AIX endian again Post: 17923

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Top Forums UNIX for Advanced & Expert Users AIX endian again Post 17923 by Perderabo on Thursday 21st of March 2002 08:16:30 AM

03-21-2002

Administrator Emeritus

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.

Perderabo

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LEARN ABOUT OSF1

swap_lw_bytes

swap_lw_bytes(9r)														 swap_lw_bytes(9r)

NAME

       swap_lw_bytes, swap_word_bytes, swap_words - General: Perform byte-swapping operations

SYNOPSIS

       unsigned int swap_lw_bytes(
	       unsigned int buffer ); unsigned int swap_word_bytes(
	       unsigned int buffer ); unsigned int swap_words(
	       unsigned int buffer );

ARGUMENTS

       Specifies a 32-bit (4 bytes) quantity.

DESCRIPTION

       The  swap_lw_bytes  interface  performs a longword byte swap. The swap_word_bytes interface performs a short word byte swap. The swap_words
       interface performs a word byte swap. Many computer vendors support devices that use a big endian model of byte  ordering.  Because  Digital
       devices	support the little endian model of byte ordering, there is a need for these byte-swapping interfaces. In addition, some buses (for
       example, the VMEbus) can have specific or implied byte ordering that may require the use of these interfaces.

       Given that a longword is equal to 4 bytes; a short word is equal to 2 bytes; and 1 byte is equal to 8 bits, these interfaces swap bytes	as
       follows: The swap_lw_bytes interface takes the 32-bit quantity specified by the buffer argument and swaps all 4 bytes.  The swap_word_bytes
       interface takes the 32-bit quantity specified by the buffer argument and swaps the individual bytes that make up each word  of  the  32-bit
       quantity.  The swap_words interface takes the 32-bit quantity specified by the buffer argument and swaps the two 16-bit words.

       The following illustration compares the byte swapping performed by these interfaces.

			 31		  0
			  +---+---+---+---+ Starting value:    | a | b | c | d |
			  +---+---+---+---+

			  Long word byte swap
			  (swap_lw_bytes)

			 31		  0
			  +---+---+---+---+ Ending value:      | d | c | b | a |
			  +-------+---+---+

			  Short word byte swap
			  (swap_word_bytes)

			 31		  0
			  +---+---+---+---+ Ending value:      | b | a | d | c |
			  +---+---+---+---+

			 31		  0
			  +---+---+---+---+ Starting value:    |   ab  |   cd  |
			  +---+---+---+---+

			  Word byte swap
			  (swap_words)

			 31		  0
			  +---+---+---+---+ Ending value:      |  cd   |  ab   |
			  +---+---+---+---+

RETURN VALUES

       Upon successful completion, these interfaces return the swapped bytes.

																 swap_lw_bytes(9r)