4tH isn't the only Forth compiler that runs on several systems. However, 4tH takes portability a whole step further. Some C-based Forth compilers need a special C-compiler in order to compile. 4tH doesn't. It has been compiled using Turbo C, Turbo C++, LCC, DJGPP, GCC, XL C and a bunch of other compilers we probably don't want to know about. 4tH runs on most Operating Systems, including MS-DOS, MS-Windows (both 16 bit and 32 bit), Linux, Coherent, AIX, SunOS, BOS, BOS-X, BSD, Apple OS/X, BeOS, RISC-OS, etc. A full 4tH compiler (including VM, RPN calculator, editor, C-source generators) has a typical footprint between 35K and 65K (depending on compiler, optimizations and OS).