Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10 Apollo; site apollo.UUCP
Path: utzoo!linus!decvax!wivax!apollo!alan
From: alan@apollo.UUCP (Alan Lehotsky)
Newsgroups: net.flame
Subject: Re: Godless 68000 instruction set - (nf)
Message-ID: <199@apollo.UUCP>
Date: Thu, 8-Dec-83 15:46:00 EST
Article-I.D.: apollo.199
Posted: Thu Dec  8 15:46:00 1983
Date-Received: Sat, 10-Dec-83 02:39:08 EST
References: <2546@hp-pcd.UUCP>
Organization: Apollo Computer, Chelmsford, Mass.
Lines: 51

Yeah, I agree completely about the abortion that
MOTOROLA produced (aka the 68K). You should notice that you
can also do:

        CMPA.W  #0,An   which only requires 4 bytes.

Further, the fastest, cheapest way to clear an A register
would be self-evident to an old time IBM-360 programmer, it's

        SUBA    An,An       2 bytes, 8 u-cycles
where-as

        MOVEA.W #0,An       is 4 bytes and 16 u-cycles

Having been responsible for writing the BLISS compilers
at DEC, especially for the VAX-11, it is ALWAYS painful
to discover what a sh**ty programming model the turkeys
in Austin, Tx. have.  [See what happens when you let hardware
people design machines....  Just like that old joke about the
pope... "you no playa the game, you no makka the rules.  Since
machines are programmed primarily by compiler-writers these days,
they should be designed with the compilers in mind!]


Just consider all the other brain-damage available...

    o No indirect thru memory.

    o The branch-displacements are specified in byte-deltas,
      not word deltas (like on the PDP-11), so branches have
      1/2 their potential range available.  The low bit is ALWAYS
      zero!

    o Instructions like LEA and PEA don't accept the full set of "memory"
      addresses.  For example, something like

            PEA (A4)+

      is illegal!

    o The BIT-test instructions are totally inept! Being restricted
        to testing one of 8 bits in a single byte is almost worthless

I heard a rumor once that the 68k was originally a reasonable machine, but that
the first implementation wouldn't fit onto the available real-estate, so they
just kept pruning until it fit.

p.s.    The fact that CLR, ST and SF do read-modify-write memory cycles has been fixed
        in the 68010.  Fat lot of good that does when there are thousands of 68000's
        which we need user-mode code to be compatible with!