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From: Eric Smith on 6 May 2008 03:05
Someone asked about state machines using encoding similar to one-hot but
with "forking" where multiple states make be active simultaneously,
and I wrote:
> DEC used that style of design in the PDP-16 Register Transfer Modules.
> Possibly also in the control units of some of their asynchronous
> processors such as the PDP-6 and KA10.

Kevin Neilson wrote:
> That's interesting--I'm not even familiar with an "asynchronous
> processor". What does that mean? -Kevin

There's no central clock. At any given time, one particular "unit"
in the computer is active. When it completes its work, it sends a
pulse to the next unit that needs to do something, thus handing off
control.

In some situations, a unit might trigger two other units. Usually
in such a case, a later unit implements a "join" between the two paths,
by waiting for both to complete.

The logic implementing such a control system looks just like a flowchart.

There were quite a few asynchronous computers in the old days, but
the world settled on synchronous designs for various reasons. In recent
years there has been a resurgence of interest in asynchronous designs,
partly due to the possibility of power savings. There are still no
mainstream asynchronous processors, though.
From: glen herrmannsfeldt on 7 May 2008 15:21
Eric Smith wrote:
(snip)

> Kevin Neilson wrote:

>>That's interesting--I'm not even familiar with an "asynchronous
>>processor". What does that mean? -Kevin

> There's no central clock. At any given time, one particular "unit"
> in the computer is active. When it completes its work, it sends a
> pulse to the next unit that needs to do something, thus handing off
> control.

Sometimes also known as "self timed logic", and probably easier
to search under that name.
(snip)

> There were quite a few asynchronous computers in the old days, but
> the world settled on synchronous designs for various reasons. In recent
> years there has been a resurgence of interest in asynchronous designs,
> partly due to the possibility of power savings. There are still no
> mainstream asynchronous processors, though.

There are rumors of asynchronous functional modules, such as
multipliers or dividers. That might make more sense in current
systems than a completely asynchronous design.

-- glen

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