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Adjust MMC1 power-on state to fix Populous #788

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Adjust MMC1 power-on state to fix Populous #788

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2 changes: 1 addition & 1 deletion src/boards/mmc1.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -169,7 +169,7 @@ static void MMC1CMReset(void) {
for (i = 0; i < 4; i++)
DRegs[i] = 0;
Buffer = BufferShift = 0;
DRegs[0] = 0x1F;
DRegs[0] = 0x0F; // Populous (unreleased) doesn't initialize this and expects 8KB CHR mode
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@thor2016 thor2016 Nov 10, 2025

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Will this break other ROMs? What has been tested here?

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@negativeExponent negativeExponent Nov 10, 2025

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correct reset value should be: DRegs[0] = 0x0C, similar behavior when load register writes value & 0x80 to issue reset which writes to Control Register with (Control OR $0C)

https://www.nesdev.org/wiki/MMC1#Control_(internal,_$8000-$9FFF)

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@awjackson awjackson Nov 11, 2025

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It is very unlikely that there are other games that depend on MMC1 initial state in the way Populous does. Populous simply never writes to MMC1 register 0 at all, which it can only "get away with" due to a unique combination of being (a) a game with no scrolling whatsoever, (b) that uses CHR RAM, and (c) is an unreleased beta that hasn't gone through Nintendo's checks.

For what it's worth, Mesen initializes this register to a random value | 0x0C.


DRegs[1] = 0;
DRegs[2] = 0; // Should this be something other than 0?
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