MIT wanted to charge IBM $0.02 per bit royalty on core memory. Whirlwind I computer with an interactive exhibit explaining core memory. It had 2,048 40-bit words of ferrite core memory. The computer used MOS memory instead of core memory. This used twistor memory for ROM and core memory for RAM. An Wang was an important contributor to the development of magnetic core memory. Both were made obsolete by the invention of magnetic core memory. Large core memory holds data ready to transfer to small core memory. Two key inventions led to the development of magnetic core memory in 1951. The 704 also marked the transition to magnetic core memory. As expected, the cores were much larger physically than those of read-write core memory. The oldest PLCs used non-volatile magnetic core memory. Early core memory cost one dollar per bit. Another form of core memory called core rope memory provided read-only storage. The Mark IV was all-electronic, replacing the remaining mechanical components with magnetic core memory.