CIA TOD Alarm Bug

Date: Wed, 08 Mar 2017 18:45:01 +0100
Message-ID: <3476337.CHqXkKiiIM@rakete>
hy fellow hackers!

i am investigating differences in the CIA behaviours (NMOS vs HMOS versions, 
ie 6526 vs 8521) at the moment. one thing that is pretty unclear to me, is a 
supposed bug in the TOD alarm behaviour. i can find some references to it on 
the web, which somehow look like one copied from the other to me - without 
ever mentioning the exact details:

"Due to a bug in many 6526s the alarm IRQ would not always occur when the 
 seconds component of the alarm time is exactly zero. The workaround is to set 
 the alarm's tenths value to 0.1 seconds." 
"There was also a problem with the Time of Day clock alarm interrupt, which 
 would not occur if the alarm time tenths-of-seconds was set to zero. The 
 solution was to set the tenths to anything other than zero."

... "we" have created a bunch of test programs [1] in the past already, and 
they indeed show some odd behaviour under certain conditions (including the 
alarm not triggering when it should) - but so far these tests behave exactly 
the same on "old" and "new" CIAs.

so i am asking you guys a) if you have heard about the above mentioned 
problem, and can tell details on the condition needed to reproduce it (so i 
can make it test program - or perhaps even you can provide one) and b) if this 
is really a problem that should only show on one of the two CIA variants 
(probably the older one). also if you know and references/documents that 
mention this problem, please tell!



Wer im Glashaus sitzt hat immer frische Gurken.

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Received on 2017-03-08 18:00:07

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