NES-EVENT

NES-EVENT is the MMC1-based board used for Nintendo World Championships, a multicart including timed versions of Super Mario Bros., Rad Racer, and Tetris. Like SUROM and SXROM, it repurposes the MMC1 CHR address bits, and MMC1 wired this way is mapper 105.

The menu music resembles selected tracks from Hello Kitty World.

Parts

• 256K of PRG ROM
• 8K of CHR RAM
• 8K of PRG RAM
• Several 4000-series and 7400-series discrete ICs

Register

MMC1 register $A000 is mapped like this:

43210
|||||
||||+- Not used (ordinarily selects bottom or top half of CHR RAM)
|+++-- 0-3: Select 32 KiB bank in $8000-$FFFF from lower 128KB ROM
|      4-7: Normal MMC1 behavior from upper 128KB ROM
+----- 0: Run timer
       1: Reset timer

The first 32 KiB is hardwired until the timer is started (write 0 then 1 to bit 4) for the first time.

The other ICs implement a 30-bit up counter clocked by M2 that fires an IRQ when it reaches a high enough value. DIP switches set bits 28-25 of the counter's initial value. This results in a range of roughly 5 to 10 minutes on an NTSC console.

References

• Kevin Horton's reverse engineering of NES-EVENT