Weekly Classic Computer Trivia Question (20150112)

Vincent Slyngstad vrs at msn.com
Mon Jan 12 11:19:02 CST 2015

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.

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.


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