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Full Discussion: AIX endian again
Top Forums UNIX for Advanced & Expert Users AIX endian again Post 17923 by Perderabo on Thursday 21st of March 2002 08:16:30 AM
Old 03-21-2002
Endian-ness in a cpu arises because we need to specify multi-byte integers with a byte address.

In the '60's I worked on an IBM 1130. It was a 16 bit machine and had no byte addressing. Address 0 was a 16 bit word. Address 1 was the next 16 bit word. There is no way to determine an endian-ness on a system like this.

I have never heard of any cpu that even allows bit addressing. If there was one, and if it required the programmer to specify a bit address for a byte, then we would need to worry about the endian-ness of a single byte.

The only time that byte endian-ness becomes apparent is when a byte is transmitted across a serial data line. TCP/IP is an example of this, and in TCP/IP, bytes are big-endian. (or more accurately, octets are big-endian, since TCP/IP does not assume that bytes are 8 bits). Contrary to your comment, this is what I regard as "normal". Most other serial protocols are also big-endian, at least most of the modern ones are.

On the other hand, RS-232 is little-endian. So is the old current-loop interface. The earlier teletype protocols are before my time, but I think that they were little-endian (but I'm not sure). These protocols never need to transit a multi-byte integer and they need to support both 7 bit and 8 bit characters.

So unless you are designing an i/o card to transmit bytes in a bit serial fashion, you can safely ignore this issue with every cpu that I have heard of.

But it's hard to be sure that something doesn't exist, so if someone out there knows about a cpu that requires a bit address for addressing its bytes, I would be very interested in the details of it.
 

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BYTEORDER(9)						   BSD Kernel Developer's Manual					      BYTEORDER(9)

NAME
bswap16, bswap32, bswap64, be16toh, be32toh, be64toh, htobe16, htobe32, htobe64, htole16, htole32, htole64, le16toh, le32toh, le64toh, be16enc, be16dec, be32enc, be32dec, be64enc, be64dec, le16enc, le16dec, le32enc, le32dec, le64enc, le64dec -- byte order operations SYNOPSIS
#include <sys/endian.h> uint16_t bswap16(uint16_t int16); uint32_t bswap32(uint32_t int32); uint64_t bswap64(uint64_t int64); uint16_t be16toh(uint16_t big16); uint32_t be32toh(uint32_t big32); uint64_t be64toh(uint64_t big64); uint16_t htobe16(uint16_t host16); uint32_t htobe32(uint32_t host32); uint64_t htobe64(uint64_t host64); uint16_t htole16(uint16_t host16); uint32_t htole32(uint32_t host32); uint64_t htole64(uint64_t host64); uint16_t le16toh(uint16_t little16); uint32_t le32toh(uint32_t little32); uint64_t le64toh(uint64_t little64); uint16_t be16dec(const void *); uint32_t be32dec(const void *); uint64_t be64dec(const void *); uint16_t le16dec(const void *); uint32_t le32dec(const void *); uint64_t le64dec(const void *); void be16enc(void *, uint16_t); void be32enc(void *, uint32_t); void be64enc(void *, uint64_t); void le16enc(void *, uint16_t); void le32enc(void *, uint32_t); void le64enc(void *, uint64_t); DESCRIPTION
The bswap16(), bswap32(), and bswap64() functions return a byte order swapped integer. On big endian systems, the number is converted to little endian byte order. On little endian systems, the number is converted to big endian byte order. The be16toh(), be32toh(), and be64toh() functions return a big endian byte ordered integer converted to the system's native byte order. The return value will be the same as the argument on big endian systems. The le16toh(), le32toh(), and le64toh() functions return a little endian byte ordered integer converted to the system's native byte order. The return value will be the same as the argument on little endian systems. The htobe16(), htobe32(), and htobe64() functions return an integer in the system's native byte order converted to big endian byte order. The return value will be the same as the argument on big endian systems. The htole16(), htole32(), and htole64() functions return a integer in the system's native byte order converted to little endian byte order. The return value will be the same as the argument on little endian systems. The be16enc(), be16dec(), be32enc(), be32dec(), be64enc(), be64dec(), le16enc(), le16dec(), le32enc(), le32dec(), le64enc(), and le64dec() functions encode and decode integers to/from byte strings on any alignment in big/little endian format. SEE ALSO
byteorder(3) HISTORY
The hto*() and *toh() functions first appeared in FreeBSD 5.0, and were originally developed by the NetBSD project. The encode/decode functions first appeared in FreeBSD 5.1. BSD
April 29, 2002 BSD
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