(PL/S) An IBM machine-oriented language derived from PL/I, in the late 1960s, for the IBM 360 and IBM 370. PL/S permitted inline assembly language and control over register usage. Previous IBM 360 operating systems such as OS/MFT and OS/MVT had been written entirely in assembly language. The first IBM OS that had any significant portion written in PL/S was MVS, followed by OS/VS1, OS/VS2 and OS/SVS. PL/S was used internally and never released to the public. It is documented in various IBM internal ZZ-? publications.