On one hand, all these mini interpreters and compilers are cool. I have a soft spot for extensible systems. On the other hand, all these things are a huge security problem. When every subsystem and data format is carrying around its own Turing complete bytecode and JIT, they all need to be secure and bug free for the system to be secure and bug free. And that far more code surface to keep clean.
Mitch Bradley created OpenFirmware. It started at Sun as OpenBoot (informally "SunForth") on the SPARCstation 1 in 1989, was standardized as IEEE 1275-1994, and was renamed OpenFirmware at that time. Its lineage runs back through Mitch's earlier Forthmacs (Bradley Forthware, early 80s), which ran on 68k Macs, Sun-2/3, Atari ST, and Amiga. Mitch credits Henry Laxen and Michael Perry's F83 and Glen Haydon's MVP-Forth as the public-domain ancestors.
The metacompiler can target many platforms, word sizes, CPUs, and threading models, and produce stripped ROMable images. It can build the kernel as direct-threaded (DTC), indirect-threaded (ITC), subroutine-threaded (STC), or token-threaded (TTC), with 16, 32, or 64 bit cells. Shipping kernels are DTC native code with cell-sized xt pointers: 32 bit on the original SPARC and PowerPC machines, 64 bit on modern PPC64, SPARC64, and ARM64 builds.
Peripheral expansion cards ship a separate, portable, variable-byte token format called FCode. The kernel interprets FCode at boot/probe time and recompiles it on the fly into the live native dictionary. After probe, FCode-loaded drivers run as ordinary native Forth words. That two-stage design (fast native runtime, portable FCode transport) is what let Sun ship one card PROM image that worked across CPU generations.
FCode was designed for SBus on the SPARCstation 1, with cross-CPU portability built in. Sun's earlier and contemporary buses were not interchangeable with SBus (Sun-2 used Multibus, Sun-3 used VMEbus, the Sun386i "Roadrunner" used AT-bus), so the cross-architecture payoff arrived later, when IEEE 1275-1994 standardized OpenFirmware and PCI allowed FCode in option ROMs. After that, the same expansion-card PROM image could boot on Sun SPARC, Apple PowerPC Macs, IBM PowerPC servers (CHRP), and the OLPC XO.
Interview with Mitch Bradley (he's like the Woz of Forth):
In parallel with the OpenBoot work, Mitch also developed an extremely portable C-based Forth (the public version is "C Forth 93"). It runs a switch-threaded inner interpreter over packed tokens, with configurable cell width (16, 32, or 64 bit) and configurable token width (pointer-sized by default, 16 bit with the T16 build flag for tight flash budgets), plus a small hand-rolled FFI built around a fixed-arity 12-argument marshalling trampoline driven by a format string. It is now the embedded variant used in OLPC's OpenFirmware and in PlatformIO targets including RP2040, Teensy, ESP32, ESP8266, and STM32:
I ran EForth under the Subleq from Howe R.J at https://github.com/howerj/muxleq (the subleq one) first at QuickJS (trivial tasks, almost a 1:1 map from the C code, made in a hurry) and under... jsinterp.py from the infamous yt-dlp but using arrays instead of printing functions. But... if yt-dlp's "mini-JS" implements some captcha input functions... you can add I/O with ease and run EForth with what they call (not me) a "Not totally functional interpreter".
Not totally... until people there run the 110 rule program, Conway's Life, Subleq+EForth...
All regular expressions are deterministic final automata https://en.wikipedia.org/wiki/Deterministic_finite_automaton (finally, a use for my CS course); the extent to which that counts as a virtual machine varies. Some of the regex syntaxes extend it in ways which don't fit in a DFA and do count as a VM; Perl-compatible RE used to be popular (e.g. in Exim).
It's easier to construct NFAs directly from regular expression definitions (rather than DFAs) because implementing the choice operator is easier. We can convert from NFA to DFA with worst-case exponential blowup.
Interesting. Not that surprising that it works like this. But isn't it a little surprising that things like regexes, printf syntax and other DSLs aren't mostly handled and parsed at compile time in 2026?
Kind of language-dependent since regexes are normally specified as strings and most languages are pretty weak at "run this code at compile time". One of the things Rust users are fond of.
I have also built a C# source generator myself (XML parser generator), but the developer experience is a bit of a hill to climb compared to what it could be.
I've got subleq+eforth (https://github.com/howerj/muxleq) running in JS
which is dead simple to do. No input but I could output
ASCII mapping values to an array.
[1] https://formats.kaitai.io/python_pickle/
[1] https://docs.oracle.com/cd/E19957-01/802-3239-10/sbusandfc.h...
https://en.wikipedia.org/wiki/Threaded_code#Token_threading
Mitch Bradley created OpenFirmware. It started at Sun as OpenBoot (informally "SunForth") on the SPARCstation 1 in 1989, was standardized as IEEE 1275-1994, and was renamed OpenFirmware at that time. Its lineage runs back through Mitch's earlier Forthmacs (Bradley Forthware, early 80s), which ran on 68k Macs, Sun-2/3, Atari ST, and Amiga. Mitch credits Henry Laxen and Michael Perry's F83 and Glen Haydon's MVP-Forth as the public-domain ancestors.
