John Nall wrote:

> The datasheet for the 18F452 (table 4-1, page 45) shows that there are 27
> "unimplemented registers" in the Special Function Register (SFR) map.  A
> note at  the bottom of the page indicates that they are "read as 0."
>
> Does anyone know if this is indeed the case?  (That is, that you can't
> write  them, and if you read them you just  get a zero?)   Trying to
> check
> it out by using the MPLAB simulator indicates you cannot change them, but
> they may just be following  the wording of  the manual.  Seems to me that
> it would be a lot more trouble in the actual manufacture of the chip to
> have  these random skips than it would be to just go ahead and make them
> usable.
>
> Does anyone know?  (If no one does, then I will find out by trying it
> with
> an actual chip.  But before going through all that trouble, thought I
> would
> check here first).

Don't go through that trouble, because it's simply impossible ;-)
You can show yourself (and us) that it will read 0 for N times, but what
will it do when you read it N+1 times ?

Her lies definitly a task for the assembler/compiler builders:
A good compiler should give a warning if you try to read from an
always-constant-location or write to an only-read-location.  As even M$
windows can do this (at a file level) it shouldn't be such a hard task.

I've just located an error in my simulator, which occured when using
Basic87x.
It was quiet hard to find, because also a crack like Myke Predko (writer
of Basic87x) thinks he can clear the RCIF bit  directly ;-)
So even MPASM doesn't perform this task ;-(

Stef Mientki

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