The TX-2 was one of the first computers to have quite a few program 
counters (I think over 20) and zero-overhead taskswitching according 
to various signals, including I/O ones.

Cheers,

Alan

At 01:29 PM 5/4/2007, Bert Freudenberg wrote:

>On May 4, 2007, at 16:20 , Jon Hylands wrote:
>
>>On Fri, 4 May 2007 15:58:40 -0400, Bert Freudenberg
>><bert at freudenbergs.de>
>>wrote:
>>
>>>I'm pretty sure this is not true, but more like what Damien wrote.
>>>Basically all that HT does is having a second register set so you can
>>>hold the state of two processes in the CPU at the same time.
>>>Execution units are shared. Since one process seldomly utilizes all
>>>execution units (due to "pipeline bubbles") the other process can use
>>>the free units. This simply enhances throughput, but it's not like in
>>>one process you only ever get half the speed.
>>
>>Okay, well perhaps I misunderstood. I know a couple years ago we did a
>>bunch of research into this, because we were running 4-processor
>>servers
>>with hyperthreading in each processor, and we were running 4 VisualAge
>>server images on each machine. By turning off Hyperthreading, we
>>got a very
>>significant speed increase (not quite double, but close). With
>>hyperthreading on, we only ended up ever using 50% of the total CPU
>>available.
>
>Well, then I'd take your experience over Intel's marketing I guess ;)
>
>The idea actually is rather old, IIRC the Alto was 16-ways
>"hyperthreaded".
>
>- Bert -
>
>