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small-lisp
A very small lisp interpreter, that I may one day get working on my 8-bit AVR microcontroller.
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sectorlisp implements a dynamically scoped Lisp for 8086 (yes, it will still run on a x86-64 machine as they boot into 8086 mode). Once I considered shrinking it further by using a bytecode for the Lisp interpreter (making it two interpreters in one, yes) which ideally would be smaller than 8086 instructions, but I never got it to work.
The smallest I've come across is manually building https://github.com/kristianlm/small-lisp with gcc which came out to 18kb. If anyone has seen anything smaller I'd love to hear about it. I'd imagine the only way to really beat 18kb is with some smart linker magic or using asm (I've never seen an asm lisp for x86).
This may not be smallest and not specifically for x86 Lisp, but Teemu Kalva's Lisp500 (LIsp in 500 lines of C) is a lot more complete than the above small-lisp. It has working macros, garbage collection and even a compiler.
For a real answer, other replies have already mentioned KiloLisp, but there's also femtolisp. Also, not exactly what you're asking for, but Maru is a very compact and elegant self-hosting lisp (compiles to x86).