Wimp_SlotSize

I was recently playing around with some code that omitted setting R1 of Wimp_SlotSize to -1. As BASIC sets unspecified registers to zero, this meant that the Next slot was, appropriately, set to zero.

I began to wonder if it wouldn’t be better if Wimp_SlotSize simply quietly ignored if the size was given as zero, as I can’t imagine that it would ever be a legitimate value.

• Next slot, when set to zero:
  • may have the effect of creating spurious “Out of memory” errors when ‘seeing’ some applications for the first time (there must be some things that are done prior to setting a WimpSlot – a bit naughty)
  • will report likewise for directly running applications that don’t have !Run files to set up slots
  • may cause problems with those that do – Manga crashed with PC set at &730 which appears to have come from usb_upcall_hook in EtherUSB. At least, that’s what it appears to be in the disassembler dump, as that’s the first thing in the SVC stack).
  • creates “interesting” TaskWindows that can’t do much, can’t even start BASIC. Given what Justin Fletcher thinks of TaskWindows, I quit it before it blew up. ;-)

• Application slot, when set to zero:
  • I can only imagine this to be an utterly dumb masochistic thing to do.
    So I tried it, dropping a call into a random BASIC application prior to it’s polling loop.
    It died with an abort, leaving me with a blank grey screen and a little bit of command line text in the middle. Like the one what pops up for backtraces, only it didn’t bother to draw anything window-like.
    I realised pretty quickly that it wasn’t the app that crashed, but everything. The desktop was unrescueable.

With this in mind, dropping in two instructions to quietly convert ‘0’ to ‘-1’ (the SWI returns sizes on exit, so this is non-destructive) would probably be a good idea from a system stability point of view as I can’t see any logical reason to set either slot to zero.

Zero-sized application slots are perfectly valid. IIRC, it’s the sole criteria that the task manager uses to differentiate between “application tasks” and “module tasks”.

To read them, yes. But to set them?

If you remove the desktop backdrop (by *Backdrop -Remove or otherwise) Pinboard will set it’s Wimp Slot to zero size.