Re: [TUHS] screen editors / machine load

[Adam Thornton <athornton@gmail.com>]

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A) The original WD UART chip had very limited buffering.   The timing was
such that as high rates it could not empty accept a second character
without the first being overwritten.  This was a long-standing issue for
many UARTs long in the 1990s.  The original chip NS built and IBM used on
the PC (the NS8250) was notorious for the same problem.  By the time of
Motorola's 6881, it had 8 characters of buffering IIRC.

Great, now I'm having flashbacks to upgrading my 4-port serial card with
16450s and then 16550s in the early 90s.


On Fri, Jan 10, 2020 at 8:49 AM Clem Cole <clemc@ccc.com> wrote:

>
>
> On Fri, Jan 10, 2020 at 10:00 AM Mary Ann Horton <mah@mhorton.net> wrote:
>
>> Yes, it was a real concern. Physical memory on the shared PDP-11 was
>> limited, and if everyone had a separate copy of vi running the machine
>> would swap itself silly.
>>
>> This only mattered if everyone had their own separate copy of vi
>> installed. The fix was to put vi in a single system directory, such as
>> /usr/ucb or /exptools. The instruction part of its memory would be
>> shared among all the users, resulting in much less swapping.
>>
> Actually it was much worse than that...
>
> What Mary Ann points out was mostly true of your PDP-11 had DH11's
> installed; which had deeper hardware buffering and 16 character DMA on
> output.   But these were expensive from DEC and also took up a 'full system
> unit' in the CPU for 16 lines.   Until Able (much later) released the
> DMAX-11 (*a.k.a.* DH/DM) product of a DH11 clone on a single board, many
> university sites did not use them; but used multiple DL-11/KL-11's instead.
>
> If your system was configured with DL/KL11s or similar (CMU had it's own
> called 'ASLIs' - a synchronous line interfaces) each character took one
> interrupt for each either input or output.  Moreover, the UARTS that DEC
> used which were made by Western Digital had 2 >>or less<< characters of
> input buffering, so they could drop chars[1].  The ASLI's used a later chip
> with a slightly better buffer IIRC but I admit I've forgotten the details
> (Jim Tetter probably remembers them).
>
> So if you had a single line, the interrupt load was huge on a PDP-11.  For
> this reason, a lot of sites limited glass TTYs to speeds like 2400 or 4800
> baud, not the full 9600.
>
> DEC later released the DZ-11 which worked on units of 8 ports per board.
> Unfortunately, it was not DMA and the buffering was still pretty shallow.
>  Joy did a lot of work on 4.1BSD in the DZ driver to cut down the
> interrupts because 9600 baud DZ lines could swamp a vax and when running
> the BerkNet between systems (before UCB had ethernet), 9600 baud serial
> lines were standard.
>
>
> [1]  Two things
>  A) The original WD UART chip had very limited buffering.   The timing was
> such that as high rates it could not empty accept a second character
> without the first being overwritten.  This was a long-standing issue for
> many UARTs long in the 1990s.  The original chip NS built and IBM used on
> the PC (the NS8250) was notorious for the same problem.  By the time of
> Motorola's 6881, it had 8 characters of buffering IIRC.
>
> B) As I understand the history, Gordon developed the original idea of the
> UART at DEC for the PDP-1. But I'm not sure of the patent details. He does
> not list the UART patent on his web site although he does mention inventing
> it.   I have been under the impression CMU was somehow mixed up in the
> patent and licensing of it, *i.e.* WD got the license from CMU to make
> them not DEC; which was part of why we had the ASLI's.  Again, IIRC, we got
> the UART chips from WD at cost and could make the ALSI's locally much
> cheaper than DL-11s.  >>I think<< the story was that one of Gordon's
> student's designed a chip, which WD fabbed and licensed.  Before that DEC
> had built UARTs on boards from transistors and later logic gates.
>
>

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