Weekly Classic Computer Trivia Question (20150112)

Johnny Billquist bqt at update.uu.se
Mon Jan 12 12:17:32 CST 2015

On 2015-01-12 18:19, Vincent Slyngstad wrote:
> From: Doug Ingraham: Monday, January 12, 2015 8:10 AM
>> What is the source of the greatest latency in the interrupt system on a
>> PDP-8.
> One answer might be "the skip chain".  Since interrupts weren't
> vectored, it was necessary to query each device in priority order to see
> if it was currently requesting interrupt service.

Ah. Good point. I forget to include this in the possible interpretations 
of interrupt latency.

> Another might be "disk I/O".  Since most operating systems' device
> drivers ran with interrupts masked, this meant that when the buffer
> filled and had to be written, significant real time (milliseconds to
> seconds, depending on the device) could be involved.  The alternative
> (interrupt driven mass storage) meant replicating significant O/S code,
> and meant you had to write your own drivers to change storage devices.

Uh? Disk I/O is normally DMA. It is not performed while you are in the 
device driver. The device generates an interrupt when it finishes, at 
which point you get back into the device driver. While the transfer is 
going on, you have interrupts enabled, and are running out in "user 
space" normally.

Interrupt driven mass storage was (and is) pretty uncommon, because it 
requires so much CPU. But of course, for those who do, you transfer only 
at interrupt time, so you are not bound by the disk speed anyway.

And of course, you also have things like OS/8, which runs with 
interrupts off at all times, to which the answer to the original 
question would be "OS/8". :-)


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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