Most recently I've been adding the code for thrust and as a result once you start a game, your ship appears and you can move around.
One nuance of porting is the distinction between zero-page accesses and memory accesses. Looking at the original 6502 source listing, there's no indication which is which. On the 6809, all labels for direct page variables are .EQU statements, and the operand prefix is '*'. If you forget the asterisk, the code assembles but doesn't work as planned. Somewhat fortuitously though, the way I have the memory map configured, you'll get stray pixels on the top few lines of the video, and that's easily trapped in the MAME debugger since - atm - Asteroids never renders there (see below).
The other issue I touched on last post is the vertical resolution. After some experimentation in the C port, which is rendering 'vectors' in 1024x1024 coordinate space, I've confirmed that Asteroids uses approximately 788 "lines" of the display space, effectively leaving the top and bottom 118 lines (or 10% each) blank. When you reduce the resolution to 192 pixels, that 20% blank space is quite significant.
Unfortunately 192 doesn't quite divide into 788 nicely, and 192*4 (768) crops the score and copyright messages. The Coco3, however, conveniently has a 200-line mode, which would greatly simplify the scaling (right-shift by 2) and allow use of most of the display. The only issue is that the graphics were, IIUC, originally designed by Norbet for a 192-line display. I'll have to experiment to see if and how they could be adapted for a higher resolution.
But back to porting code for now...
UPDATE: Tonight I ported the code that handles all the collisions. I need to update my disassembly comments in a few areas here, as the routines actually handle all collisions between all objects, whilst my comments suggest it's only the collisions between shots and other objects.
There's a decent chunk of code involved, so not surprisingly it doesn't yet work. Worse yet, I've actually broken what was working before, in the process of 'fixing' a few bugs that I discovered porting the new code. It's too easy to forget that the 6502 X,Y registers are 8-bit vs 16-bit on the 6809...
Looking purely at object (binary) code size, the porting is now roughly 50% complete.