Other ports followed for Apple III, the Zilog Z80-based Tandy TRS-80 Model I, Model II, Model III, Model 4, and Sony SMC-70. The TRS-80 Model I and Sony SMC-70 ports were the only versions of VisiCalc without copy protection. The Sony SMC-70 port was the only CP/M version. Most versions were disk-based, but the PET VisiCalc came with a ROM chip that the user had to install in one of the motherboard's expansion ROM sockets. The most important port was for the IBM PC, and VisiCalc was one of the first commercial packages available when the IBM PC shipped in 1981. It quickly became a best-seller on this platform, in spite of being severely limited in order to be compatible with the versions for the 8-bit platforms. It is estimated, that 300,000 copies were sold on the PC, bringing total sales to about 1 million copies.