🚀 go-pugleaf

> I apologize if the question is trivial/stupid; my VAX architecture knowledge
> is rusty. I also had trouble deciding which vmsnet newsgroup should I post it
 to:
> I selected .internals because all other newsgroups seemed less appropriate.

To use that logic, then you must know for a fact, that there is an appropriate
vmsnet group for your question :-)

> I have the following C code in my program:
>
>   for (i = 0; (i<SYMBOL_NAME_SIZE - 1) && (*source != '\0'); i++)
>     *dest++ = *source++;
>
> which, when compiled (using gcc 1.39) results in :
>
> %SYSTEM-F-ACCVIO, access violation, reason mask=04,
> virtual address=175C4, PC=25C8A,PSL=03C00000
>
> The machine code that generates the error is
>
> 025C8A:         movb (r1), (r0)
>
> with r0=95684(=0x175c4), r1=94591.

>  TFM says that reason mask 04 (4(dec) = 0100(bin), so that bit 2 is
> on ?) means that I attempted a read-modify operation---am I right
> here?  But the source and destination addresses do not seem to
> overlap, so why exactly does my computer complain?

As several other answers has pointed out, then this error is a ordinary
write access-violation, which means that *dest is not valid for writing.
Why it is not valid for writing depends a lot on the rest of the code !!!!

>                                    I can manually deposit values at
> location pointed to by dest/r0 (using debugger 'deposit *dest=...'),
> so it is not a invalid address.

But now for the interesting !

The above test do NOT test what you think it tests at all !

The debugger is capable of depositing values in NOWRT psects !

Small example:

$ fortran/obj=z sys$input
      PROGRAM Z
      CALL DUMMY(1)
      END
*
      SUBROUTINE DUMMY(I)
      WRITE(*,*) I
      RETURN
      END
$
$ link z
$ define/nolog/user dbg$process NONE
$ run/debug z
set mode noscreen
dep 512=2
go
exit
$ exit

(And for the less FORTRAN knowledgable: constants are passed by reference and
the $PDATA psect is NOWRT !)

This little feature has been around for all the VMS versions I have
used and can be quite usefull.

I once got the explanation that it is due to the fact, that the NOWRT
psect actually means user-mode:readonly and the debugger runs in
supervisor-mode !

Does anyone know if this stuff is documented anywhere ?

                                                          Arne

Arne Vajhxj                             local DECNET:  KO::ARNE
Computer Department                     PSI:           PSI%23831001304030::ARNE
Business School of Southern Denmark     Internet:      ARNE@KO.HHS.DK