To enforce this agreement, Lexmark cartridges included a computer chip that included a 55-byte computer program (the "Toner Loading Program") which communicated with a "Printer Engine Program" built into the printer. The program calculated the amount of toner used during printing: when the calculations indicated that the original supply of Lexmark toner should be exhausted, the printer would stop functioning, even if the cartridge had been refilled. In addition, if the chip did not perform an encrypted authentication sequence, or if the Toner Loading Program on the chip did not have a checksum matching exactly a value stored elsewhere on the chip, the printer would not use the cartridge.