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CRTC Screen sizing

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IndyUK:
Hi folks

I've been experimenting with manipulating the CRTC registers using BASIC to try and understand how it works. So I tried the following BASIC instruction


--- Code: ---OUT &BC00, 6:OUT &BD00, 20
--- End code ---

Worked fine. However, what I noticed was although the screen display shrunk (20 character lines), the physical screen didn't. How do I set the screen so that both the display and physical match?

Thanks

roudoudou:

there is 272 lines visible => so 34 lines of char
96 bytes width instead of 80 as default

you may have a borderless screen (even with Basic running) but as the visible screen is (approx) 24K, the memory will loop
also you need to move the beginning of the screen so there is more registers to set

andycadley:
I'm not sure I follow what you mean when you say the physical size didn't change.


There are always the same number of actual scan lines (well more or less, you can fiddle it slightly and get away with a mistimed frame flyback but not by much). The result of this is that the size of a pixel row isn't ever going to change. If you reduce the height of the screen in characters, you just get more border at the bottom of the screen - it doesn't scale up the characters to fit the space previously, if that's what you were hoping for.

IndyUK:
My apologies. I probably did a bad job in trying to explain things.


If you see the attached screenshot. There you will see the shorter height screen (20 chars lines worth). That part is fine, howver, the issue is that when you take the cursor past the bottom of the (new) bottom height (pressing the down cursor), it doesn't scroll. What I found is that the physical chars lines are still 25 i.e. I still have to move 5 lines down after the cursor disappears for the bottom of the screen to scroll up. In other words, the view has (virtually) changed but the physical lines are still 25.

I hope that makes things a bit more clearer.

Thanks

roudoudou:
you may define a WINDOWin mode 1
WINDOW 1,40,1,20
and the 5 extra-hidden-lines will be ignored

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