🚀 go-pugleaf

>
>Actually using this is a bit tricky. Brian Brunswick has working code which
>uses this in Memphis (possibly not the released version). The biggest
>problem is that when the FreeSpace module calls your handler routine, you
>are in User mode and R13 points to the FreeSpace module's private stack in
>it's workspace. This is of zero gauranteed depth ie. you can't use it, so
>you have no stack (unless you want to over-write random bits of RMA). So you
>need to supply your own stack, or not use any stack, or call SWI OS_EnterOS
>to get the SVC stack. You can ease the problem by doing as little work
>yourself as possible eg. call SWI OS_FSControl 49 (get free space) to do all
>the work for you for free space reason code 2 (the other option for IDEFS
>is to call IDEFS_DescribeDisc).
>

 This is not quite as bad as you try and present it ...

 By the same token, you are never guaranteed that there is any free space on
the SVC stack, and so should check every time before you push a register.

 The routines are called in USR mode, WITH SUFFICIENT STACK SPACE !

 Sufficient stack space means that you should not go around allocating
buffers out of the stack, or using deeply recursive algorithms, but you
should have no problem if you are just pushing a few registers on the stack.

 In fact, I'd be surprised if you ever get called when there are less that
256 bytes free on the stack, but don't quote me on it :-)


 In fact, from having a second look at the code, for some of the reason
codes you may be called in SVC mode, so don't go round allocating a stack
and making R13 point to it, or you may end up corrupting the SVC stack
pointer!


>Owen.

        Ran.


-----                    
RMokady@acorn.co.uk          |
Ran Mokady                   |  "We're children, Needing other children
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Acorn Computers              |   Acting more like children than children"
Cambridge, UK.               |