How to change tile buffer to 60x60?

[slobu]

So, I'm trying to make an experimental scrolling engine in BasiEgaXorz. I've set it up so that it only needs to load map data and blast tiles to the buffer every 20x20 tiles of movement. Trouble is the extra tiles left over when using the default buffer size.

The Charles MacDonald VDP guide seems to suggest you can change the buffer size somehow. Is this possible? Is it simple enough that I can find or someone can suggest example code to do it? BasiEgaXorz allows inline assembly so any clues would be greatly appreciated!

Thanks in advance

 

[Chilly Willy]

The pattern name table (the correct name) can be only 32, 64, or 128 wide, and CANNOT be 32 unless in 256 wide mode. It can also only be 32, 64, or 128 tall, and CANNOT be 128 tall if it's also 128 wide.

256x224/256x240(PAL) : 32x32, 32x64, 32x128

256x224/256x240(PAL) or

320x224/320x240(PAL) : 64x32, 64x64, 64x128, 128x32, 128x64

Those are the only sizes the scroll layers can be. They are also the same size - layers A and B cannot be set separately.

I have no idea how BEX sets this... you'd best look at the BEX examples or ask over at Devster's. To set it via assembly, set VDP register 16.

b5-4 = vertical size, 00 = 32 tiles, 01 = 64, 10 = reserved, 11 = 128

b1-0 = horizontal size, 00 = 32 tiles, 01 = 64, 10 = reserved, 11 = 128

The other bits are reserved and should be set to 0.

Remember that the tiles DISPLAYED by the layers depends on the resolution.

256x224 = 32x28 tiles

256x240 = 32x30 tiles

320x224 = 40x28 tiles

320x240 = 40x30 tiles

Where that block of tiles comes from in the pattern name table is set by the vertical and horizontal scroll values. When the scroll values are 0, the displayed tiles start at 0,0. If you have a 40x28 display in a 64x32 pattern name table, you can scroll up to 24 tiles horizontally, and up to 4 tiles vertically. When you reach a border, you might move the pattern names back over top of the ones scrolled off the screen, then write new names into the empty positions while setting the scroll values back to 0,0. There are plenty of examples of scrolling in the BEX forum over at Devster's.

Note that even though the pattern name table is 64x64 (for example), you don't HAVE to use all 64 tiles in each direction. You could use 60 is each direction, but remember that the stride for the table is 64 words. Once you scrolled up to 60, then reset back to 0 and draw in the new data. So you can use any value larger than the display size (say 40x28), and no larger than the pattern name table size (say 64x64).

 

[slobu]

@Chilly Willy

Thanks! That gives me alot to go on. I'm just aping the terminology I've heard so far so it's good to know it's a "pattern name table" instead of a tile buffer.

 

[Chilly Willy]

Well, as long as people know what you mean, it really doesn't matter. SEGA has certain names they used, so if you also use them, people should know what you mean. SEGA called tiles "patterns", and the table of tile indexes is the "pattern name table" where "name" means index+flags. But I think most folks will understand if you use tiles and tile buffer.