Saturday, January 31, 2015

Feature Sweep

The end of RetroChallenge 2015/01 is nigh! But is Fahrfall running on the Apple II? Well, sure it is (at least for the IIe and above)! The last couple of days have seen a flurry of feature development, but I think the result is looking fairly good...

Urge To Merge

My last post talked about adding color patterns to the platforms and using another LFSR to change the platform colors. While writing that code, I discovered that I was close to the edge of a performance barrier for drawing during the vertical blanking interval. I thought that was resolved by some code optimization, but the issue returned when I attempted to merge the changes into the main game code. Fortunately I was able to find a few more minor tweaks that allowed me to merge the color changes and to still draw everything within the allotted time constraints.

Making Introductions

Games generally have a starting and an ending, and it is appropriate to mark those points prominently. Like any game, Fahrfall needs to introduce itself to the player and it needs to tell the player how well they performed. The Apple II offers a video mode that allows for graphics on most of the screen while providing four lines of text at the bottom of the screen. Fahrfall now uses this mode to display some colorful graphics while communicating to the player through text at the bottom of the screen both before and after a game is played.

Keeping Score

With the "intro" and "game over" screens in place, it was finally time to add scoring. The score itself increments as a timer while the game is in progress. Since the score is displayed as a decimal number, this seems like a perfect application for the BCD support built into the 6502! This allows for maintaining the score just like any other binary number, while making conversion of the score into screen data almost painless.

I have been using a joystick while developing the Fahrfall code, and using a joystick is the preferred method of play. Not everyone will have a joystick, so supporting the keyboard is desirable as an alternative. Unfortunately, the keyboard repeat rate is too slow to support use of the arrow keys for controlling Fahrve. (Maybe I just don't know how to get around relying on the keyboard to report repeated keys?) However, unlike most of the keys on the Apple IIe keyboard the "Open Apple" and "Closed Apple" keys are accessed directly through "soft switches". This lets me poll those keys directly at any time, so I am using those keys for "left" and "right" when keyboard access is selected. (FWIW, the keyboard access selection happens when the <spacebar> key is used to start the game.)

The game ending "sizzle" is like a Fahrfall trademark. Some people hate it, but it always gives me a chuckle to watch a new player nearly jump out of their skin the first time they end a game! The sound itself is just a short burst of noise, implemented by using the LFSR to determine when the speaker is toggled. This addition is a nice "final touch" to make Fahrfall seem complete.

Well, I think that is "pencils down" for my RetroChallenge 2015/01 entry. I now know a lot more about programming the Apple II and the 6502 processor, and I have an Apple II port of Fahrfall to show for my efforts. I reckon this counts as a success! I'll probably be back later today or tomorrow with a wrap-up post, so I hope you will stay tuned... :-)

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