Exercise 1: What LC-3 and the 6502/Z80 Would Need to Add — Possible Solution
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WHY THE CONCEPT CAN'T EXIST TODAY IN THESE ARCHITECTURES
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Per this chapter's own explanation, "an instruction is forbidden in
ring 3" requires the CPU itself to (1) track WHICH privilege level
the currently-running code is operating at, and (2) actively check
that tracked privilege level against a per-instruction rule before
executing certain instructions, refusing to run them and raising an
exception if the check fails. Neither LC-3 (per assembly1) nor the
6502/Z80 (per cpu8bit1) have ANY hardware state that tracks "what
privilege level is currently running" at all — there's no register,
flag, or mode bit anywhere in either architecture's own design that
records this.

WHAT WOULD NEED TO BE ADDED
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For LC-3 or the 6502/Z80 to support this concept, each would need,
at minimum:

1. Some new piece of hardware state recording a current privilege
   level (analogous to x86-64's own ring tracking) — none of these
   architectures' documented registers (LC-3's PC/registers/condition
   codes; the 6502's A/X/Y/SP/P; the Z80's main/shadow/index
   registers) serve this purpose at all.

2. A way to mark SPECIFIC instructions as privilege-restricted, and
   hardware logic that checks the current privilege level against
   that marking before allowing the instruction to execute.

3. A defined exception/fault mechanism to trigger when a restricted
   instruction is attempted at an insufficient privilege level — none
   of these chips have anything resembling x86-64's own General
   Protection Fault; the closest any of them come is the 6502's own
   distinct interrupt vectors (cpu8bit1-9), which respond to external
   or software-triggered events, not privilege violations.

WHY THIS WORKS AS AN ANSWER
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It identifies precisely which pieces of state and logic the concept
actually depends on (privilege tracking, per-instruction restriction
checks, a fault mechanism), confirms none of them exist anywhere in
either prior architecture's documented design, and lists concretely
what each would need to add before "forbidden in ring 3" could even
be expressed.