addm's save registers

11.29: sci.addm/sharing registers sub's efficiently:

. when an act'rec saves registers
couldn't this be done more space efficient by
having each subprogram bak its own?
main has act'rec hold all var's
but while in control,
it copies its hot var's to reg's .
. when it calls a sub,
it first updates the stack with the current reg'val's
then calls a sub' that can do the same thing .
. in case of concurrency,
the task frame needs a space to hold reg's
in a place where the scheduler can find them .
. this could be an unwise idea
depending on what the efficiencies are
and the ways for registers to be moved to stack .
. as a routine proceeds, its use of reg's varies;
so there's less code used in the idea of having
a single register_save.rec for each act'rec .
. they use one code to save vector-given regs
to curr's reg'save.rec [as is typical of asm instructions
where each register is represented by a bit
telling the reg'save.instruction which reg's to save .]
. whatever's right,
depends also on how much info is sharable .
. the caller of module cannot know
what the module's reg needs are,
so it must assume worst-case:
all used regs must be saved .
. in a local subsystem,
the sub's function type can indicate which reg's its using,
this is a good idea in a modular system .
. perhaps there can be codes for
2 styles of calls in {modules, subordinates} .