Add a page on the M161 mapper (#419)

Co-authored-by: Eldred Habert <[email protected]>

Author: CasualPokePlayer

src/M161.md

@@ -0,0 +1,43 @@
+# M161
+
+The M161 is a simple multi-cart mapper by Mani (Nintendo's official Chinese distributor). 
+This mapper only allows for 8 separate 32 KiB banks, with a limit of 1 bankswitch, preventing any bankswitches afterward.
+This gives 1 bank for the main menu, and a maximum of 7 different 32 KiB games.
+This mapper does not support SRAM, so only purely MBC-less titles can be used.
+
+This mapper is only known to be used in a single cartridge: Mani's 4 in 1 cartridge, which contains Tetris, Tennis, Alleyway, and Yakuman.
+All later Mani 4 in 1 cartridges use [the MMM01 mapper](<#MMM01>), so the M161 mapper could be considered the predecessor to the MMM01.
+
+The mapper is really just a simple off-the-shelf 74HC161A used a flip-flop.
+The 74HC161A contains 4 input pins and 4 output pins, and another pin for latching the input pins to the output pins.
+The first 3 of the input pins are connected to the lower 3 bits of incoming write data, with the 4th pin kept high.
+The first 3 of the output pins are connected to the ROM addressing pins (RA15-RA17).
+The 4th output pin (which corresponds to the always high input pin) is connected to the latching pin.
+Bit 15 of the write address is also connected to (OR'd with) the latching pin.
+The latching pin latches the input pins to the output pins on a transition from high to low, so when a ROM write comes through (bit 15 = 0), the input pins will be latched to the output pins.
+Since 4th output pin will be high, the signal cannot become low anymore, thus preventing further bankswitches.
+
+## Memory
+
+### 0000-7FFF - ROM Bank $00-$07 (Read Only)
+
+This area contains a 32 KiB bank of ROM.
+On startup, this will contain the first 32 KiB of ROM (bank 0).
+
+## Registers
+
+### 0000-7FFF - ROM Bank Number (Write Only)
+
+This 3-bit register (range $00-$07) selects the ROM bank number for the $0000-$7FFF region.
+Like other mappers, the high bits are discarded.
+
+Only 1 bankswitch is allowed per session, once any write to this register occurs (even if set to 0), all future writes will be ignored until the mapper is powered down.
+
+## Operation Notes
+
+Because the full 32 KiB range is switched over, caution should be taken, as the entire ROM range will change.
+It may be best to have the bankswitch code placed right before $0100, so as to fall through to [the game's init sequence](<#0100-0103 - Entry Point>).
+
+### References
+
+- Source: [M161 Schematic](https://pics.tauwasser.eu/image/schematic-dmg-m161-m01-v10.pdDz) on tauwasser pics

src/SUMMARY.md

@@ -66,6 +66,7 @@
   - [MBC6](./MBC6.md)
   - [MBC7](./MBC7.md)
   - [MMM01](./MMM01.md)
+  - [M161](./M161.md)
   - [HuC1](./HuC1.md)
   - [Other MBCs](./othermbc.md)