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Best[1] I could do was a non-networked forth that easily fit in a jumbo frame. Maybe one day I'll think of a trick to fit it in a normal MTU, but it's more likely that networking will widen in the meantime.

[1] I chose the constraint that CODE words were fine, but the kernel had to already contain the dictionary implementation: no "magic numbers" known to both the forth and kernel source. The motivation for that constraint was the "three instruction forth" which I would call a "monitor program" instead.