VerticalScrollingMode7

From Retrosoftware

Revision as of 21:09, 24 June 2011

I read on one of the posts today that someone was looking for a mode 7 scrolling game and that started me thinking - how hard would a vertically scrolling mode 7 routine be and would anyone want it anyway ;-)

This code should show four things:

1. How to set the gfx chip up for mode 7

2. How to set up a v-sync interrupt for flicker free scrolling

3. How to set the mode 7 screen up for pixel graphics

4. How to scroll the mode 7 screen vertically

So, what does it show?

1. I didn't write, just run with the demo in a non-scrolled mode 7 screen.

2. Doesn't seem to work correctly, so .vsync_wait begins with an RTS - please feel free to let me know where I went wrong!

3 Works, by setting the first character of each row to a gfx excape character - see the BBC User Guide for details

4 Works, but I haven't optimised it.

The code is reasonably fast, but you could save a few cycles by copy-n-pasting the decode bit (lda (&80),y..sta (&70),y) three times to remove two thirds of the bpl next, saving 26 bpl * 3 cycles * 40 lines which if anyone was going to use the code, I would recommend. You need to unroll the bpl next loop in threes to make sure that the last one is still the 39th, so the next unroll would be 13 and that would be too big IMO.

The data is stored in a similar way to the original mode 7 gfx characters, but uses the full 8 bits for 8 pixels
[__1][__2]
[__4][__8]
[_16][_32]
[_64][128]
The row consists of 39 bytes - giving a 78x4 pixel block.

The four decode sections are for the four alignments of a 78x4 block with the 78x3 screen row. top0 uses the first (lowest) 6 bits of each byte in the bits_to_gfx look-up, top1 - the last and the other two or two data lines to get the index for the look-up.

With some small changes, you could leave gaps at the top and bottom for scores and gauges (and not scribble on the first few bytes of ROM). but there is still quite a bit of work to do and so, other than a straight racer, you would need to run at 25fps.

If you wanted to do a game with horizontal scrolling, I would suggest having two copies of the data offset by a pixel and then just copy alternately.

Please feel free to comment on the forums