After Burner II (Genesis)

After Burner II
Developer: Dempa Publishers: Dempa (JP), Sega (US/EU/AU), Tec Toy (BR) Platform: Genesis Released in JP: March 23, 1990 Released in US: March 22, 1990 Released in EU: 1990 Released in AU: 1990 Released in BR: 1991 This game has unused text. This game has a hidden level select.

While this port of After Burner II was decent for its time (being released early in the console's lifespan and that Dempa was the one doing it), the Genesis could surely do better. Also the first Genesis/Mega Drive game with support for custom 3rd party controllers.

To do: XE-1AP versions of cheat codes.

Max Out Missiles/Secret Message

At the start of various levels, the game will stop the action and send a ship which will restock your missiles; it only gives you 50. If you use a certain button combination before your ship makes contact with that ship (that is, before the RELOAD WEAPONS message appears), you will see a secret message and the ship will give you 100 missiles instead, maxing out your stock. Taken together, all the secret messages tell some story completely unrelated to the game. It seems to be describing drawing mangaː Tone and G-pens are traditional manga drawing tools, while "simekiri" refers to a deadline and "bottsu" is rejection of a submitted piece.

Level	Button Code	Message
3	Hold Left, then press B.	PRINCESS CAPTURED DARK FORCE. RESCUE PRINCESS MIA,TOM!!
5	Hold Right, then press B.	THE NAME OF DARK FORCE IS = SIMEKIRI =
9	Just press B.	TOM FIGHT THE FORCE AT G-PEN AND TONE.
11	Hold Right, then press B.	DARK MASTER USES SPELL,"BOTTSU!!". TOM FEELS EVIL POWER.
13	Hold Left, then press B.	OH! IT IS SO DANGEROUS!
16	Hold Left, then press B.	"I NEVER GIVE UP!" TOM USES ITEM. "YUNKKELUU!!"
19	Just press B.	TOM DRAWS AND DRAWS. PRINCESS MIA ONLY PRAYS...
21	Hold Right, then press B.	HURRY UP !! THE MORNING COMING SOON.

Unused Related Text

Levels 1 and 23 (the cutscene-only level used for the ending) also have strings associated with this cheat, but the cheat does not work for those levels. Had it worked, you would just press B to activate it.

Level	String	Address in Japanese ROM	Address in US/Europe ROM
1	== ............... ==	$4DD2	$4EC4
23	+100 MISSILES !!	$4F86	$5078

For the addresses, the first byte indicates the button combination, the next three bytes position (and possibly color? TODO) the text, and after that is the null-terminated string. The three bytes+string would be repeated if there were more text, however since these are the only strings printed here, the byte $FF ends printing. Most, if not all, text in the game takes the three bytes-string-...-$FF form.

(Source: andlabs (entire section))

Demo recording?

To do: These are for the Japanese ROM; the code is still in the US/Europe ROM but I need to rewrite this paragraph to cover it.

There is unused code at $6ACE that runs a special version of the main game loop that logs all controller input to the (invalid for this game) address range [$80000, $80400] for some time, then freezes with the message "READY TRACE!!" (at $6C3C in the above-mentioned string format). Pausing is disabled in this special game loop. The freezing is presumably caused by the opcode $FFFF (triggering a line 1111 emulator handler) at $6BBC; the game does jump into the (shared by most errors) handler. My (andlabs) best guess is that the debug hardware allowed grabbing the data from the address range above and placing it elsewhere for the demos.

You can jump into this game loop and play with patch codes 006350:4EF8 and 006352:6ACE (which replaces the actual game loop with this new one), however I do not know how to extract the data (it could just be changing the two addresses to be in SRAM for emulators that automatically create SRAM) or how to play your demos back (as I have not yet located the demo data in ROM).

(Source: andlabs)

Miscellaneous

This needs some investigation. Discuss ideas and findings on the talk page.

The trap #8 vector (at $A0) is set to point to the out of bounds ROM address $F0CE0 in the US/Europe ROM rather than the standard error handler in the Japanese ROM. The reason for this, as well as what this would point to, is unknown.

(Source: andlabs)

Unusual Sound Playback Method

After Burner II uses an atypical method for playing back audio samples by using the Genesis' PSG chip. To explain why this is so strange, we need to explain how most Genesis games handle digital samples.

The YM2612 chip inside the Genesis has a PCM mode, which replaces the sixth FM channel with a programmable DAC. Just like the previous Sega System 16 and its YM2151, the YM2612 uses the Z80 as the sound CPU to assist with audio. Unfortunately, there is a crucial design flaw; just like the System 16, the Z80's RAM still uses an glib 8 kilobytes. This was fine since the System 16 (which used a even smaller 2 kilobytes) used a separate sample chip and ROMs for the audio data, but now with only one ROM to get data from, the Z80 has to fight with both sample data and song data.

Nearly everyone who programmed for the Genesis had to battle with this problem. Games either had to compress the audio data, stream the data to the Z80 on the fly, or use the 68000 to "manhandle" the YM2612 while also handling the game logic. Included in this mess was sample data, and because of bandwidth problems with either CPU, samples couldn't play without stopping in tiny intervals. This is the cause of the infamous "scratchy" samples the Genesis was often associated with.

Dempa decided to forego the PCM mode in the YM2612, and instead went for a idiosyncratic approach. By setting the SN76489's three channels to output supersonic frequencies and streaming data to their volume registers, After Burner II manages to have three samples that can play at once throughout gameplay. The main drawback with this method is that the PSG's sound playback is very quiet, so the whole game had its volume adjusted while also compensating for the low volume distortion bug that plagued early Genesis consoles. Later Genesis consoles without the LVD bug have a quiet output, with instruments missing as a consequence.

It's worth noting that while this is unusual for the Sega Genesis, it was a common method for sample playback for games made for the Sega Master System. Games such as Alex Kidd: The Lost Stars, Alien Syndrome, Trap Shooting, Altered Beast, Space Harrier, and even the Master System's After Burner, all use the SN76489 for sample playback.

Level Select

At the title screen, hold A + B + C. Start the game for the level select.