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| From: Bionic (all posts)
Date: 2000-08-28
Subject: Re: TEST bit
|
> Hi there...
>
> > Anybody following the thread on the cbm-hackers list, about the TEST bit
> > in the C64? They basically found a way, to affect the raster counter by
> > setting the TEST bit, so they can now make effects, which were doable on
> > TED only... I'm wondering, if the TEST bit on the TED could have similar
> > effects...
>
> On #c-64 some guyz were talking about a real interlace with VIC-II. It had to do with the TESTBIT aswell. I fooled around with it some years ago...
while playing samples and stuff like that... I haven´t really understood its effects since they differed from time to time... APOS did some experimenting with the TEST BIT aswell. He told me something like he "cleared" RAM by using the testbit...
The test bit will freeze the TED and therefore the DRAM is not refreshed anymore and will be set into the state it had at the beginning.
(usually 00 FF) Depending on the ram type it can take quite a while -
even up to several minutes. However CPU accesses do also refresh the ram. Therefore you can refresh it by accessing a full 256 byte page of ram each 4ms. I once used this for a high speed digizizing program. It reached 130000 samples/sec.
Doing real interlacing should indeed be possible with the TED.
The difference between even and odd PAL frames is the vsync which differs by half a line. Since $ff1e is also writable (isnt it ?)
it might be possible to get the TED to output just a half of a line. Careful timing is required.
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