Hi,

>However, for whatever reason, the most significant word of a longword
>was stored first in memory; I don't remember if that was a software
>convention, or a hardware thing.

    Funnily enough, I happen to have the RSX-11M 4.1 vol 5 
    i/o drivers development and ref manual on my desk. (Nov 1981)
    
    A rather superficial skim of this yields,
    
    Appendix A (development of the address double word) says;
    
    "The addressing in a mapped system uses virtual addresses
    and memory mapping hardware. I/O transfers, however, use 
    physical addresses 18 bits in length. Since a PDP word size
    is 16 bits, some scheme is needed to represent an address 
    internally...
	
     Section 4.1.4.1 (4-32) says 
     For NPR device drivers the layout is as follows.
     
     Word 1 
      bit  0 	Go bit
      bits 1-2  function code
      bits 4-5  Memory extension bits 
      bits 6    interrupt enable
      bits 7-15 MBZ
    
     Word 2
      bits 0-15 The lower 16 bits of the address.
      
     So, i/o hardware. Probably what happened is that they stuffed
     the extra 2 address bits into the "setup word" that preceded it,
     as a wild guess. 
     
     Don't know if that has anything to do with the price of tea in
     china, but hope it helps :-).
     
     		Andy
		
-- 
atp@nojunk-mssl.ucl.ac.uk             |        Dr. Andy Phillips
phillips@nojnk-isass1.solar.isas.ac.jp| Mullard Space Science Laboratory
a.phillips@nojunk-ucl.ac.uk           | "It's the late 1990s, This is a spam   
atp@nojunk-coralcay.demon.co.uk       | protected .sig. You know what to do"