Dump GB/GBC cartridges via GBA multiboot
Power on the GBA without a cartridge inserted, then click "Send Multiboot & Dump". After the multiboot completes and the GBA switches to GBC mode, insert your GB/GBC cartridge. Some cartridges have a voltage supervisor that resets the GBA on insertion — placing a small piece of kapton tape over pin 30 (/RESET, 3rd pin from the right) on the cartridge prevents this. Inserting slowly can also help. Once the cartridge is detected, press:
Use "Dump Only" if the GBA is already running the dumper ROM.
Idle
Disconnected