[Coco] Newbie doubt OS-9 Level 1 and the CoCo 2

[gene heskett]

On Thursday, May 31, 2012 06:30:32 AM Juan Castro did opine:

> I never used OS-9. Does it multitask on the CoCo 1/2?

Yes.

> If so, how is it able to? CoCos, pre-Coco 3, didn't have timer 
interrupts.

Having walked around in the clock module, one of the first modules I ever 
attacked in the last half of the '80's. that is not true,  The clock module 
is where the processing takes place, at nominally 60 ticks a second and 
schedules which process gets to run for the next tick.  This is also where 
hardware interrupts are serviced, with the searching for the interrupt 
source order being determined by the priority value passed in at the time a 
module registers its individual IRQ service routine.

I have measured the IRQ latency of a coco1/2/3, once the mpi was strapped 
to pass the interrupts, at a maximum of 15 microseconds, which, considering 
the 1/2 ran at .889 mhz, is very good.  This is from the instant the ACIA 
in an RS-232 pack pulled the IRQ down/true, to the data register in the 
ACIA being read, which cancels that IRQ.

> Was it cooperative multitasking?

No, see above.  My foray into the clock module was brought about by the 
clock chip in the B&B-XTC HD interface, it needed a relatively slow to 
execute access 'password' shifted into it in order to gain access to the 
time registers.

Originally written to update the coco's time from the RTC chip every 60 
seconds, this slow, shift the password into it took several milliseconds to 
execute, with the IRQ's disabled, and played hell with serial 
communications even at 300 baud, driving the early versions of rzsz out of 
its skull since it had a 1 minute pipeline drain before it requested the 
restart of the file download that was errored out by this locked out IRQ 
causing several characters to be lost.  I was lucky to get an effective 
download speed of 75 baud under those conditions.

By the time I got done with that particular clock module, most of the 
software only clock had been transplanted into it and the tick values fine 
tuned such that it only did this at bootup and midnight, only getting off 
15 seconds or so in that time frame.

Self defense, this was back in the days before Judge Green, and it was long 
distance charges for me to call my ISP, 11 miles of noisy phone line away.  
So it was a problem that HAD to be fixed when it was costing me $120+ a 
month to go online with my coco.

> Juan
> 
> --
> Coco mailing list
> Coco at maltedmedia.com
> http://five.pairlist.net/mailman/listinfo/coco


Cheers, Gene
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
My web page: <http://coyoteden.dyndns-free.com:85/gene>
...and scantily clad females, of course.  Who cares if it's below zero
outside.
	-- Linus Torvalds