BugID: #01326

madcrash (marvins.c driver)

There is an ingame bug if you set the "Occurence" Dip Switch to "1st, 2nd then every 2nd" ...

Here are some useful addresses :

  - 0xc663-0xc666 : player score   (BCD coded, MSB first)
  - 0xc303-0xc306 : 1st bonus life (BCD coded, MSB first)
  - 0xc307-0xc30a : 2nd bonus life (BCD coded, MSB first)
  - 0xc670-0xc673 : next bonus life in case of more than 2 occurences (MSB first)


In fact, only these addresses interest us : 0xc663-0xc664, 0xc307-0xc308 and 0xc670-0xc671 ...

I assume that you've set the "Bonus Lives" Dip Switch to "20k, 60k" ...

When you start a game, here are the contents of the addresses :

  - 0xc663 = 0x00 and 0xc664 = 0x00
  - 0xc307 = 0x00 and 0xc308 = 0x06
  - 0xc670 = 0x00 and 0xc671 = 0x00

When you get your 1st life at 20000 points, this code is executed :

[BEGIN]
1300: 21 07 C3    ld   hl,$C307
1303: 11 70 C6    ld   de,$C670
1306: 01 04 00    ld   bc,$0004
1309: ED B0       ldir
[END]

So the contents of the addresses are now :

  - 0xc663 = 0x00 and 0xc664 = 0x02
  - 0xc307 = 0x00 and 0xc308 = 0x06
  - 0xc670 = 0x00 and 0xc671 = 0x06

But when your score reaches 60000 points, this code is executed :

[BEGIN]
1352: 11 0A C3    ld   de,$C30A
1355: 21 73 C6    ld   hl,$C673
1358: 06 04       ld   b,$04
135A: 37          scf
135B: 3F          ccf
135C: 1A          ld   a,(de)
135D: 1B          dec  de
135E: 8E          adc  a,(hl)
135F: 77          ld   (hl),a
1360: 2B          dec  hl
1361: 10 F9       djnz $135C
[END]

If you watch carefully, you'll notice that there is NO 'daa' ("decimal ajust", needed when you add/sub BCD numbers) after the 'adc  a,(hl)' at address 0x135e ...

So the contents of the addresses are now :

  - 0xc663 = 0x00 and 0xc664 = 0x06
  - 0xc307 = 0x00 and 0xc308 = 0x06
  - 0xc670 = 0x00 and 0xc671 = 0x0c  !!!

When your scores reaches 100000 points, 0xc663 = 0x00 and 0xc664 = 0x10 ...

And 0x0010 > 0x000c ... So you get ANOTHER life and the buggy routine is called again !

So the contents of the addresses are now :

  - 0xc663 = 0x00 and 0xc664 = 0x10
  - 0xc307 = 0x00 and 0xc308 = 0x06
  - 0xc670 = 0x00 and 0xc671 = 0x12  !!!

And this means that another life will be awarded when your score reaches 120000 points (0xc663 = 0x00 and 0xc664 = 0x12) !

I could continue for ever ;) I hope that you've understood everything ...