Friday, 29 November 2013

Insert Coin

Neo Kong is coming along nicely, and now it displays the 1st attract mode screen - the 'INSERT COIN' screen - in its entirety. It doesn't seem like much, but there's a decent amount of infrastructure code behind the scenes; as well as the main program flow I described last post, there's also the routines for displaying general messages, credits and scores, for example.

Donkey Kong (left) vs Neo Kong (right)

One of the more interesting areas I reversed-engineered last night was the mechanism for queuing functions to be executed in the foreground loop. Up to 32 functions can be scheduled by storing the function's jump-table offset, together with a 1-byte parameter, in a queue. Typical functions that may be queued include messages and bonus & score updates. Perhaps not surprisingly, the code for this mechanism has deviated the most from the original Z80 code.

With a reasonable amount of the infrastructure now in place, I'm hoping that there's more eye-candy to show (i.e. attract mode screens) for relatively little more effort.

And here I may have found a 'bug' in MAME. "L=00" isn't supposed to be updated in attract mode; but the code to initialise the attract-mode flag after power-on is executed from the VBLANK interrupt. In MAME the foreground loop gets executed at least once before the 1st VBLANK interrupt, which results in the level being displayed. Not so on the Neo Geo (I've fudged it for comparison). I'm wondering what the real hardware does!?!

EDIT: Seems I was a little too quick to blame MAME. Turns out that the update of "L=00" is a side-effect of the routine being called before the variables are initialised; the score is compared against the bonus life setting ('00,000') and an extra life awarded, causing the level to be displayed. I hadn't finished coding the routine that is responsible for the erroneous update.

I'll skip the game-play demo - which appears after the 'INSERT COIN' screen - and start on the title screen next (sans sprites).

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