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| Defining screen mode using hardware registers http://www.retrosoftware.co.uk/forum/viewtopic.php?f=73&t=71 |
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| Author: | DaveF [ Thu Apr 03, 2008 4:49 pm ] |
| Post subject: | Defining screen mode using hardware registers |
Hello I'm trying to define my screen mode by writing into the CRTC registers and the video ULA. The problem I'm having is that it appears to work for a few frames, then it would appear to have the teletext bit set again in the video ULA. If I re-write my value into the video ULA each frame it sort of works but occasionally there'll be a flash of teletext(!). Does anyone know what causes the BBC to do this? I don't really want to write into that register every frame. Cheers. |
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| Author: | FrancisL [ Thu Apr 03, 2008 4:51 pm ] |
| Post subject: | Re: Defining screen mode using hardware registers |
Do you have some sample code to illustrate the problem? Also, are you trying this on a real BBC or under emulation? Kind regards, Francis. |
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| Author: | DaveF [ Thu Apr 03, 2008 4:53 pm ] |
| Post subject: | Re: Defining screen mode using hardware registers |
It's under (BeebEm) emulation. I'm pushed a bit today at work but I'll try and get some sample code knocked up tomorrow. |
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| Author: | TomW [ Thu Apr 03, 2008 8:29 pm ] |
| Post subject: | Re: Defining screen mode using hardware registers |
The OS rewrites the ULA control every few frames for flashing colours. Either set a graphics mode before rewriting all the registers (rather than doing it from mode 7) and put up with colours 8-15 flashing, disable all interrupts, or stop the vsync interrupt from getting to the OS (using IRQ1V). |
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| Author: | RichTW [ Thu Apr 03, 2008 8:44 pm ] |
| Post subject: | Re: Defining screen mode using hardware registers |
Or use *FX 154,n to write to the Video ULA, which also updates the OS's copy that is rewritten every flash period. |
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| Author: | DaveF [ Fri Apr 04, 2008 8:58 am ] |
| Post subject: | Re: Defining screen mode using hardware registers |
Aha! Cheers wizards! |
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