Endianism Considerations (from HP document)

 http://h71000.www7.hp.com/doc/82final/6673/6673pro_008.html

Users who are porting from OpenVMS Alpha or OpenVMS VAX to OpenVMS I64 do not need to be concerned with endianism issues; OpenVMS on all platforms uses little-endian for data storage and manipulation. This chapter is being made available more for reference to allow the reader to be aware of issues, in general, when porting between different hardware architectures where endianisms are different.

Endianism refers to the way data is stored and defines how bytes are addressed in multibyte data types. This is important because if you try to read binary data on a machine that is of a different endianism than the machine that wrote the data, the results will be different. This is an especially significant problem when a database needs to be moved to a different endian architecture.

There are two types of endian machines: big-endian (forward byte ordering or most significant first, [MSF]) and little-endian (reverse byte ordering or least significant first, [LSF]). The mnemonics "big end in" and "little end in" can be useful when discussing endianism. Figure 7-1 and Figure 7-2 compare the two byte-ordering methodologies.

Figure 7-1 Big-Endian Byte Ordering

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Figure 7-2 Little-Endian Byte Ordering

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The Alpha microprocessor provides a little-endian environment for operating systems such as OpenVMS and Tru64 UNIX, while the PA-RISC microprocessor provides a big-endian environment for the HP-UX operating system.

On a little-endian operating system, such as OpenVMS, the little end, or least-significant byte is stored at the lowest address. This means a hexadecimal like 0x1234 is stored in memory as 0x34 0x12. The same is true for a 4-byte value; for example, 0x12345678 is stored as 0x78 0x56 0x34 0x12. A big-endian operating system does this in reverse fashion, so 0x1234 is stored as 0x12 0x34 in memory.

Consider the number 1025 (2 to the tenth power plus 1) stored in a 4-byte integer. This is represented in memory as follows:

 

little endian: 00000000 00000000 00000100 00000001b   big endian: 00000001 00000100 00000000 00000000b

The Intel Itanium processor family architecture supports both big-endian and little-endian memory addressing. There are no problems reading and exchanging data between OpenVMS Alpha and OpenVMS I64 running on Itanium processors. If there is a need to exchange data between OpenVMS on an Alpha or I64 system and HP-UX on PA-RISC or the Itanium processor family, then byte swapping might be required.

 

 

Endianism Considerations