Peter Relson noted... >Historically, the PL/x compiler variant was created from the pre-existing assembler, not the other way around.
>When the PL/x variant existed first (or for new parts) prior to the tool, the assembler was hand-generated. As I remember, the oldest ancestor of PL/X was BSL, "Basic System(s?) Language". A talk on it was given at SHARE by Bob Brittenham, who led the team that developed it. Back in those days IBM shipped many modules with source code, and in several cases the "shipper" forgot to include only the "clean" generated assembler source and included a version with the embedded BSL statements. Gio Widerhold (of Stanford Med Center's ACME time-sharing system -- written in PL/I, as I remember) and I (at the Stanford Linear Accelerator Center at the time) published a paper in SIGPLAN Notices called "Inferred Syntax and Semantics of BSL". Things tightened up quickly after that, and much less was said externally about BSL and its successors, the first of which I believe was PL/S. I assume Peter means the control blocks were hand-generated, not the BSL compiler itself.
