Installing PDP-11 UNIX w. no tape - solution

Greg Lehey grog at
Sun Feb 1 12:45:58 AEST 1998

On Sat, Jan 31, 1998 at 01:18:24PM -0500, Allison J Parent wrote:
>> From: "Steven M. Schultz" <sms at>
>> The DL-11 to which the TU58 was attached (could it be hooked up to
>> something a bit better?  I would think so but don't know for sure)
>> had no buffering/silo - at 9600 there was only 1 millisecond to get
>> the character and that's cutting things a bit too fine on a ~.5 mips
>> machine, especially if other things are going on at the same time.
> I've run z80s/4mhz at 19.2k with no errors it's was the structure of the
> driver and a total level of hardware buffering of one byte.

Sure.  UNIX drivers have never been particularly optimized for high
async interrupt performance.

>> Ummm, 'PC's I'm used to don't seem terribly upset at 10 or 20 thousand
>> interrupts per second - that should be sufficient to handle any 9600
>> baud serial line I'd think.
> they can only because the '450 has a silo of 4 bytes 

The 16540 has only one byte buffer.

> and the 550 it's either 16 or 32 bytes.

The 16550 has 4 bytes, and the 16650 has (I think) 16 bytes.

> That's a whole lot of time before you must service it and then there
> is the matter of a few dozen mips of cpu behind it.

That's more to the point.  Don't forget that a high-end Pentium is
probably 1000 times the speed of an 11/20.  I regularly get 50-60k
interrupts per second when downloading fonts to my PostScript printer,
and I think the printer is the limiting factor there.

>> Not 'overhead' as much as just 'slowness'.  An 11/44 is about .6 mips
>> (an 11/73 is about 15% less) - that's quite a bit less than even a
>> 286.
> No comparison.  My 11/23 runs just fine with the TU58 running at 38.4k.
> the difference is the 11/23 is not using a console processor inbetween.
> In that case the DLV11j is the higest priority in the bus.
> My 11/73 also uses the TU58 at 38.4 but the disks (RX02, RQDX3, RL02)
> are all lower priority.  Again there is no problem unless the system is
> real busy and then the tu58 will do rereads for blocks that were not
> ack'd.

Have you modified the kernel?  Normally disks will preempt ttys.

> Its byte timing.  the bits are handled at the uart.  But your right
> my systems are lightly loaded and generally run RT11FB or XM.



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