Thanks again for helpful info. Research indicates that it is not safe
to call MP services at interrupt level, which I assume would include
DeferredTask on OS9. OTAllocMem is safe at interrupt, by drawing from
special pools that are replenished by ordinary allocations at safe
times. Probably better to just do that myself, rather than learning
to use OT memory properly. Or, re-write my DeferredTask as an MP
Task.... not sure if there are any other advantages there.
Also, on my original concern regarding va_start() and vsprintf() at
interrupt time, I did find Apple sample code, debugf.c, for
interrupt-safe formatted printing. It uses va_start, but
re-implements vsprintf. So that must be where the interrupt problems
I ran into before came from.