Re: Trying to make complete sense of the video scanner, HBL, and the floating bus

[Linards Ticmanis <ticmanis@gmx.de>]

On 02/11/2012 05:07 PM, BLuRry wrote:
> I was sure that I had implemented everything right, but for some strange reason things aren't right yet.  If you all would indulge me, I'm going to brain dump the various things I've gathered on the subject of floating bus.  Sather does an awesome job at talking it up from the hardware side, but after re-reading his notes a couple of times I still can't quite make sense of what exactly happens during HBL, except when he talks about what happens on the left-most fringe, where the previous byte in memory affects the left edge (e.g. the upper-right pixel at $2027 affects the middle-left pixel 64 rows down at $2028).

Just a small note here, the artifact at the left edge of the screen does
not happen in IIe and higher machines, only in the II/II+.

> I get kind of confused on these points, because when I re-read Sather, he talks about the HBL and VBL accounting for the left, right, top and bottom margins.  But effectively, there are only left and bottom margins so to speak, because Y is incremented at the very end of the visible scanline, and VBL stops the moment it begins rendering row 0.  Am I reading too much into this?

Well, the scanner resetting to "column 0" and the electron beam going to
the next line don't don't happen at the same time, and the same is true
for VBL, the scanner reloading the start value and the beam going back
to the top don't coincide either. So of course the picture that you see
on-screen does have left, right, top and bottom margins... even if the
"logical picture" in memory might not.

Not sure whether you got the rest right, I'd have to read it again too.

There's one thing that you have to watch out for: there's a delay
between the time a byte of graphics data is read from memory and the
time it's displayed on screen. You really have to look hard at the
schematics and scratch your head a bit to understand it.

By the way there's something odd about your line formatting, you use
basically one line per paragraph, which causes quotation to not work as
it should.

Regards,
-- 
Linards Ticmanis