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I have a 4in1 multicart, but I don't know how it works. What's its Mapper? Who can help me analyze it?Thank you very much

The attachment contains the rom file.

PCB says "K-3043" on the back.

74'161 latches A0-A3
74'161 cleared on reset.
74'161 clocked by MMC3 clone /WRAMCE
74'161 latch enable is MMC3 clone /WRAMWE

74'161 Q0 goes to both ROM A17.
74'161 Q1 goes to both ROM A18.
74'161 Q2 and Q3 not obviously connected to anything.

This is almost oversize mapper 47, but it uses the address bus instead of the data bus. I don't see a mapper in FCEUX that is this hardware.

There is a solder pad and an unused spot for another 74 chip. What would that one do if installed?

uh...
pin 1 = Q1 (input)
pin 2 = MMC3 PRG A17 (input)
pin 3 = Q0 (input)
pin 4 = PRG ROM A17 (output)
pin 5 = MMC3 CHR A17 (input)
pin 6 = Q0 (input)
pin 7 = CHR ROM A17 (output)
pin 8 = ground
pin 16 = +5V
pin 9-15 - floating

I don't recognize that. It's not any standard 2-, 3-, or 4-input logic gate.

The game would run as oversize mapper 47, since the menu writes the outer bank both to data and address. The PCB is a compatible subset of mapper 344, which I just updated.

The missing chip seems to be to switch between 128 KiB and 256 KiB outer banks.

... The missing IC is a 74'157. If populated, and the two traces on the back cut, it changes the geometry to instead be one 256/256 and two 128/128 games. (in that order)

Theoretically pin 15 should be explicitly grounded in order for the '157 to work, but I don't see that, so it may not be functional even if populated.

thanks