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Comment by rprospero | Hacker News Reader

Thank you for the explanation about the microcode. Many years ago, someone was explaining to me that Lisp machines were Lisp “all the way down”. There were no layers that you couldn’t reprogram with Lisp. When pressed for details, he claimed that even the band gap potential in the transistors of the machine could be reprogrammed, because the transistors were just an abstraction over more Lisp.

Your explanation makes far more sense.

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In the early days the CPUs had loadable microcode. With new operating system versions came language extensions for Lisp. Then one also got a new version of the microcode, which then had been extended for, say, inclusion of an instruction for OOP method dispatch. One also could compile some code to microcode.

Later the CPUs were actually Microprocessors (from TI and Symbolics). At least for the Symbolics Ivory microprocessor, the microcode no longer was loadable.

don't forget that besides microcode much of the lisp machine logic was implemented with PALs, which were themselves programmed in Lisp. the simple gates, mixers and flip-flops were describe with lisp code, so that the logic could be tested on the lisp machine. I don't know if they built the test tables using the lisp code, or wrote them out manually, but you essentially had multiple sources of truth that could be mutually verified: lisp code, test tables, and the behavior of the specific PAL.

Ironically we are back to loaded microcode and memory tagging.

The turns the world does instead of adopting good ideas from the get go.

What kind of memory tagging are you referring to? I can think of W^X, but I don't see how well it compares to lisp stuff.

Don't forget undocumented "hide tags on ECC bits" of POWER 64bit ISA, for use with AS/400

I can put on my ‘pjmlp hat and talk about how SPARC ADI is being reborn today as CHERI and MTE.

Armv8.5-A Memory Tagging Extension (MTE)?

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