Re: VIC Article

From: Hatch (
Date: 2005-02-10 05:51:17

----- Original Message ----- 
From: "Jim Yopat" <>
To: <>
Sent: Thursday, February 10, 2005 5:48 AM
Subject: VIC Article

> 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?

These numbers are for the internal 9 bit vertical raster counter in the VIC,
the reason they don't make much sense is that the counter doesn't start at
line 1.  The counter is offset so that the number in the counter is not the
same as the physical raster line as far as the TV is concerned.  Vertical
counter with 0 doesn't = raster line 1 (Not as far as the  TV is concerned).
There also a 9 bit Horizontal counter which is also offset.  These counters
are largely to do with sync signals and enabling the display, as far as
coding goes it shouldn't worry you.  I think (from my poor memory) the
person  who wrote the VIC article also wrote something like- when designing
the PAL version the engineers added 25 lines from above and below the
picture information, or something like that, which would explain why 300 and
12 (25 raster lines difference above the picture). The NTSC C64 only has
around 260 raster lines and the PAL C64 has 312.

I hope that cleared up that question and not made thing more complicated.

> Thank you
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