There are 2 main adresses :

  - 0x10009e.l which is always written to 0x700034.l (coin handler according to MAME driver)
  - 0x100078.l which is the number of credits (range 0x0000-0x0013)


When game boots, this code is executed :

003DCA: 33FC 000C 0010 009E        move.w  #$c, $10009e.l
003DD2: 33FC 000C 0070 0034        move.w  #$c, $700034.l

So both coin slots are unlocked.


When you insert a coin and get additional credit(s), this code is executed :

003E7A: 0C41 0013                  cmpi.w  #$13, D1
003E7E: 6510                       bcs     $3e90
003E80: 4268 0006                  clr.w   ($6,A0)
003E84: 0279 FFF3 0010 009E        andi.w  #$fff3, $10009e.l
003E8C: 323C 0013                  move.w  #$13, D1
003E90: 3281                       move.w  D1, (A1)
003E92: 4E75                       rts

So both coin slots are locked due to code at 0x003e84 if you have 19 or more credits.


When you press a START button during the "attract mode", this code is executed :

001846: 5379 0010 0078             subq.w  #1, $100078.l    player 1
00184C: 0079 000C 0010 009E        ori.w   #$c, $10009e.l
...
0018A0: 5379 0010 0078             subq.w  #1, $100078.l    player 2
0018A6: 0079 000C 0010 009E        ori.w   #$c, $10009e.l
...
00190C: 0079 000C 0010 009E        ori.w   #$c, $10009e.l   player 3
001914: 0839 0000 0010 005B        btst    #$0, $10005b.l
...
001960: 5379 0010 0078             subq.w  #1, $100078.l    player 4
001966: 0079 000C 0010 009E        ori.w   #$c, $10009e.l
...

And when you press a START button when you are playing, this code is executed :

005900: 5379 0010 0078             subq.w  #1, $100078.l    any player
...

You can easily notice the difference : 0x10009e.l is NOT updated when you are playing.
So, if you insert enough coins to lock the coin slots when you are playing,
you won't be able to insert any more coins until you go back to the "attract mode" and start a new game !

Was all this done on purpose so people couldn't end the game with more than 20 credits is another story ...