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Byte Ordering on Big Endian and Little Endian Platforms


Integer values are typically stored in one of three sizes: one-byte, two-byte, or four-byte. The ordering of the bytes for the integer varies depending on the platform (operating environment) on which the integers were produced.

The ordering of bytes differs between the "big endian" and "little endian" platforms. These colloquial terms are used to describe byte ordering for IBM mainframes (big endian) and for Intel-based platforms (little endian). In the SAS System, the following platforms are considered big endian: AIX, HP-UX, IBM mainframe, Macintosh, and Solaris. The following platforms are considered little endian: AXP/VMS, Digital UNIX, Intel ABI, OS/2, VAX/VMS, and Windows.

How Bytes are Ordered Differently

On big endian platforms, the value 1 is stored in binary and is represented here in hexadecimal notation. One byte is stored as 01, two bytes as 00 01, and four bytes as 00 00 00 01. On little endian platforms, the value 1 is stored in one byte as 01 (the same as big endian), in two bytes as 01 00, and in four bytes as 01 00 00 00.

If an integer is negative, the "two's complement" representation is used. The high-order bit of the most significant byte of the integer will be set on. For example, -2 would be represented in one, two, and four bytes on big endian platforms as FE, FF FE, and FF FF FF FE respectively. On little endian platforms, the representation would be FE, FE FF, and FE FF FF FF.

Writing Data Generated on Big Endian or Little Endian Platforms

SAS can read signed and unsigned integers regardless of whether they were generated on a big endian or a little endian system. Likewise, SAS can write signed and unsigned integers in both big endian and little endian format. The length of these integers can be up to eight bytes.

The following table shows which format to use for various combinations of platforms. In the Sign? column, "no" indicates that the number is unsigned and cannot be negative. "Yes" indicates that the number can be either negative or positive.

SAS Formats and Byte Ordering
Data created for ... Data written by ... Sign? Format
big endian big endian yes IB or S370FIB
big endian big endian no PIB, S370FPIB, S370FIBU
big endian little endian yes S370FIB
big endian little endian no S370FPIB
little endian big endian yes IBR
little endian big endian no PIBR
little endian little endian yes IB or IBR
little endian little endian no PIB or PIBR
big endian either yes S370FIB
big endian either no S370FPIB
little endian either yes IBR
little endian either no PIBR

Integer Binary Notation and Different Programming Languages

The following table compares integer binary notation according to programming language.

Integer Binary Notation and Programming Languages
Language 2 Bytes 4 Bytes
SAS IB2., IBR2., PIB2., PIBR2., S370FIB2., S370FIBU2., S370FPIB2. IB4., IBR4., PIB4., PIBR4., S370FIB4., S370FIBU4., S370FPIB4.
IBM assembler H F
C short long

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