Don’t do this

MOV R10,#OS_CallASWI
SWI OS_CallASWI    ; Go directly to #1 Infinite Loop

Nor the obvious alternative. Amusingly, you can do either version of this:

MOV R10,#OS_CallASWIR12
MOV R12,#OS_WriteI+7
SWI OS_CallASWI   ; Use CallASWI to get CallASWIR12 to call WriteC

It would be a really good idea to not change Basic to use OS_CallASWI unless Basic (or preferably CallASWI) spots the infinite loop and errors safely.

Both of the CallASWIs should reject both of the CallASWIs, really.

Both of the CallASWIs should reject both of the CallASWIs, really.

RISC OS sanity checking inputs? What’s this heresy?

It’s explicitly stated in the documentation to not do that:

You cannot use this SWI to call itself, that is OS_CallASWI or OS_CallASWIR12, since there is no defined final target SWI in this case.

It’s explicitly stated in the documentation to not do that:

There’s a warning on the bottom end of power pylons stating danger of death – people still try to climb them so it’s no surprise Nemo is testing the equivalent of the lower level licking a 9 volt battery.

The questions are:

1. Can anything be done to stop people doing that (and similar things around the OS)
2. Does anyone feel inclined to make the alterations, or are they too busy on other things?

I say leave it. If somebody wants to use a SWI to call itself and cause a universal meltdown, then let’s count that as a lesson to be understood and learned…

I mean, of we’re going to talk about dumb things not to do – don’t treat the base of the RMA as a beginning of a sprite area… There’s loads of stuff not to do. ;-)

Can anything be done to stop people doing that

RTFM

Does anyone feel inclined to make the alterations

A pointless exercise I’d say, you’d end up adding so much input validation to so many SWI. Developer time would be better spent rewriting SWI’s into C.

A pointless exercise I’d say, you’d end up adding so much input validation to so many SWI. Developer time would be better spent rewriting SWI’s into C.

It was a rhetorical question. Of course there’s the question whether the rewrite in C would be like for like or be enhanced, but we can leave that for another day.

The merit in leaving the SWIs as they are now is that computers should do what they are told. You might be stupid to ask the computer to do that, but we learn from our mistakes.

My point, not sufficiently clear it seems, is

It would be a really good idea to not change Basic to use OS_CallASWI unless…

Because, regardless of RTFM, SYS"OS_CallASWI" currently calls OS_Find or OS_ValidateAddress because of its entry conditions, whereas if Basic actually used OS_CallASWI then the above would become an infinite loop that stiffs the entire machine. And I don’t think we should be adding any of those.