Originally all System/360 operating systems were written in assembler language, and all system interfaces were defined by macro definitions. Access from high-level languages (HLLs) was restricted to what that language supplied., Anyand other system calls had to be coded as assembler subroutines called from HLL programs. Also, IBM allowed customization of OS features by an installation thru what were known as ''Exits''—user-supplied routines that could extend or alter normal OS functions. These exits were required to be coded in assembler language. Later, IBM recoded OS/360 in a systems programming language, [[PL/S]], but, except for a short trial, decided not to release the PL/S compiler to users. As a result of these factors, assembler language saw significant use on IBM systems for many years.
