Mike Raiford wrote:
> A sequence of one or more control signal sets configured to execute the 
> desired instruction. 

I guess, depending on how you define "control signal sets" and "configured" 
and "sequence", sure.  *I* consider it microcode if the configuration is ROM 
instead of hardwired logic, and "sequence" implies having something 
counter-like to sequence it.

The idea being that all you need to change what instructions a microcoded 
CPU supprts is to change the ROM itself without rewiring other gates.

People used to build computers that would load different microcode depending 
on the program you were running. The first Smalltalk machines had a 
Smalltalk microcode set, so there weren't any "primitives" - the CPU ran 
Smalltalk code directly. Other machines could switch between FORTRAN and 
COBOL microcode.  The original 8" floppies were designed to hold the 
microcode for those mainframes.


> n mine, for example, the microcode rom is 
> basically a big LUT allowing for 16 microcode steps each instruction, 
> since an instruction can have more than one phase:

And how do you step thru those instructions?  A program counter type thing? 
Then it's likely microcode in my book. :-)

Don't think I'm trying to be authoritative on this one.  There's probably 
some actual official definition out there that's relatively arbitrary due to 
things on the edge of the definition.

-- 
Darren New, San Diego CA, USA (PST)
    Eiffel - The language that lets you specify exactly
    that the code does what you think it does, even if
    it doesn't do what you wanted.

Post a reply to this message