Aside from their large size, computers based on electron tubes had other drawbacks. The heating filaments in the tubes made the computers hot, and the filaments would often "blow". But in 1947, the development of the transistor by scientists at Bell Telephone Laboratories transformed the computer landscape. These tiny components were made from crystals of semiconductors such as germanium and silicon, and could do everything an electron tube could do, but were smaller and morereliable. Cheaper, more compact computers were soon in production, although some still occupied a whole room. Along with the developments in hardware, there were changes in software. Originally, all instructions for computers were written in binary code, "machine code". In 1951, a programmer named Grace Hopper proposed "reusable software", code that could be assembled according to instructions written in a "higher-level language" - something more closely resembling English. Hopper further proposed the concept of a compiler - a program that would translate instructions written in a higher-level language into machine code. FORTRAN, the first fully-fledged language, and its compiler were introduced in 1956. During this period, the punched cards and tape used to input data into computers were gradually replaced by magnetic tape and disks.