VIC Article

From: Jim Yopat (
Date: 2005-02-09 20:48:38

I've been reading an article on the C64 VIC chip that's located here:
But I'm having some trouble understanding some of it so I need some help from one of you C64 gurus
1) In section 3.4, there is a line that reads: "To keep the graphics aligned with the window, X/YSCROLL have to be 0 and 3 for 25 lines/40 columns and both 7 for 24 lines/38 columns."
Can someone elaborate on this, I don't understand why X/YSCROLL has to be 0 and 3 or 7.
2) Just below that line is a table with some values in it. Taking the PAL version of the VIC chip as an example, I understand were the number of lines and the number of cycles came from (312 and 63) but I'm having trouble understanding how the author came up with 284 (visible lines) and 403 (Visible pixels/lines). Can someone explain were these numbers came from? 
3) Then the table right below that are some more values. I have know idea were these values came from. Like why is the NTSC First vblank of 13 drastically different from the PAL version of 300? Can someone explain were these numbers came from?
Thank you

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