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"Juergen Stuber" <stuber@loria.fr> wrote:
> "Mark Riley" <markril@hotmail.com> writes:
> >
> > It also turns out that there are 8 bytes of external
> > RAM that peek through at addresses 0xff80-0xff87.
> > Writes to these locations also affect the motor
> > control register. I suppose there's a small advantage
> > in using these registers to control the motor since
> > accesses to these locations can use the processor's
> > 8 bit addressing modes. And, because of the
> > "shadowing" by the RAM, the various bit instructions
> > seem to work fine. I bet the H8 designers left this
> > little hole just so these instructions could be used
> > on external devices.
>
> In that case, why didn't the RCX designers use an 8 input NAND
> to decode their addresses, to leave more of the RAM usable?
> Maybe there werent aware of this possibility?
As it is, it seems[1] that two of the three NAND gates in the
74HC10 are being used as a driver for speaker output (it's odd
that it's two, not one though). So, this leaves one three input
NAND gate to do the decoding. Now that I think about it, you
only need to look at A12-A14 since A15 is always high on
external memory accesses, so this makes sense.
To answer your question, the circuit board is pretty crammed
as it is and this is just a trade-off in terms of cost and circuit
complexity vs. the ideal programming environment.
Mark
[1] I say "seems", because I'm using some hi-res photos I took
of the circuit board as a reference. It can be somewhat
challenging to follow some of the traces on the board this
way. Perhaps if someone has a dead RCX, it could be donated
to the "cause" so we could find out where some of the traces
underneath the various chips go. ;-)
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