Tuesday, 20 June 2017

Insert Coin. Press Start. Player 1.

Ordinarily I wouldn't have another update yet, but my 2 yr old came into our bed this morning at 4am and shortly before 5:30am, not having had a minute of sleep since then, I gave up and went out do some more Asteroids.

The plan was to use the MAME debugger to ascertain which DVG ROM subroutines were yet to be implemented. As I expected, the first to reveal itself was the copyright message at the bottom of the screen. The routine itself draws some discrete vectors (presumably for the © symbol) before calling the character routines for the remainder of the message.

I had two choices here; simply do the same and explicitly call my own character routines in sequence for the entire message, or implement some mechanism to allow me to simply point to the DVG ROM routine and recursively execute DVG instructions. I figured the latter wasn't worth the effort - and would be slower - so I implemented the former.

Someone had also 'complained' about the flickering graphics after I posted my last video. Of course this being purely a development aid I wasn't concerned, but knowing the Apple II had two text pages, curiosity got the better of me. And I'm not claiming to be breaking any new ground here, but I did manage to implement double buffering without any conditional logic involved in the process at all.

There's not a lot more to see in attract mode alone, so I decided it was time to properly initalise some dipswitches and hook up some crude control panel inputs. I settled on two hook routines, apple_reset and apple_start, that get called at the end of the original reset and start routines respectively.

In apple_reset, the hardware I/O locations - such as dipswitches - can be initialised. Since they map to normal Apple II RAM locations, all that is required is to write the appropriate value to the respective address. Thus far I set the coinage and the number of starting lives.

In apple_start, the Apple-specific initialisation code is run. Here I'm currently setting up the page flipping logic, and clearing both text pages.

As I've mentioned in the past, the NMI routine in the arcade code handles the coin switch inputs. Other inputs are read in the main game loop, once per frame. For the moment though, I simply added a few lines to read the Apple II keyboard at the end of my frame rendering routine. Pressing <5> will insert a coin by simply incrementing a zero page shadow value, and pressing <1> will start a game by setting a bit in the hardware I/O location (mapped to Apple II RAM) - for 1 frame. That's enough to get a game started and running.

I then added the display of the remaining ships, mainly because it was trivial. Unfortunately with only 16 lines on the screen, they overwrite the score, but the point is that it's more evidence that things are running as expected. The next obvious object to implement was the player ship...

video


...and here is where things start to get more complicated. The DVG ROM indeed has a table of 17 subroutines for drawing the ship (and optionally the thrust), not unlike other objects. However, these 17 ships only cover 90 degrees of rotation. As a result, the 6502 can't simply add a JSR to the player ship routines into the display list.

Instead, the 6502 copies the component instructions (vectors) from the above-mentioned DVG ROM routines into the display list, adjusting each on-the-fly for the current direction. So when the Apple II rendering code comes to the player ship, it's simply a list of CUR and VEC instructions - nothing decidedly identifiable as the player ship object!

So how do we solve this? In a rare coincidence, the solution is actually an optimisation as far as an Apple port goes - and there are also a few options. The most straightforward is to replace the 6502 routine for the player ship entirely, bypassing the display list and directly rendering the appropriate bitmap on the Apple display. One step removed from that is to 'tokenise' the display list entry; rather than add the component vectors, simply add a 'token' command to display the player ship that the Apple rendering engine can parse. Both have pro's and con's.

At this point though, I think I've taken the text-based proof-of-concept engine as far as I need to. It's time to make the switch to the 2.8MHz IIGS, consider writing the rendering engine in native 65816, start working in graphics mode, and decide how best to solve the latest issue.

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