[TUHS] non-blocking IO (morphing to "The serial I/O loop")

Paul Ruizendaal pnr at planet.nl
Tue Jun 2 19:34:54 AEST 2020

> On 5/31/2020 9:46 AM, Warner Losh wrote:
> > Sorry to top post, but LSX or Miniunix had non blocking I/O as well. 
> > It was in one of the documents that Clem scanned in the last year. It 
> > specifically was an experiment into how to do it.
> >
> > Warner

> I did  add a few new features to LSX to deal with contiguous files
> and to handle asynchronous read/write's for real time applications.
> They are described in the LSX paper in the 1978 BSTJ on the
> UNIX Time-Sharing System.
> Heinz

Thanks for highlighting this!

The realm here is async I/O to disk, my original scope was limited to “communication” files (tty’s, pipes, network connections). Still, I find it an interesting topic.

For others, the paper that you refer to can also be found here:
If I read correctly, the async functionality was available only in the stand alone program version of LSX. Is that correct? In any case, the source code appears lost.

From another paper in that set, I get the impression that the async functionality for LSX builds on earlier work for a very early version of Unix:

- - -

When reading through the papers in that TUHS directory, something else caught my attention: early networking at the labs. For a while I have been puzzled by the “serial I/O loop” in use at the labs in the early seventies. Last Fall I found some 1970/1971 BSTJ papers about it, but there it stopped.

I see in the memo on Glance (https://www.tuhs.org/Archive/Documentation/TechReports/Heinz_Tech_Memos/TM-75-1352-3_GLANCE_Terminals_on_UNIX_Time-Sharing_19750303.pdf) that D.R. Weller continued to work on it up to 1973 at least and that it was integrated with Unix in some way. Is that correct? Did the two memo’s referred to (MM 70-1384-1 and TM 73-1356-8) survive?

Then the memo on satellite processors is very interesting (https://www.tuhs.org/Archive/Documentation/TechReports/Heinz_Tech_Memos/TM-78-3114-2_A_Minicomputer_Satellite_Processor_System_19780322.pdf). This appears to show the serial I/O loop in use as late as 1978, with a very intriguing use case involving system call forwarding over the network.

Can you tell me more about the serial I/O loop and its use cases?

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