Exercise 3: RIP-Relative vs. LC-3's PC-Relative — Same Mechanism, Different Reason — Possible Solution
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WHAT THEY HAVE IN COMMON MECHANICALLY
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Both compute a memory address by adding an offset to the CURRENT
instruction's own address (or the address just past it), rather than
embedding a fixed, absolute address directly in the instruction.
Per assembly1-3, LC-3's LD used "PC + SEXT(offset9)"; per this
chapter, x86-64's RIP-relative mode uses "[RIP + offset]" — the exact
same underlying idea of expressing a target address relative to
wherever execution currently is, rather than as a fixed, standalone
number.

WHY LC-3 ACTUALLY NEEDED IT
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Per assembly1-3's own explanation, LC-3 needed PC-relative addressing
because its instructions were a FIXED 16 bits wide, with no room left
over to also embed a full, separate 16-bit address inside a single
instruction. PC-relative addressing was a way to reach memory despite
that hard bit-budget limitation.

WHY X86-64 ACTUALLY NEEDS IT
------------------------------
Per this chapter's own explanation, x86-64 does NOT have that same
bit-budget problem — its instructions are variable-length (established
back in assembly2-1's own lineage discussion), so embedding a full
address directly is a real, available option in a way it never was
for LC-3. x86-64 uses RIP-relative addressing instead for a completely
unrelated, distinctly modern reason: POSITION-INDEPENDENT CODE — a
program whose internal data references need to stay correct no matter
where in memory the operating system actually loads it, a real
security and shared-library requirement that simply didn't exist as a
concern for LC-3 or the 8-bit chips.

THE BROADER POINT
------------------------------
The same technical mechanism (relative-to-current-instruction
addressing) was independently useful to two very different
architectures, decades apart, for two genuinely unrelated reasons —
one a hard hardware constraint (LC-3's fixed instruction width), the
other a software-and-security requirement (x86-64's relocatable
code) that only became a real concern much later in computing's
history.

WHY THIS WORKS AS AN ANSWER
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It identifies the shared mechanism precisely (relative-to-PC/RIP
addressing), then explains each architecture's own DIFFERENT
motivating reason using each chapter's own stated explanation, rather
than treating the resemblance as coincidental or the reasons as
interchangeable.