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Every 8 lines scrolled horizontally
Posted: Mon Mar 24, 2014 1:50 pm
by M-374 LX
Why does the VDP scroll every 8 lines horizontally when I try to scroll only one?
Posted: Mon Mar 24, 2014 3:55 pm
by r57shell
Wrong scroll settings.
There are three different types of scrolling:
1) Whole screen
2) Cells (by 8 lines at once)
3) Lines
Dig into VDP settings (registers).
Posted: Mon Mar 24, 2014 4:19 pm
by M-374 LX
I have found the problem: register 0x0B was set to 1. Changing it to 3 solves the problem.
Nevertheless, according to Charles MacDonald's VDP document, it should work if the register is set to 1. So, this seems to be a mistake in that document.
Posted: Mon Mar 24, 2014 8:25 pm
by Charles MacDonald
M-374 LX wrote:I have found the problem: register 0x0B was set to 1. Changing it to 3 solves the problem.
Nevertheless, according to Charles MacDonald's VDP document, it should if the register is set to 1. So, this seems to be a mistake in that document.
There's two line scrolling modes, 1 and 3. Mode 1 re-uses the first 8 words of the scroll table and applies them to all display lines, mode 3 maps the scroll table words to display lines like normal.
Section 13 has a more in-depth description of how the scroll modes work, but you're right, I should have clarified how they differ in the register 0x0B description.