"In the ROM based systems the cartridge or ROM cards can be used directly as memory by the machine.
ROM uses to be very slow compared with standard dynamic RAM (DRAM) or even more with static RAM (SRAM) but at the time the ROM cartridge system was used CPUs were not so fast to have many problems with it. The code was executed from the ROMs as well as the data for sound and video.
The small RAM memory was used for generation or modifications of the sound and graphic data and for the game variables."
Found in "Study of the techniques of emulation programming" paper.