Weekly Classic Computer Trivia Question (20150112)

[Johnny Billquist]

On 2015-01-13 03:19, Rick Murphy wrote:
> At 01:17 PM 1/12/2015, Johnny Billquist wrote:
>> 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". :-)
>
> The OS/8 FORTRAN runtime (FRTS) runs with interrupts enabled. OS/8 in
> general doesn't care if interrupts are on, which is how OS/78 (or was it
> OS/278?) symbionts worked. So, you can't blame OS/8 for getting into the
> way. There's nothing typically in OS/8 that requires interrupts to be
> disabled, but it's true that most of OS/8 doesn't do anything to handle
> interrrupts.

Well, not entirely true. Yes, FRTS runs with interrupts enabled. But it 
disables them before jumping into OS/8, unless I remember wrong.
It's been a very long time since I mucked around inside FRTS...

But the problem is that if you have interrupt enabled, and OS/8 do some 
I/O, you will get the interrupt. Meanwhile the OS/8 device driver is 
sitting in a polling loop waiting for the interface to indicate it is 
finished. The problem is that whenever it is finished, you get the 
interrupt. The only way to dismiss the interrupt is to remove the finish 
flag, which means the OS/8 device driver will never see it set. And you 
"hang".

The other potential problem being that you might get interrupts from 
whatever OS/8 is playing around with. Some of which you might not 
actually be prepared to handle. Although this is possibly less of a 
point, since there is a risk those devices might generate an interrupt 
as soon as you enable interrupts anyway. Depends on the device.

> Interrupt latency? Either you're looking for the length of the "skip
> chain", or interrupt blockers like CIF instructions.

Continue reading the thread. Seems he's looking for the hardware 
response to the raising of the interrupt pin on the CPU, and I assume 
under the condition that interrupts are enabled.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
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