Saturday, January 30, 2016

RC2016/01 Wrap-Up

Here we are at the end of RC2016/01, and what have we got? Well in my case I have a port of Xmas Rush for the TRS-80 MC-10 -- very exciting stuff! At the end of my last post on the topic, there was still some timer stuff to handle and some "odds and ends". So, let's see how that went...

Pause To Experiment

Early on I had thought that precision timing might not really be necessary, that perhaps the video glitches would be spread so thin that it would hardly be noticeable. Later, someone else made a similar suggestion. So I conducted an experiment that changed the game's time base to just a bit more than the time required for a single frame of video. This did have the predicted effect of spreading the video glitches around so as to make them less objectionable, but they were still both noticeable and constantly recurring. While I did not pursue any algorithmic changes to the drawing routines that might have made this even less objectionable, the experiment cemented my belief that stable timing synchronized with the video hardware remained a worthwhile pursuit.

Help From Afar!

I implemented the timing synchronization by enlisting help from the user to position a marker off-screen. This marker represented the time the video hardware was not actively drawing the screen, and moving it off-screen equated to synchronizing the program to do its animation while the hardware was not actively drawing. This solution is quite effective, and I don't think it is objectionable for the user to do this once when starting the game.

The timing I am seeing here at home is the roughly 60Hz timing of NTSC video. Of course, in much of the world the video standards are timed for 50 Hz vertical refresh rates. By adding a user choice between 50Hz and 60Hz and a corresponding variable used for timer setting, I was able to accommodate that situation. However, I don't have an MC-10 that uses PAL video or one of the French MC-10 clones, the Matra Alice. Luckily I was able to reach out for testing help on the Yahoo MC-10 group. There I found confirmation that my 50Hz timer synchronization was working too.

One Last Bug...

Along with that testing came a report that in some cases the snowmen would overlap and disappear! Worse, even in this vanished state they could still capture the player, causing him to lose...what was going on?

The moving objects in Xmas Rush are drawn using an XOR operation against the existing screen data.  The primary reason for this is to highlight the case when a snowman captures the player, but a side effect is that when two snowmen exactly overlap then they cancel each other out on the screen and seemingly disappear. In order to counteract that I have a series of tests in the snowmen movement routines that were intended to prevent an exact overlap. However, those test were clearly not working...

The original 6809 code for the snowman position tests used a CMPD instruction to do a 16-bit comparison against the D register (i.e. the A register and B register together). The 6803 lacks such an instruction, so in the conversion process I compared 8-bits against A and the other 8-bits against B. This changed the flow just enough that I confused myself, and I ended-up checking each snowman's position against the position of only one other snowman, allowing overlaps to occur with the other two! Once I found that problem it was easy to correct, and now snowmen should generally be unable to eclipse one another.

That's A Wrap

So anyway, that's about it for Xmas Rush on the MC-10. I'll get the MC-10 binaries added to the Xmas Rush web page and then I'll call it a month. I am really happy to have finally taken the time to learn more about the MC-10 and to put it to a decent use, and I hope you have enjoyed monitoring my progress along the way. I will probably have Xmas Rush on display at the 25th annual "Last" Chicago CoCoFEST! this April, if you want to stop and give it a go.

Until next time, I hope you will find your own excuse to get out your retro computing gear and make it do something fun, useful, or otherwise enjoyable. And as always, I hope you will stay tuned!


  1. Those snowmen do have unique personalities. If I can just figure them out more clearly perhaps I can make it past 3! Is there a 4th?

    1. There are 4 total! They aren't too smart, but they can be hard to get around... ;-)