[stella] Visual questions

Subject: [stella] Visual questions
From: Glenn Saunders <cybpunks@xxxxxxxxxxxxx>
Date: Tue, 25 Jul 2000 04:00:17 -0700
Two visual-related questions.

When talking about a byte, the numbering is like this, right?

128 64 32 16 8 4 2 1

What about the bit numbering? Does it go from left to right or right to left?

The memory map diagrams and everything else indicate bit0, bit1, bit2, bit3, etc... The actual location of the bits address numbers is important in visually representing the memory map, since many registers have special values in individual bits or pairs of bits.

So when drawing player sprites for use in memory by using graph paper, what you see is what you get on the screen if you feed it numbers like the above, ala:

X X X X X X X X = 255
X 0 0 0 0 0 0 0 = 128
X 0 0 0 0 0 X 0 = 130


Also, how I'm going to explain VDEL is that it has the net effect of shifting sprites down a single scanline. So if you want to move a sprite up a single scanline, you move it up a full scanline (since it's a 2 line kernel) and enable VDEL, the next frame you don't enable VDEL so it has the effect of moving it another single scanline, and so on.

If you want to move a sprite down a single scanline, just position the sprite at the same place and enable VDEL. If you want to move it down two scanlines, then move it to the next line in your two line kernel and don't use VDEL.

Also, I've noticed that a lot of source code stores sprite data upside down. Any reason why? Do they scan memory by decrementing a counter or something?

Glenn Saunders - Producer - Cyberpunks Entertainment
Personal homepage: http://www.geocities.com/Hollywood/1698
Cyberpunks Entertainment: http://cyberpunks.uni.cc

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