Making programs for the computer was difficult. Making the computer ready for one single program could take many days or even weeks. This was because the programming had to be done by pulling wires from one place to another. This was not hard for small programs, but if a large program was wanted, it was very hard work. Programming by pulling wires was used until 1948, when a special type of memory was added (called Read Only Memory (ROM), because the computer could read it but not write it). After that, programming was done by using switches, which only took hours instead of days.