Re: [colorforth] multitasking

["Jeff Fox" <fox@xxxxxxxxxxxxxxxxxxx>]

> I was curious about a couple things.
>
> On a stack machine, how is multitasking acheived? 'pause' in colorForth
> will exchange between two sets of stacks by changing the stack pointers in
> the pentium registers. That is not an option for a stack machine.

Your definition of stack machine is very narrow. Many stack machines
have had the ability to exchange registers inluding stack pointers.
Some have even had alternate sets of registers so that single cycle
is all that is needed to change all the registers including the
stack pointers.

Now Chuck's VLSI stack machines have had neither stack pointers
into memory nor automatic spill/fill mechanisms like those he
used on Sh-Boom.  So perhaps you mean't the VLSI chips that
Mr. Moore has done rather than the generic term stack machine.

Multitasking also does not have one single definition that fits
all.  What we call multitasking in Forth is generally called
multithreading by most people becuase tasks generally share
the same memory space.  So we have heavywiight, medium and
lightweight multi-tasking (multithreading implementations
that share/co-ordinate different amounts of resources.

Medium is exchange stack pointers and user pointer and/or
any stack items cached or stored in on-chip registers.
Heavyweight, like we had in 4OS also passed ownership of
resources. Tasked owned allocated dynamic memory, files,
i/o devices, and used semiphores and channels to symchronize
tasks.  Still compared to most systems it had a very small
and very fast multitasker.  Lightweight exchanges a minimum
of registers and may require that the tasks clean up stack
use themselves at pause.

But in addition to many ways to implement multi-tasking
in software on most any and all hardware there is the
issue that stack macines may have custom hardware.  They
can use any hardware trick that they want to make faster
or easier multitasking if that is important.

> Is the second task in colorForth really needed?

It is for many of the things in colorfort to work the
way they do.  You wouldn't see source blocks and memory
dumps being updated in the background if their was not
that background processing going on.  Is it important to
you?  Maybe not.

> It seems the display can
> clean up after itself, perhaps if you wanted to break the rendering into
> chunks a 'pause' might be useful, the same effect could achieved by a
> state variable however.

Could.  Many things are possible.  It could render over a network
connection to a remote virtual monitor etc.  Everything in any
given version of colorforth does not have to be the way it is.

Best Wishes

>
> Mark
>
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