Time-sharing

The term time-sharing refers to the practice of allowing many users to access a computer or several computers, thus sharing time on a single system. It is similar to the more modern cloud computing, the main difference being that cloud systems tend to use a much larger pool of servers. Time-sharing was the most common way to run APL in the 1970s, with STSC and IPSA being the best known providers, and APL had a strong influence on time-sharing, with many time-sharing systems being written in APL.

The APL time-sharing business came into being with the release of APL\360 as an IBM Type III Library in August 1968, meaning that its source code was available for use outside of IBM. Other companies were quick to adapt the existing time-sharing model to APL, with eight companies offering this service in 1968 alone. These included I.P. Sharp Associates, which had assisted in the implementation of APL\360 since 1966 (in particular, through employee Roger Moore), and STSC, the Scientific Time Sharing Corporation. The two companies offered systems using the same APL, APL*PLUS, with IPSA focusing on the Canadian market and STSC on the United States.

Time-sharing companies and users drove the development of APL in the 1970s, with IBM adopting and unifying new features after careful consideration. Time-shared APLs were the first to have file system interfaces; IBM first offered an APL that could share resources with other programs in 1973 with APL.SV, and only supported files beginning with VS APL in 1976.

Around 1980 it became more common for companies and universities using APL to install their own computers, both because these organizations were doing more work in APL and could justify the expenses and because computers were becoming more affordable as the price of mainframes fell and the power of minicomputers grew. IPSA and STSC began offering in-house versions of their products around this time, and soon offered software for personal computers as well: STSC's APL*PLUS/PC in 1982 and IPSA's SHARP APL/PC in 1984. Although demand for APL remained, the number of APL vendors shrank as the profits from selling software only were much lower than those from time-sharing. New APL vendors in the 1980s, rather than writing an APL to run on their own system, developed portable implementations to be compiled for various machines.