Re: CIA TOD Alarm Bug

From: Segher Boessenkool <>
Date: Thu, 9 Mar 2017 16:12:46 -0600
Message-ID: <>
On Wed, Mar 08, 2017 at 06:45:01PM +0100, wrote:
> 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:

There also is 8520.  I haven't yet seen good pictures of the 8521, but
the pictures that are there show that 8520 and 8521 are nothing alike
(and neither is like the 6526, either -- in the physical layout anyway).

> wikipedia:
> "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!

I'll have a look at the die pics again, this weekend I hope.


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Received on 2017-03-09 23:00:02

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