The metacompiler can target many platforms, word sizes, CPUs, and threading models, and produce stripped ROMable images. It can build the kernel as direct-threaded (DTC), indirect-threaded (ITC), subroutine-threaded (STC), or token-threaded (TTC), with 16, 32, or 64 bit cells. Shipping kernels are DTC native code with cell-sized xt pointers: 32 bit on the original SPARC and PowerPC machines, 64 bit on modern PPC64, SPARC64, and ARM64 builds.
Peripheral expansion cards ship a separate, portable, variable-byte token format called FCode. The kernel interprets FCode at boot/probe time and recompiles it on the fly into the live native dictionary. After probe, FCode-loaded drivers run as ordinary native Forth words. That two-stage design (fast native runtime, portable FCode transport) is what let Sun ship one card PROM image that worked across CPU generations.
https://github.com/MitchBradley
https://github.com/MitchBradley/openfirmware
FCode was designed for SBus on the SPARCstation 1, with cross-CPU portability built in. Sun's earlier and contemporary buses were not interchangeable with SBus (Sun-2 used Multibus, Sun-3 used VMEbus, the Sun386i "Roadrunner" used AT-bus), so the cross-architecture payoff arrived later, when IEEE 1275-1994 standardized OpenFirmware and PCI allowed FCode in option ROMs. After that, the same expansion-card PROM image could boot on Sun SPARC, Apple PowerPC Macs, IBM PowerPC servers (CHRP), and the OLPC XO.
Interview with Mitch Bradley (he's like the Woz of Forth):
https://web.archive.org/web/20120118132847/http://howsoftwar...
In parallel with the OpenBoot work, Mitch also developed an extremely portable C-based Forth (the public version is "C Forth 93"). It runs a switch-threaded inner interpreter over packed tokens, with configurable cell width (16, 32, or 64 bit) and configurable token width (pointer-sized by default, 16 bit with the T16 build flag for tight flash budgets), plus a small hand-rolled FFI built around a fixed-arity 12-argument marshalling trampoline driven by a format string. It is now the embedded variant used in OLPC's OpenFirmware and in PlatformIO targets including RP2040, Teensy, ESP32, ESP8266, and STM32:
https://github.com/MitchBradley/cforth
OpenFirmware even has its own song:
https://www.youtube.com/watch?v=b8Wyvb9GotM
More on Mitch, OpenFirmware, and CForth:
https://news.ycombinator.com/item?id=21822840
https://news.ycombinator.com/item?id=33681531
https://news.ycombinator.com/item?id=38689282
Not totally... until people there run the 110 rule program, Conway's Life, Subleq+EForth...
- ACPI configuration for power management and platform stuff [1]
- Bitcoin transactions [2]
- TrueType fonts [3]
[1] https://wiki.osdev.org/AML
[2] https://en.bitcoin.it/wiki/Script
[3] https://learn.microsoft.com/en-us/typography/opentype/spec/t...
https://uefi.org/specs/UEFI/2.10/22_EFI_Byte_Code_Virtual_Ma...
I guess that is why you say re.Compile.
[1] https://dl.acm.org/doi/10.1145/363347.363387 -- Programming Techniques: Regular expression search algorithm
[2] https://swtch.com/~rsc/regexp/regexp1.html -- Regular Expression Matching Can Be Simple And Fast
[3] https://swtch.com/~rsc/regexp/regexp2.html -- Regular Expression Matching: the Virtual Machine Approach
[4] https://swtch.com/~rsc/regexp/ -- Implementing Regular Expressions
https://wiki.xxiivv.com/site/rewriting.html
C# is in the middle on this one, where specific features get compile-time support and regex is one of them: https://www.devleader.ca/2026/05/03/c-regex-performance-gene...
I have also built a C# source generator myself (XML parser generator), but the developer experience is a bit of a hill to climb compared to what it could be.
[1] https://github.com/fabiensanglard/Another-World-Bytecode-Int...
There is a tiny Java Bytecode VM in an insanely large list of places, you can find some of them here:
https://github.com/crocs-muni/javacard-curated-list https://en.wikipedia.org/wiki/Java_Card
Other game examples using VMs not for obfuscation: Z-machine and SCUMM-VM.
https://jxself.org/compiling-the-trap.shtml
I've got subleq+eforth (https://github.com/howerj/muxleq) running in JS which is dead simple to do. No input but I could output ASCII mapping values to an array.
https://esolangs.org/wiki/Subleq
So, yes. yt-dlp runs propietary Youtube JS code defying the original purpose.