>What does the message "Caution - file register read before written" that
>MPSIM attaches to certain instructions signify? Is it telling me anything I
>should pay attention to, ie "the simulator behavior is different from
>silicon at this point" or what? The documentation does not seem to cover
>this message.

The message is telling you that you are reading a file register before it
has been originally written to and if you are depending on this value, it's
probably in error.

The power up values of file registers are indeterminant (an individual value
*may* seem to be always a certain value, but if you were to record the power
up values everytime you powered up, you'd find that they change).  Actually,
I did this experiment (I can give you the code - just requires a few LEDs
and a switch).

Because of this, it is extremely important that all variables are
initialized at the start of a program; even if it is just to clear them, it
is better than trying to debug what is going on.

I spent six weeks in the Summer of '96 looking for an improperly initialized
variable (many people on the list will probably remember it).
Unfortunately, the variable wasn't hit until about 100,000 cycles (or 100
msec) into the program (which takes an inordinate amount of time for the
simulator to get to) and I was just to lazy to wait for it (it would have
taken about a half hour and I would have had to write a pretty complex
stimulus file for it) - but if I had, I would have found the problem pretty
quickly because of the "Register Read before Written" Message.

myke

"There are only three kinds of economists in the world.  Those who can count
and those who can't." - Eddy George, governor of the Bank of England