave according to Ada semantics. In other words, control is transferred to the handler. Control is not returned to the process context where the signal occurred. (The default behavior of a DIGITAL UNIX signal handler is to return control to where the signal occurred.)

Both the exception choice others and the predefined exception NON_ADA_ERROR can be used to handle DIGITAL UNIX signals. If a user-defined signal handler is used, it can interfere with Ada exception handling and adversely affect the behavior of the program.

Working with multiple libraries Section 7.1.3

On DIGITAL UNIX systems, DEC Ada provides program library contexts to work with multiple program libraries. See Table 7-1 for commands.

Compilation/program library
management 		Section 7.1.2

See Table 7-2 for a summary of the commands on these platforms.

Format and content of user listings On DIGITAL UNIX, the ADALISFLAGS environment variable can be used.
Errors, warnings, messages Section 7.5
Mixed-language programming Section 10.3

The mechanism DESCRIPTOR is not supported on DIGITAL UNIX systems because those descriptors have only 32 bits available for an address. Instead, dope vectors are used. (However, the mechanism dope vectors cannot be specified.)

The package C_TYPES is not supported on ULTRIX systems.

Debugging facilities Section 9.2
Package SYSTEM-defined constants Table 10-1

Note platform differences.

Additional software development tools On ULTRIX systems, DIGITAL provides DECset. On ULTRIX and DIGITAL UNIX systems, DIGITAL provides DEC FUSE, which is a Motif-based graphical programming environment.

For details, see Table 1-1.

Appendix A
For More Information

Table A-1 lists sources of additional information for the topics covered in this guide.

Table A-1 Sources of Additional Information
Topic For More Information
Lexical elements Chapter 2, DEC Ada Language Reference Manual
Types Chapter 3, DEC Ada Language Reference Manual
Object representation and storage Relevant run-time reference manual
Pragma FLOAT_REPRESENTATION Chapter 3, DEC Ada Language Reference Manual
Pragma LONG_FLOAT Chapter 3, DEC Ada Language Reference Manual
Safe numbers Chapter 3, DEC Ada Language Reference Manual
Pragma INLINE, restriction Chapter 6, DEC Ada Language Reference Manual
Pragma INTERFACE, restriction Chapter 13, DEC Ada Language Reference Manual
Pragma summary, descriptive Annex B, DEC Ada Language Reference Manual
Type representation The relevant run-time reference manuals
Tasking, concepts Chapter 9, DEC Ada Language Reference Manual and the relevant run-time reference manuals
Pragma TIME_SLICE Chapter 9, DEC Ada Language Reference Manual
Pragma PASSIVE Chapter 9, DEC Ada Language Reference Manual
Pragma PRIORITY Chapter 9, DEC Ada Language Reference Manual and the relevant run-time reference manual
Task entries and ASTs Chapter 9, DEC Ada Language Reference Manual
Exception handling Relevant run-time reference manual
Pragma INLINE_GENERIC Chapter 12, DEC Ada Language Reference Manual
Pragma SHARE_GENERIC Chapter 12, DEC Ada Language Reference Manual
Representation clauses Chapter 13, DEC Ada Language Reference Manual
Pragma COMPONENT_ALIGNMENT Chapter 13, DEC Ada Language Reference Manual
Pragma TASK_STORAGE Chapter 13, DEC Ada Language Reference Manual
Pragma MAIN_STORAGE Chapter 13, DEC Ada Language Reference Manual
Import and export pragmas Chapter 13, DEC Ada Language Reference Manual
DEC Ada additions to package SYSTEM Chapter 13, DEC Ada Language Reference Manual
Pragma INTERFACE_NAME Chapter 13, DEC Ada Language Reference Manual
Compilation options, detailed information DEC Ada Developing Ada Programs for the relevant platform
Implementation-dependent language attributes, details Chapter 13, DEC Ada Language Reference Manual
Input-output facilities The relevant run-time reference manual and Chapter 14, DEC Ada Language Reference Manual
File sharing and record locking The relevant run-time reference manual and Chapter 14, DEC Ada Language Reference Manual
Package specifications and explanation of operations provided by each package Chapter 14, DEC Ada Language Reference Manual
Predefined language attributes, summary Annex A, DEC Ada Language Reference Manual
Package STANDARD Annex C, DEC Ada Language Reference Manual
Predefined language environment Annex C, DEC Ada Language Reference Manual
DEC Ada program development environment Developing Ada Programs on DEC OSF/1 Systems or Developing Ada Programs on OpenVMS Systems
Professional Development Option Developing Ada Programs on DEC OSF/1 Systems or Developing Ada Programs on OpenVMS Systems
Smart recompilation Developing Ada Programs on DEC OSF/1 Systems or Developing Ada Programs on OpenVMS Systems
Debugging facilities Developing Ada Programs on DEC OSF/1 Systems or Developing Ada Programs on OpenVMS Systems
Debugging tasks and exceptions Developing Ada Programs on DEC OSF/1 Systems or Developing Ada Programs on OpenVMS Systems
DEC Ada diagnostic messages Developing Ada Programs on DEC OSF/1 Systems or Developing Ada Programs on OpenVMS Systems
Performance Relevant run-time reference manual
Porting Developing Ada Programs on DEC OSF/1 Systems or Developing Ada Programs on OpenVMS Systems

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