IBM's OS/360 and DOS/360 implement most system calls through a library of assembly language macros, although there are a few services with a call linkage. This reflects their origin at a time when programming in assembly language was more common than high-level language usage. IBM system calls were therefore not directly executable by high-level language programs, but required a callable assembly language wrapper subroutine. Since then, IBM has added many services that can be called from high level languages in, e.g., z/OS and z/VSE.