Without debug info, Ghidra must guess where a function ends. You’ll often see one giant function spanning 64KB. Use the view and look for BX LR (return for Thumb) or MOV PC, LR (return for ARM). Split functions manually at those points.
Check the header at 0x08000000 – bit 0 of the entry point address determines the mode: decompile gba rom
Before writing a single command, you must understand what a GBA ROM actually is. Without debug info, Ghidra must guess where a function ends