On Jan 9, 2004, at 4:26 PM, der Mouse wrote:

> ["Nathan J. Williams" <nathanw@wasabisystems.com>, replying to me]
>>> (b) the Unix PALcode
>> PALcode generally has its own set of implementation-dependent
>> registers that you can't get to from non-PAL mode anyway.
>

If memory serves me, there were some instructions that only executed
when in pal mode. Also, there are some general registers that have a
separate pal mode set, and to get at the normal set of registers you use
a different instruction that only works under pal mode. However, not all
of the general registers have duplicates, so parameters are passed in
the shared registers, and pal code doesn't have to save any registers to
do its own work unless it needs more registers than it has privately.
  At least it makes it so you don't save the entire
register set every time you enter pal mode...

> [same attributions, another message]
>> I think the short version is:
>>> (a) hardware
>> The zero register.
>>> (b) palcode
>> No registers in the ordinarly avaliable register set.
>
> I'm sure I saw one reference that spoke of one register that the
> PALcode made assumptions about in some cases.  I think it was $sp but
> I'm not sure - I can't seem to find the reference now.  For my purposes
> I have to support signal delivery, so $sp is reserved anyway, but I
> wanted to make sure there weren't any other lurking surprises.
>
>>> (d) compiler
>> This is trickier.  AT and GP are the ones most likely to get you in
>> trouble, but at this point you'll need to get a good sense for the
>> ABI, and I'm not sure how you're planning for your asm code to
>> interact with compiled code.  Within a leaf function, I think you can
>> bash everything except sp (see above) and you only need to restore
>> s0-s6 and ra.
>
> I expect to be a leaf function in most respects.  If I do have to call
> anything else, I'll either do it by returning and letting the C code
> that called me handle it or I'll save my own state first and restore
> something more like what I was called with.
>
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