So I've been doing a bit of reading on 1A and 4ESS technologies lately, getting a feel for the state of things just prior to 3B and 5ESS popping onto the scene, and came across some BSTJ references to the programming environments involved in the 4ESS and TSPS No. 1 systems. The general assembly system targeting the 1A machine language was known as SPC-SWAP (SWitching Assembly Program)[1](p. 206) and ran under OS/360/370, with editing typically performed in QED. This then gave way to the EPL (ESS Programming Language) and ultimately EPLX (EPL eXtra)[2](p. 1)[3](p. 8) languages which, among other things, were used for later 4ESS work with cross- compilers for at least TSS/360 by the sounds of it. Are there any recollections of attempts by the Bell System to rebase any of these 1A-targeting environments into UNIX, or by the time UNIX was being considered more broadly for Bell System projects, was 3B/5ESS technology well on the way, rendering attempting to move entrenched IBM-based environments for the older switching computation systems moot? For the record, in addition to the evolution of ESS to the 5ESS generation, a revision of TSPS, 1B, was also introduced which was rebased on the 3B20D processor and utilized the same 3B cross-compilation SGS under UNIX as other 3B- targeted applications[4]. Interestingly, the paper on software development in [4](p. 109) still makes reference to Programmer's Workbench as of 1982, implying that nomenclature may have still been the norm at some Bell Labs sites such as Naperville, Illinois, although I can't tell if they're referring to PWB as in the branch of UNIX or the environment of make, sccs, etc. Additionally, is anyone aware of surviving accessible specimens of SWAP assembly, EPL, or EPLX code or literature beyond the BSTJ references and paper referenced in the IEEE library below? Thanks for any insights! - Matt G. [1] - https://bitsavers.org/magazines/Bell_System_Technical_Journal/BSTJ_V58N06_1… [2] - https://ieeexplore.ieee.org/document/810323 [3] - https://bitsavers.org/magazines/Bell_System_Technical_Journal/BSTJ_V60N06_1… [4] - https://bitsavers.org/magazines/Bell_System_Technical_Journal/BSTJ_V62N03_1…