[Gmecm] What really is a 'mask'?

[DV Fagan]

The term "mask" is used to identify a pattern of bits
stored in a mask programmed ROM.  In the C3 ECM arena
the term is misunderstood, as a "mask" portion of the
program resides on the CPU chip while a part of the
program as well as the data constants (spark timing
tables, fuel vs manifold pressure tables, etc.) are
stored on a PROM chip.  This chip, an obsolete 2732A
on the C3 ECMs, could be replaced with any compatible
device.  Craig Moates and others offer adapters to use
currently available 27C256 or 27C512 devices to enable
storage and driver selection of multiple sets of data
constants.  The size, location and scaling of each
element of the data constants is fixed by the program
"mask" stored in the CPU and is non-alterable.  This
is why a $9A PROM cannot be used with a $A4 CPU board.
 An addon board by Bowling and Grippo disables the
"mask" ROM on the C3 CPU and substitutes an entirely
new program complete with variables for the ones that
GM engineers created and can be used to control 4
barrel TBI or TPI motors.
Dennis

--- Don Sauman <donsauman at cythera.net> wrote:

> My understanding is that the mask is that part a
> program on a ROM that 
> is created during manufacture for a specific purpose
> and cannot be 
> erased. I.e. non-volatile memory.
> 
> Don
> 
> davesnothereman at netscape.net wrote:
> 
> > From what I understand, the mask ID is a
> calculated number.  I believe the table locations
> and sizes as well as code length are the main
> contributors to the ID.  Ludis Langens wrote a
> program several years ago which would look at a
> calibration and calculate the mask ID, as well as
> determine whether the file was stock or modified,
> and if stock would return the BCC.
> > 
> >The BCC is the broadcast code, an identifier used
> to pick out specific calibrations as mandated by US
> law.  While the mask can be thought of as the "table
> location and code template," the various BCC's
> associated with a certain mask represent different
> calibration variables.
> > 
> >The mask ID system does not guarantee a unique
> identifier for every variation of code and table. 
> Ludis had mentioned once that there are different
> codes which will generate the same mask ID.
> > 
> >Since a similar question has come up on another
> list, I have been thinking about this.  I wonder if
> the term "mask" has a historical reference to the
> early days of prom production.
> > 
> >Zaphod


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