The Debug Mode

A short film about the debug mode in Super Mario All-Stars, voiced by Supper Mario Broth (  / mariobrothblog  )

Super Mario All-Stars checks address 7E0160 in the SNES's memory for the value 80 every time at startup. It expects the value to be set to 80 manually by a dev kit to enable debug mode in Super Mario Bros. 3. However, due to the address actually being uninitialized at the time of access, the value is unpredictable and can randomly return 80 on any SNES unit, regardless of them being dev kits.
This leads to the scenario showcased in the video, where the debug mode becomes randomly enabled for one play session, only to then never be seen again.

In theory, the chance of this address being 80 is 1/256. In practice, this is dependent on the electric signals of that particular home's electric grid, as well as the physical properties of that particular SNES, so it can be less than 1/256 for some consoles and more than 1/256 for others.

The underlying reason for the error is that this was originally a developer function in the NES version of the game, which was copied over without change into the SNES version without considering that while the address in question was manually set to 00 on the NES, it remains uninitialized on the SNES. More information can be found here: https://tcrf.net/Notes:Super_Mario_Al...