Author Topic: CRTC R12 R13 : overscan in loop  (Read 787 times)

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Offline freemac

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CRTC R12 R13 : overscan in loop
« on: 17:00, 04 December 16 »
Hi,

Since I unlocked all video layers memory (64KB) in FPGAmstrad (CoreAmstrad MiST-board), I have some problems of overoverscan... with my -normally- CRTC1.

In wiki http://www.cpcwiki.eu/index.php/CRTC , REG12/13 has a "buffer video" (and "video page") option setting 16KB or else 32KB.

Is this "buffer video" a bank limitation or else a counter maximum ?  in other terms does it takes into account the start address offset ? Can it cross three pages and then loop at 32KB step?

During SimCity scroll, I normally have :
[attachimg=1]
[attachimg=2]
But in my, it does that :
[attachimg=3]
« Last Edit: 19:48, 04 December 16 by freemac »

Offline andycadley

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Re: CRTC R12 R13 : overscan in loop
« Reply #1 on: 20:17, 04 December 16 »

You're much better off forgetting all about things like "16/32K mode" because they really a simplistic explanation intended for developers who aren't necessarily hammering the hardware that hard and just want to know the "magic incantation" necessary for getting the output they're looking for.


The CRTC is, at it's heart, just a bunch of counters and all the screen addressing (including how to get 32K displays) just falls out of doing the right manipulations on the output bits.


There was a pretty good thread on it: http://www.cpcwiki.eu/forum/emulators/crtc-question-12407/ which is well worth reading to help clear up how and why it works.