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Meh. Lisp machines still exist. They're just simulated in various Lisp's runtime environments. It turns out that a RISC machine running a Lisp interpreter or an executable compiled from Lisp source tends to perform better than a tagged/cdr Lisp Machine w/ hardware GC.

That being said... I've wanted to implement an old Explorer using an FPGA for a while. Maybe if I just mention it here, someone will get inspired and do it before I can get to it.

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Lisp machines didn't have hardware GC, though they had hardware support for read/write barriers.

Kind of. A lot of the GC support in the Symbolics 3600 architecture is on the Microcode level (current CPUs usually don't have such operations in Microcode). The word size of the CPU is 36bit. The CPU operations don't deal with the type and gc tags on the instruction level, this is done on the Microcode level. Things like "invisible pointers" are also dealt on the Microcode level.

Ephemeral GC, concentrates on garbage collecting objects in RAM: For example every memory page has a page tag, which marks it modified or not. The ephemeral GC uses this to scan only over changed pages in memory. The virtual memory subsystem keeps a table of swapped-out pages pointing to ephemeral pages. The EGC can then use this information...

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