Blits and terminal paging

[Guy Harris]

> Many people, perhaps most, ask why we didn't just make the Blit a personal
> computer.  The answer is that we didn't want personal computers,
> but the question can be rephrased as, "Why don't you turn it into
> a personal computer?" and the answer becomes more interesting, and
> unexpected:

> 	The division of function enforced by the architecture of a
> 	programmable terminal separated by an RS-232 line forces some
> 	design decisions that would not be made otherwise.  We were
> 	lucky in that many of those decisions result in better
> 	interaction, simpler function and much simpler software
> 	than what would develop were the same functionality provided
> 	on the host.

Actually, the question I'd ask is "why is the Blit different from a
personal computer?"  I consider it to be as different from what is called
a "terminal" as it is from a personal computer.  In a lot of the type of
software one runs on a very intelligent terminal, there is a lot of computation
involved in interacting with the user.  A word processor, for instance, must
recompute line endings and possibly even page breaks as the user types; a
graphics editor must move, rescale, and otherwise modify figures; and so
forth.  Most "smart" terminals provide little more than forms fill-in
capability and some local reformatting of text, but for "dumb" and "smart"
terminals, and even a lot of "intelligent" terminals, the only processing
done in the terminal is the limited rearrangement of text already formatted
at the host.

> This is hard to defend in a brief note, because the reasons are many
> small things rather than a couple of obvious advantages, but one
> exemplary effect is that the operating system running in the terminal
> (sic) has a much simpler structure than Unix -- in fact, it is a real-time
> system; getting such good interaction from Unix would require a fair
> bit of real-time functionality in the Unix kernel.  We were able to
> avoid that morass altogether.

Many desktop workstations running UNIX are now coming out; they may have
modified UNIX specifically so that it can provide this sort of real-time
response.  There is nothing in the structure of UNIX that absolutely requires
it to be oriented to multi-user time-shared operations; to some degree, that
is merely a policy decision made by the UNIX scheduler.  The simplicity of
the Blit OS is probably due to its not having to provide all the features
of a multi-user system supporting a file system and similar services.

This does, however, raise an interesting discussion about what the
appropriate architecture for "desktop computing" is; what operations should
be performed in a centralized host and what operations should be performed
in the desktop computer (which I count the Blit as, based on the assumption
that in a text editor the reformatting of text under insertions and deletions
is done in the Blit and that in graphic applications the movement of
figures is also done in the Blit).

	Guy Harris
	{seismo,ihnp4,allegra}!rlgvax!guy