Much as with the GameCube, the MX "RTC-SRAM-ROM" chip sits on the EXI bus at address 0-1. http://hitmen.c02.at/files/yagcd/yagcd/chap10.html#sec10.3
The only data contained in ROM on the Wii is two fontsets (one Latin, one Japanese). They are compressed using a proprietary format (Yay0), and generally only used in GameCube mode.
Real-Time Clock (RTC) is 32-bit value, counting time intervals in seconds. To get the real time (seconds since January 1st, 2000 12am) add the counter bias saved in SRAM.
The SRAM stores some miscellaneous settings that are only used in GameCube compatibility mode.
The custom MX23L4005 chip used on the Nintendo Wii contains the GameCube font set, RTC, and power management functions. It is listed on WiiRe as "U8: MX "E062674-MG" "MX23L4005-24C1" "RTC-RVL A" "2S08802" (RTC, TSOP28 18x8mm)".
Die shot: http://www.bunniestudios.com/blog/?p=227
Observed pin behavior:
1 connected to power button - active high (measures +3.3v while power button is pressed) 2 wii power state (+3.3v when power is on or in standby, 0v otherwise) TP113 comes on slightly after pin 3 --power on? 3 wii power state (+3.3v when power is on or in standby, 0v otherwise) (nc on PCB!) 4 Gnd 5 0? TP46 pulses occasionally to 1 6 1? TP106 green LED? 7 0? TP45 8 0? measuring this pin prevents the wii from shutting off when you hold the power button down ?!!? 9 nc on PCB 10 TP109 0? 11 TP43 0? 12 TP44 exi? 13 TP41 exi? 14 TP42 exi? 15 exi? 16 Gnd 17 3.3v when wii has power applied 18 3.3v when wii has power applied 19 TP108 measuring this causes wii to power on immediately 20 TP231 0? 21 TP110 3.3v when wii has power applied, goes low briefly when powering off 22 xtal 23 xtal 24 TP119 turns on shortly after poweron (connects to U10) -- reset? 25 TP117 +3.3v when wii has power (connects to U9) 26 TP116 0? 27 +3.3v when wii has power 28 NC on PCB
This chip appears to also have some connection with the disc drive, as it stores flags for when the eject button is pressed (0x01010101) and when a disc has been changed (0x02020202), which the System Menu depends upon for the disc channel to actually detect changed discs. These flags are accessed over EXI, with reads using 0x21000800 and writes using 0xa1000800. Implemented in Dolphin by this commit.