At power up, DOS 3.3 looks at location $E000, both on the motherboard and on the firmware card, to check which version of BASIC is available. If a Language card is present, DOS tries to jump into some random information on the card.
This jump is prevented by a machinge language routine that places a $00 at $E000 on the Language card during booting. This routine is stored on the DOS diskette at track 0, sector 9, starting at byte $D3.