[TUHS] re: Raise of interrupt level necessary

Michael Davidson MichaelDavidson at pacbell.net
Sun Dec 15 09:27:40 AEST 2002

Wolfgang Helbig wrote:

>To see what happens, I just removed those IPL raising instructions from gword,
>pword,_savu, _aretu, _copyseg, and _clearseg in m40.s. In fact, it didn't crash
>the system. At least on my configuration, ISRs don't touch user space while
>interrupting a kernel process. This rule is broken by the clock ISR, which calls
>display() unconditionally. Just curious: Is there any other reason beside
>seeing blinking lights that justifies calling display() 60 times per second
>and breaking the rule?
Well it depends on what you think that the "rule" is ...

If you think that the "rule" is "no ISR can touch user space while
interrupting a kernel process", then it is display() that breaks
the rule.

If, however, the rule is "the user PAR and PDR registers will at
all times reflect the address space mapping of the current user
process" then it is _copyseg and _clearseg that "break" the rule
by "borrowing" some of the user PARs and PDRs, and in that case
it seems appropriate that they should raise the spl level so that
nobody else will be "surprised" by seeing those registers in an
apparently inconsistent state.


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