PDP/Vax/Alpha Well Ordered Little Endians

[james hanratty]

Hi,
 
I coded on PDP-11s from 1974, on Vaxs from 1982, and have had a little to do on 
Alphas in the last few years.
 
The DEC machines are all well behaved Little-Endians, not NUXI as shown in many 
of the include files in LINUX.
 
I think perhaps the root of the mis-understanding comes from 
1) the ordering of bytes/words/double word and how they are drawn in memory 
layouts. If someone is unfamiliar with little endians, bad information is 
assumed and propogated.
2) the confusion of byte and numeric values (HEX). 1234x is stored as 3412 But 
this is not byte ordering-- this is normal little endian numbering.1234 is a 16 
bit number, not a 32 bit number as indicated in the byte ordering of PDP-ENDIAN 
= 3412. 
 
byte    word     d-word   q-word
a         
b         ba
c
d         dc        dcba
e
f          fe
g
h         gh        ghfe      ghfedcba
 
DEC labeled their dumps with the address in the midst of the dump data 
(assuming hex in place of the octal of PDP/VAX):
 
.byte 'A       41              addr
.byte 'B       42              addr+1 
 
The above text string becomes
 
Label: 
.ascii "ABCDEFGH"       4847464544434241  :addr       ABCDEFGH
           ^                                                 ^^                 
^                           /// ADDR
             ^                                          ^^                      
  ^                          // ADDR+1
 
and so forth. Label: has the address of addr and is where A resides.
 
The memory locations of the data in a string def reads reversed in the order of 
memory location from the visual order of the string. In the ascii dump, the 
order increases from the addr on the dump and is in the same order as the 
string human ordering.
 
This is all normal little-endian.
 
The several swab instructions are needed to convert from "12345678" to 
"87654321". The existentce of these instructions and the use of octal may have 
helped create the NUXI misinformation. Numeric values of 12345678 and 87654321 
do not represent the byte order. 12-34-56-78 does.
 
The PDP_ENDIAN definition of 3412 violates both the little endian nature of 
these machines where the last byte contains the even/odd (parity) of the number 
(1) and the first bit/byte the sign (4). Does 34-12x represent a number and not 
the byte order??? byte order of 1234 is really 01-02-03-04 or in DEC 
04-03-02-01.
 
I know some of ATT's unix machine from the early 70's are still 
running--although they are being replaced with Intel machines--same endian!
 
Hope this helps.
 
Jim Hanratty
 
 
 
 

                
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