Main-ROM contains mainly a set of routines designed to allow the programmer to access to the hardware (BIOS) and the BASIC interpreter.

The standard says that MSX1 must have the Main-ROM in primary slot 0 or in secondary slot 0-0. Later it was decided that the MSX2 could have the Main-ROM in any slot but this was only the case for MSX2 upgrades and for next generations it was finally decided that it would remain in slot primary slot 0 or in secondary slot 0-0.