GoodNES 3.23b has a ROM named "Duck Maze (HES) [!]" which has the same copyright-bearing title screen, but a different CRC32. The different bytes are as follows (left: GoodNES 3.23b ROM, right image: your ROM image:
- Code:
-
00005705: B1 F8
00005706: C1 B1
00005707: 71 C1
00005708: B3 71
00005709: C9 B3
0000570A: 0A 91
0000570B: 90 B3
0000570C: 02 29
0000570D: E9 10
0000570E: 0A F0
0000570F: 91 04
00005710: B3 38
00005711: 88 4C
00005712: 30 05
00005713: 06 D7
00005714: A9 18
00005715: 00 B1
00005716: 71 B3
00005717: B3 29
00005718: 91 0F
00005719: B3 91
0000571A: C8 B3
0000571B: EA 88
0000571C: 88 10
0000571D: 10 E8
0000571E: E5 D8
Hm, that section is definitely 6502 code, and the byte at 5705 (F8h) is the "SED" opcode, which enters "decimal mode". The NES' 2A03 CPU has no decimal mode. I remember reading on some web site (I do not remember the address, unfortunately) that Bit Corp. developed for Famiclones that still had the decimal mode, and had to fix their games to work on the original Nintendo console that lacked decimal mode. In other words:
- Your ROM is basically Bit Corp's original version of the game and should only work on decimal-mode-having Famiclones, i.e. "Duck (rev0)".
- GoodNES 3.23b's "Duck Maze (HES) [!]" is the updated version for original Nintendo consoles, i.e. "Duck (rev1)".
This of course means that your ROM should fail at some point in NES emulators that accurately lack decimal mode, possibly in a rather subtle way. The code seems to be called when starting a stage, and seems to have something to do with the score.
I remember the same web site specifically mentioning a decimal-mode-using version of Bit Corp.'s Othello, the existence of which now has become more likely.