chip 8 tetris

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http://12me21.github.io/sbhighlight3/#../DIFIE.txt

This is the code I wrote for chip-8 tetris. It's designed to be read by my
interpreter, written in SmileBASIC.

A few things to explain about SmileBASIC:

' starts a comment
&H and &B are the hexadecimal and binary prefixes
DATA stores values that can be read with RESTORE/READ/COPY
Labels (for GOTO, DATA, etc.) are prefixed with @
In an expression, @LABEL is a string, equvilant to "@LABEL"


My addresser:

Normal (positive) data values are read directly as instructions
Negative numbers have special meanings:
-1                   : end of program
-2,label             : define label
-3,instruction,label : instruction with address.
-4,instruction,label : instruction with address, offset by 1 byte.

Comments:

Most of the lines are commented in my own variant of chip-8 assembly. I
probably forgot to update some of the comments so they might not match
the actual instruction.


The Program:

While making a chip-8 interpreter, I was reading the code of some existing
games, and tetris had a lot of flaws (It relied on the interpreter running at a
specific speed rather than using the delay timer for everything, and was
generally very slow).

Around this time I found out about the chip-8 contest (octojam 4), so it was
the perfect time to make something in chip-8.

I designed tetris to be as small as possible (without sacrificing
gameplay, of course). It would probably run a bit faster if I put some
subroutines inline, but the main speed limitation is the sprite drawing, not
the code, anyway. The entire game is around 730 bytes, and 180 of those are data while the rest is code.

One of the lesser-known and VERY important restrictions on the original Cosmac
VIP interpreter is that you can only draw 1 sprite per frame. This means that
limiting sprite draws is the biggest thing you can do to improve speed. For
example, rather than doing 10 or 20 sprite draws to check if a row is filled, you only need to do either 2 or 4 by checking 5 pixels at once (Tetris is 10
blocks wide, so you have to use 2 sprites).

Originally I was going to write the entire thing by hand, but around halfway
through I got really tired of having to update all the addresses every time I
added something (Some of the comments still have my addresses in them like
'48A LD v1,1B). I designed a system to automatically calculate addresses which
made everything a lot easier. By the time I was finished, I had memorized many
of the instructions, though by now I've probably forgotten most of them.