> Mainframes, because they have saner systems programming languages than C.

Are you claiming that PL/X is saner than C? What is your reasoning?

Less UB, bounds checking, proper strings and arrays?

The PL/8 papers related to IBM's RISC research are quite interesting, before they decided to go with UNIX on RISC instead, as the best way to recover their money, thus switching to C.

Also not all mainframes were written in PL/X variants, there is also NEWP, which already had the notion of unsafe blocks in the late 60's.

PL/X and PL.8 may have various technical advantages over other languages, but it is hard to judge given IBM's long-standing refusal (going back decades) to publicly release compilers and other tools for them. From a software engineering viewpoint, developing your software in proprietary/in-house/unreleased programming languages – much of z/OS is written in PL/X, while some of the firmware/millicode is in PL.8 – seems to me a rather bizarre decision – not to mention other secret dialects of PL/I IBM has cooked up over the years, such as the PL/MP and PL/MI that were used on the AS/400s, and PL/DS on the IBM 8100. Why not release the languages, try to build some sort of community around them – wouldn't that be good for their long-term health, and the long-term health of the platforms written in them?

(Probably it is way too late for this, but if IBM had been willing to publicly release PL/S tools back in the 1970s, or at least not use their lawyers to stop others from independently doing that, PL/S and its descendants could have become much more significant systems programming languages.)

Those were the rules of the game back in those days, languages were tied to OS vendors.

UNIX and C only became widespread, because contrary to IBM, DEC, Wang, and many others, AT&T was forbidden to sell UNIX and they started offering it alongside source code for a symbolic price of $100, which was like being free given the prices of other systems.

In any case, that doesn't make C technically better than the alternatives outside AT&T walls, it was just cheaper to get hold of.

The folks calling for IBM to release a PL/S compiler were not demanding IBM hand it out for free. They would have been satisfied by IBM offering it for sale on usual commercial terms, for a similar price to IBM's other compilers (PL/I, COBOL, FORTRAN, etc).

But IBM wasn't willing to sell its customers a PL/S compiler.

One reason was that (up until the OCO announcement in 1983) IBM gave its customers the PL/S source code, but not giving them a PL/S compiler discouraged customers from changing the source code, since they couldn't recompile it. Customer modifications had to be made in assembler, which was a lot harder to maintain.

Another reason was to make life harder for competing vendors selling IBM-compatible mainframes, such as Amdahl, Fujitsu and Hitachi.