[TUHS] TMG and B

scj wrote: Sorry for straying from direct Unix history, but this remark spurred a lot of memories. TMG (from "transmogrify", defined in Webster as "to change or alter, often with grotesque or humorous effect") was imported from Bob McClure, erstwhile Bell Labs person, then at Texas Instruments. And an interesting import job it was. McClure had written TMG for the CDC 1604 in machine language. He sent me green coding sheets hand-transliterated into 7090 code. Interesting debugging: one knew that the logic of the code was sound, but the opcodes might not always be right. Sometimes, for example, the wrong one of the two accumulator-load instructions, CLA and CAL, was used. Clem Pease converted TMG to the GE 635 for Multics by the artifice of defining 7090 opcodes as 635 macros--sometimes many instructions long to slavishly emulate the 7090's peculiar accumulator (which mixed 38-bit sign-magnitude with 37-bit twos-complement and 36-bit ones-complement arithmetic). It's amusing to speculate about the progressive inflation of TMG had McClure sent me a similar translation for the 7090. TMG had a higher-level language written in TMG, which evolved during the Multics project into something considerably more elaborate than McClure's original, including features like syntax functions, e.g. seplist(a,b) denoted a sequence of a's separated by b's, for arbitary syntactic categories a and b. Syntax functions took TMG beyond the domain of context-free languages. Multics was to be written in PL/I, a compiler for which was commissioned from Digitek. They had brilliant Fortran technology, but flubbed PL/I. When it appeared that the Digitek compiler was hopeless, Bob Morris proposed that we write a quick and dirty one in TMG. Despite being slow (an interpreter running on an emulated 7090) and providing only three diagnostics, this compiler carried the project for a couple of years. When Unix came along, we were again faced with how to bootstrap TMG across machines. This time I wrote a bare-bones interpreter in PDP7 assembler, then by stages grew the language back to the Multics state. Ken, in a compliment I still treasure, once called this the most amazing program on the original Unix machine. I believe TMG was involved in the initial evolution of B, but the real tour-de-force in B was the ability of an interpreted version to exploit software paging and transcend the limited memory of the PDP-11. The following scenario was to be repeated several times in the early days of Unix. When the native version of B ran out of steam, the interpreted version would be used to introduce some new optimization that would squeeze the native version back to fit. (Bigger input, smaller output!) Subsequently we saw the same thing happen with C and the kernel. When the kernel grew too big, a new optimization would be introduced in C to squeeze it. (And to squeeze the compiler, too. The compiler, though, never enjoyed B's advantage of being able temporarily to run in a bigger arena.) Doug