Question about context switching on SPARC

[Shun Yan Cheung]

I have two questions about the Sparc assembly instructions:

1. When is the WIM updated ? It seems to me that SAVE and RESTORE should
   mark and unmark a window as used and unused respectively.  In Cypress
   Sparc User's guide, it seems to imply that one can implement many
   ``windowing policies'', which means that the OS is responsible for
   maintaining WIM on window over and underflow.  Is this guess right ?

2. I want to save context myself to implement a process based simulation
   system, but there do not seem to be any user instructions for doing that.
   To save context, I have to know how many windows are used by reading the
   CWP in PSR, but RDPSR is privileged.  What the deal here ? Or is there a
   trap that I can call to save my register windows ? If so, is that 0x3
   (ST_FLUSH_WINDOWS) ? What exactly does this trap do ? Other the one line
   in the Sun 4 assembly language manual, I can't find other reference to
   this trap.

Thanks in advance for any help.

Shun Yan Cheung     |  cheung at mathcs.emory.edu                  Internet
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