Other languages like Rexx, awk, Perl, and some Unix shells can also be implemented in this fashion on many calculators of this type. The GCC development suite is available for several models of Casio, HP, and TI calculators, meaning that C, C++, Fortran 77, and inline assembly language can be used to develop a program on the computer side and then upload it to the calculator. Projects in development by third parties include on-board and/or computer-side converters, interpreters, code generators, macro assemblers, or compilers for Fortran, other Basic variants, awk, C, Cobol, Rexx, Perl, Python, Tcl, Pascal Delphi, and operating system shells like DOS/Win95 batch, OS/2 batch, WinNT/2000 shell, Unix shells, and DCL. Many TI, Casio, Sharp and HP models have Lua interpreters which are part of the default configuration or can be optionally added. Some calculators run a subset of Fortran 77 called Mini-Fortran; the compiler is on the calculator so connecting to a PC to put programs onto the machine is not needed. The OnCalc C Compiler for the Casio fx-9860 series is now available. The Sharp PC G850V pocket computer has an on-board C compiler in addition to an assembler and a Basic interpreter. One important feature of programmable calculators is the availability of some form of persistent memory. Without persistent memory, programs have to be re-entered whenever power is lost, making the device cumbersome. Persistent memory can be internal or on a separate device. Some programmable calculators employ both schemes. Magnetic card readers were among the first persistent memory options available. 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 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. 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.