What does the unimplemented trap bomb error message mean?

Hex Star hexstar at gmail.com
Wed Feb 7 00:30:53 CST 2007


On 2/6/07, Ethan Dicks <ethan.dicks at gmail.com> wrote:
>
> On 2/6/07, Hex Star <hexstar at gmail.com> wrote:
> > well what's a software trap? something that traps data?
>
> Nope.  Something that traps program execution.  If you dig into the
> Motorola documentation for the 68000 processor, there's a number of
> pages (a whole chapter?) dedicated to how the trap architecture works.
> To attempt to summarize, in a simple architecture like the 6502, you
> have 3 fixed vector addresses in memory - all up at the top of the
> 65536-byte address space - I don't recall what order they come in, but
> one is the reset vector, one is the IRQ vector, and one is the NMI
> vector.  In this context, a vector is an address to some code, not the
> code itself.  So... when the reset pin is asserted or the interrupt
> request is asserted or the NMI (non-maskable interrupt) pin is
> asserted, the processor grabs 16 bits from a specific place at the top
> of memory and (leaving out a few details) stuffs that value in the
> program counter and starts executing at the address based on what was
> stored at the top of memory.  With the 68000, it's *much* more
> complex.  In addition to hardware interrupts doing something similar
> (but pulling from _low_ memory), events such as trying to execute from
> an odd address (it's OK to jump to $10000 but not $10001) or divide by
> zero or a variety of other pre-defined events, will cause unique
> vectors to be invoked, allowing all of these events to be handled in
> software, not just crashing the processor and forcing a reset.  In
> addition, there are officially defined "trap" instructions as part of
> the 68000's instruction set - when the processor hits a TRAP
> instruction, a similar chain of events is set into motion - in other
> words, the processor acts like some hardware event happened, but
> _really_, it was a software event - the OS or application programmer
> _wanted_ the program flow to drop what it was doing and, using a fixed
> location in memory as a reference, jump to some routine elsewhere in
> memory.  If you don't stuff the right 24-bit number (for a 68000) into
> your software trap vector addresses in low memory, the processor is
> going to end up going to some default routine that means, essentally,
> that code flow jumped to something that hasn't been defined yet.
> Rather than do nothing, the MacOS throws up a graphic and tells you
> that you ended up somewhere that was never changed from some catch-all
> default.  It wasn't a hardware event, so the term "software trap" is
> entirely correct.
>
> > I don't think it has
> > to do with the normal meaning of trap which is kind of confusing...IMO
> it
> > would've been better if they didn't use trap for these errors...
>
> The word "trap" here is being used in a jargonistic sense, not a
> conversational English sense.  For someone that knows how the 68000
> works, the term is clear and completely correct.  To someone who isn't
> technically savvy with processors at a very low level, it probably
> does look like gibberish.
>
> -ethan
>

ah wow...sounds like the detailed descriptions would be interesting to read
(and the processor is alot more complex then one would think)...where can I
find the documentation on the 68000 processor? thanks! :-)



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