Re: 747 Rom Emulator / design

[Ludis Langens <ludis@cruzers.com>]

thergen@svn.net wrote:
> 
> Usually, you can't reliably read and write the same address at the same
> time on dual port rams.
> 
> Note that it can get interesting when updating a two byte value if the ECM
> gets half of the old value and half of the new value.

The solution is to have two banks of dual port RAM (or actually, one
double size DPRAM chip).  The ECM runs out of one bank while the other
is slowly changed.  Once the new ROM image is in place, the two banks
are switched while the ECM isn't looking.

This avoids any problems with two byte values.  It also allows a change
like bumping up values in one VE table while bumping down values in another.


Daniel N wrote:
> 
> Also, a little on the Emulator board, isnt there a simpler way of doing it ?
> I mean, i'm not saying its bad or anything, but compared to the Kalmaker
> emulator we have here for the Holden Delcos.. it looked much smaller /
> simpler.. but then again have i missed something?

Yes, the Kalmaker method is simpler, at least from a hardware
perspective.  However, it requires assistance from the ECM software.  A
true ROM emulator needs no help from the ECM.  This means that somebody
like Bruce can be quickly up and running with it on a new ECM.  A true
emulator is also important for the older C3 ECMs which have about 80% of
their program code in unchangeable mask ROM in the ECM - it would be
very hard to modify them for a Kalmaker style emulator.

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/

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