Can anyone explain why this happened?

["Norm Dresner" <ndrez@att.net>]

Inside a function called by a real-time task, I coded a
"simple" routine to read the contents from a port.  Because
I wanted not to even try to read from the address if the
corresponding board wasn't there, I put an if-statement in
front of the i/o read.

The code I wanted to write was:
    ...
    if ( BoardOK[ boardno ] )
        return inw( basePort );
    else
        return( -1 );

But instead I screwed up and wrote
    ...
    if( BoardOK[ basePort ] )  // basePort ~ 0x300 while
boardno ~ 1-4
        return inw( basePort );
    else
        return( -1 );

And the computer would *hang* every time the function was
called.  I'm obviously addressing memory beyond the end of
the array and there's probably not enough static data in
the driver to allow addressing 0x300 * sizeof( int ) beyond
the end of it without running out of the address range
assigned to my driver's data segment either.  [I know that
the hang in this routine and not after it returned data
because I had already commented out that code in an attempt
to isolate the problem.]

If I was addressing beyond the end of the virtual address
space for the kernel, I'd expect to get a segfault, not a
hang.  And if there was memory there, why didn't I just
read a value instead of hanging so solidly that only the
reset or power switches would have any effect?

I don't know how rtlinux handles a sigfault so I'm hoping
that someone else does and can  explain why this happened.

TIA
    Norm