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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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HTON(3) 						     Library Functions Manual							   HTON(3)

hton, htons, htonl, ntohs, ntohl - host to network byte order conversion SYNOPSIS
#define _MINIX_SOURCE 1 #include <stddef.h> #include <sys/types.h> #include <net/hton.h> u16_t htons(u16_t host_word) u32_t htonl(u32_t host_dword) u16_t ntohs(u16_t network_word) u32_t ntohl(u32_t network_dword) u16_t HTONS(u16_t host_word) u32_t HTONL(u32_t host_dword) u16_t NTOHS(u16_t network_word) u32_t NTOHL(u32_t network_dword) DESCRIPTION
These macros convert 16-bit and 32-bit quantities to and from the network byte order used by the TCP/IP protocols. The function of the macros is encoded in their name. H means host byte order, n means network byte order, s means a 16-bit quantity and l means a 32-bit quan- tity. Thus htons converts a 16-bit quantity from host byte order to network byte order. The difference between the lower case and upper case variants is that the lower case variants evaluate the argument at most once and the upper case variants can be used for constant fold- ing. That is, htonl(f(x)) will call f(x) at most once and HTONS(0x10) will be equivalent to 0x10 on a big-endian machine and 0x1000 on a little-endian machine. SEE ALSO
ip(4). AUTHOR
Philip Homburg (philip@cs.vu.nl) HTON(3)

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