The entered programs are stored on magnetic strips. Those were easy to transport, and the reader/writer was compact in size. However, the reader/writer as well as the magnetic strips were quite expensive. The last and most notable devices to use magnetic strips were the HP-41C and TI-59. Continuous memory Continuous memory does not lose its content when the calculator is switched off. With continuous memory the user can, for example, change batteries without losing the entered programs. Cassette tape Compact Cassettes offered a simple, inexpensive alternative to magnetic cards. Usually, an interface module, such as the Casio FA-1, was used to connect the calculator to an ordinary cassette recorder, and digital data were encoded as frequency-shift keyed audio signals. Sharp and Hewlett-Packard also sold dedicated micro- or mini-cassette recorders that connected directly to the calculator. These set-ups, while being more practical and reliable, we're also more expensive. Semi-continuous memory As memory demands rose, it became more difficult to create true continuous memory and developers sought alternatives. With semi-continuous memory content was only preserved if specific battery-changing rules were observed. The most common rules were: A special backup battery would ensure that the memory was not lost while the main batteries were changed. Battery removal and replacement had to be completed in a relatively short time. For example, with the HP 35s, battery replacement had to be completed in less than 2 minutes after removal. At least two main batteries were used and could only be changed one at a time. PC-connection Programs and data are transferred to a personal computer for storage. The transfer is done by the following connection methods (chronological order of appearance) RS-232, IrDA and USB. This method has the advantage of being very cost-efficient and is usually faster than the cassette interface. These advantages are offset by the need for a personal computer. An early example of a PC connection is the Casio FX-603P in conjunction with the Casio FA-6 interface. In this set-up, transfer was done in plain text so the program and data could be stored and edited with a standard text editor. Flash memory Programmable calculators and pocket computers Throughout the 1980s and the beginning of the 1990s, programmable calculators stood in competition with pocket computers, with high-end calculators sharing many similarities. For example, both devices types were programmable in unstructured BASIC and with few exceptions featured QWERTY keyboards. However, there were also