Re: 8520/21

From: Pasi 'Albert' Ojala <*a1bert_at_iki.fi*> Date: Thu, 05 Mar 2015 22:51:11 +0200 Message-ID: <54F8C1BF.1070105@iki.fi> · a1bert_at_iki.fi

On 03/05/15 20:25, Gerrit Heitsch wrote:
>
> So they had that 6526 in NMOS and when the Amiga design came around 
> ('85), they wanted a CIA for it as well but decided against the 
> TOD/RTC from the original design, probably wanting the plain counter 
> instead
As far as I remember, the event counter of the 8520 is by default 
required by the Amiga, it is used to count scanlines, and used by the 
graphics subsystem to time screen refreshes so that no 'tearing' or 
other update effects become visible.

You could replace the 8520 with 6526, but you needed a little patch code 
to disable the feature, otherwise the graphics system would lock up 
waiting forever for any line that the TOD registers are not able to provide.

-Pasi

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