cpu_step &= 31;if(cpu_step == 5) cpu_step=0;
Tapper is a 1983 game by Sega. It has some pretty nice sounding music for a 2600 game. This game also happens to be an excellent emulation test case as it uses several hardware edge cases that I haven't seen in other games.
It didn't work at all in BizHawk 1.11.6. This was because an undocumented 6532 instruction was not emulated correctly. Fixing this at least let's the game run.
It also has some odd mapping going on. Apparently some of Sega's games have banks that are physically flipped, so even though this game uses a common mapper, the initial bank that gets pointed to is incorrect. But somehow through apparently shear luck the game still runs. This will eventually need to be fixed somehow, but I'm not sure exactly how yet.
More seriously, in the above pic we see the middle mug is missing. The reason is due to the uderlying clocking delays in the TIA. When a 'start drawing the player' signal is generated, it actually happens 5 clocks before a pixel appears. What Tipper does is reset the player on the same clock that the signal is generated. On hardware this is fine, but without proper low level emulation of the start signal, this results in the player not being drawn at all in BizHawk. This is going to require quite a significant modification of the code to fix. I guess there is still a lot of work to do!
while (true) do
cam_x=memory.read_u16_le(0x5900)
sonic_x_low=memory.read_u16_le(0x59E8)
sonic_x_high=memory.read_u16_le(0x59EA)
sonic_x=sonic_x_high*65536+sonic_x_low
gui.text(50,20,string.format('%d',sonic_x))
gui.text(90,50,string.format('%d',cam_x))
gui.text(90,50,string.format('%d',cam_x))
memory.write_u16_le(0x5900,math.floor((sonic_x-30000)/256))
emu.frameadvance();
endWell, Andy, that's actually wrong, Kanata posted a picture on Twitter where we can clearly see the graphical glitch on screen and he's on N64 (https://twitter.com/kanata2s/status/724783344601292800?lang=fr) so for now we just have no idea how he got it, and testing on emu is so annoying the result is always random (sometimes I just get absolutely nothing... So for now I'll try to do that on my N64 PAL but it will take time and I'll also ask some other people who have a N64
I also took one more look at Cosmic Ark stars, but all I could come up with is that they are displayed differently based on different TIA revisions. I don't think this can be further resolved without analyzing the circuits, so I'm just going to set this aside and focus on known issues.