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From: melvin@ji.Berkeley.EDU (Steve Melvin)
Newsgroups: comp.arch
Subject: Re: QM-1 or like machines???
Message-ID: <20312@ucbvax.BERKELEY.EDU>
Date: Thu, 27-Aug-87 14:03:00 EDT
Article-I.D.: ucbvax.20312
Posted: Thu Aug 27 14:03:00 1987
Date-Received: Sat, 29-Aug-87 17:22:37 EDT
References: <63900006@convex>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: melvin@ji.Berkeley.EDU.UUCP (Steve Melvin)
Organization: University of California, Berkeley
Lines: 31
Summary: The M68000 is *NOT* like the QM-1

In article <63900006@convex> graham@convex.UUCP writes:
>
>There was once a company named Nanodata which built a machine called the
>QM-1.  The QM-1 had two levels of "micro-code" (one called nano-code and the
>other call micro-code).

oster@dewey.soe.berkeley.edu.UUCP replies:
>
>The Motorola 68000 has two levels of "micro-code" (one called
>nano-code and the other call micro-code).

alan@pdn.UUCP replies:
>
>The M680x0 family has a nanocode/microcode architecture, and has been
>used to emulate other architectures.

Actually, the QM-1 and the M68000 are not the same at all.  The QM-1 has
a *true* two-level structure while the M68000 doe not.  In the QM-1, a
microinstruction generates a dispatch into nanocode, and a sequence of
nanoinstructions is then executed to "interpret" that microinstruction.
Thus, a two level interpretive structure exists.  The nanocode
interprets microcode and the microcode interprets macrocode.  In the M68000
however, there is only one level.  There, a microinstruction contains 
within it a field which indexes into a ROM to produce a "nanoinstruction".
Each microinstruction specifies only one "nanoinstruction" and there is
only combinational logic between the microinstruction and the
"nanoinstruction".  One advantage of this scheme is that it can save
control store space.  So, the M68000 really has only one level of
interpretation, the only reason that the second group of microcode is
called "nanocode" is that the designers of the 68000 didn't understand
the use of the term. :-)