the tyranny of the clock
In AlAn tUring’s day logic was slow and costly but, rela- tive to logic, wires were fast and almost free. Since then the costs of logic and wires
have reversed: modern logic is fast
and almost free but, relative to logic,
wires are now slow and costly. They
are costly in three ways. 1) The wires
in a modern circuit chip cost most of
its area; the transistors in a chip hide
underneath a thick bed of tiny wires.
2) The wires in a modern circuit chip
cost most of the delay. 3) Worst of all,
moving electric charge onto and off of
wires wastes most of the energy. The
cost of logic and memory dominated
Turing’s thinking, but today, communication rather than logic should
dominate our thinking.
Does communication dominate
My question applies equally to hardware, software, and theory.
Today’s digital design paradigm, the
“clocked” design paradigm, depends on
a rhythmic clock signal. The clock signal breaks time into discrete time steps.
The designer knows exactly his intent
for all the actions of each time step and
can check that all the necessary precursors for the actions of each time step
happen in earlier steps. Discrete time
steps simplify the design task.
PhotograPh by erIk Velldal
Before the telegraph, there was
no easy way to synchronize time over
distance. Fortunately, there was little
need outside navigation to know what
time it is somewhere else. “Simulta-
neous” did not need to apply between
Chicago and New York; each city could
be its own time zone. The railroad
a clock signal “simultaneously” be-
comes smaller and thus the number
of “clock zones” must increase. The
clock beat of each zone differs from
the beat of its neighbors in phase and
often in frequency as well. A large chip
may have hundreds or even thousands
of separate clock zones. The clocked
design paradigm helps within each
zone, but only within the zone.
changed that: passengers wanted to
know at what local time their train
would arrive, and dispatchers wanted
to avoid collisions. Fortunately, the
telegraph could provide a notion of
“simultaneous” from New York to
Chicago so that schedules could be
kept. Like a railroad the clocked design paradigm makes designers want
a concept of “simultaneous” so that
clock periods can begin and end everywhere simultaneously.
As transistors and wires get smaller, the area over which one can deliver