David Noon wrote in a message to John Poltorak:

 DN> I was also brought up on IBM mainframe assembler -- over 20
 DN> years ago! However, in the 1970's I also encountered the
 DN> worst range of machines ever built, the DEC PDP-11, and
 DN> learned its assembler, Macro-11. 

 DN> Since the designers at Intel all worshipped DEC, the Intel
 DN> has the same dumb little-endian architecture, very similar
 DN> instruction repertoire, same word = 2 bytes, and its
 DN> assembler language is almost a clone of Macro-11.

 DN> So, I guess the PDP-11 was useful after all. It gave me some
 DN> preparation for assembler under PC-DOS, Windows and OS/2.

Little-endian operation is by no means dumb, certainly not if you are
making the hardware.  This stuff was not designed for the purpose of
confusing you.

The motivation for having varying word sizes in the CPU was to allow
different units to be made with reasonable cost.  For example, many
machines of that era allowed 16-bit load and store operations but only
8-bit arithmetic and logical operations.  Look at the difference in price
between the 6800 and the 68000 in those days: the 68000 was introduced at a
per-unit price of hundreds of dollars.  Similarly, the motivation for
little-endianism was to allow the smaller ALU to chain operations on longer
words more efficiently.

JP> You wouldn't have come across a book such as 'PC Assembler 
JP> for mainframers' would you? It would be handy having such a 
JP> book...

 DN> No. I used Harley Hahn's book that came bundled with MASM.

 DN> Just wait until you try floating point. Extended precision
 DN> has only 10 bytes on an Intel. It is the "contracted" shade
 DN> of meaning of the word "extended".

The Intel floating point format follows the IEEE-754 standard, which
handles reals as 64 bits.  As it happens, the native Intel 80-bit real
significantly exceeds the precision required by IEEE-754.  If you need more
precision on reals than 80 bits, then you should have no expectation that
the hardware is going to support you.
 
-- Mike


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