Theatre Design & Technology - Summer 1993 - 47

Multitasking:
Don't
Try This
at Home!
CHARLIE
RICHMOND

recendy wenr our of rown for 12
days and rerurned ro find abour
125 new messages on "Computers
in rhe Theatre" in an E-mail discussion forum I regularly arrend.
Since rhis subjecr is admirredly
dear ro my hearr I plunged in, eager ro read
orhers' opinions. Discourse ran all rhe way
from rhe old "Mac vs. IBM" warhorse ro
TO scheduling sofrware, box office, and
ricker programs. I 0 discussions, however,
focused on computers involved wirh rhe
acrual producrion or running rhe show.
This is easy ro undersrand since rhe
mosr familiar com purer plarforms would
make extremely poor show or media controllers. To see "mulrirasking" defined, jusr
warch any srage manager. The average compurer user, however, has only recently
become familiar wirh mulrirasking. When
computers help run a show, everyone has ro
feel complerely comfonable-wirh live
conrrol sri II ulrimarely in charge of everyrhing. Only genuine preemprive mulrirasking software makes rhis possible.

Operating Systems and
Multitasking
ow, new versions of old operaring sysrems
(or "OS"-rhe software which makes compurers run) are rouring "mulrirasking"ofren in an abbreviared form allowing rhe
user ro easily access anyone of several software applicarions. This way, various programs can be used concurrendy (bur nor
necessarily simultaneously) and applicarions can be chosen quickly. Popular examples are: MS-DOS 6.0, Windows 3.1 and
Macinrosh System 7.1.
Terms which signify such incomplere
implememarion-"message driven," "rime
slice" and "rask swap"-are somerimes used
wirh or insread of "mulrirasking" (see
comparison chan). These are quire differenr from rrue (preemprive) mulrirasking
where each program has irs own ser of
rasks-each wirh irs own prioriries-and
where rhe
can acrually decide whar program and which rask needs ro be serviced
based on a variery of needs.
These lesser OS's use "cooperarive"
mulrirasking-Iacking rhe speed and real-

rime response rhar normally comes wirh a
well-designed preemprive
Almosr all
software wrirren for older OS's, from which
rhese newer variams have sprung, predares
rhis cooperarive fUncrionaliry and rherefore
has no inherem mulrirasking abiliry. In rhis
case, rhe newer
musr guess rhe needs of
rhe software ir is running and swirch fUncrions cooperarively-bur nor necessarily
efficiently. By comparison, preemprive
mulrirasking allows rasks ro be serviced by
rhe
according ro rheir needs, wirh or
wirhour human inreracrion.
More complex OS's incorporare preemprive multirasking via mulrirhreadingmosr effecrive in an expensive multiprocessor environmenr, in which individual
"rhreads" or "parallellighrweighr processes"
are assigned ro separare processors. When
mulrirhreading wirh a single processor,
extra overhead for addirional scheduling
and comexr swirching is incurred compared
ro a non-mulrirhreading
Speed and
realtime response suffers yer again. Popular
examples are: Windows NT, OS/22.1,
NeXTstep and UNIX.
In preemprive multirasking, any rask
may "forbid" orner rasks ro preempr ir umil
ir "perm irs" such preemprion again. Also,
each rask is assigned a "rask prioriry" value
which indicares ro rhe
how imporram
rhar rask is ro rhe program using it. Each
applicarion program uses any number of
rasks, and many programs can be acrive
simulraneously. Since each rask is a separare
emiry, mulriple programs can use a single
rask insread of mulriple idenrical rasks.
Sophisricared mulrirasking OS's have powerful messaging capabiliries allowing rhese
"library" rasks ro be insrantly accessible
rh rough srandard sysrem calls.
True preemprive mulrirasking OS's are
hardly rhe mosr popular ones, even rho ugh
rhey've been around for years. And, viceversa, rhe leasr powerfUl mu!riraskers have
become rhe mosr popular OS's. Why'
AJmosr any programmer knows:

as.

I. Few programs inherently require mulrirasking (show conuol is a norable
exceprion).
2. Mulrirasking OS's have complex rules
which musr be followed even rhough
TD&T

1993 SUMMER

Table of Contents for the Digital Edition of Theatre Design & Technology - Summer 1993