Exercise 3: Choosing the Right Addressing Mode for a Runtime Array Index — Possible Solution
====================================================================

THE SCENARIO
------------------------------
The array's starting address is already sitting in R3 at runtime, and
we need to read its 4th element, where the exact index isn't known
until the program actually runs.

THE RIGHT MODE: BASE+OFFSET (INDEXED)
------------------------------
LDR R1, R3, #3   ; address = (value in R3) + 3

Base+offset is built exactly for this situation: it computes an
effective address as a chosen register's current value plus a small
offset. R3 already holds the array's base address, and #3 selects the
4th element (0-indexed), matching the chapter's own description of
base+offset as "the mode that makes arrays and data structures
practical."

WHY THE OTHER THREE MODES FAIL HERE
------------------------------
- Immediate: has no addressing step at all -- it embeds a VALUE
  directly in the instruction, not an address to look up. It can't
  express "go read whatever is at this computed location" at all.

- PC-relative: computes its address relative to the PC -- a fixed
  point tied to where the INSTRUCTION lives in the program, not to
  wherever the array happens to sit in memory. It has no way to
  incorporate a runtime value like the one sitting in R3.

- Indirect: also computes its pointer address relative to the PC
  (PC + offset), then follows a pointer stored in memory at that
  fixed location. Like PC-relative, it has no mechanism to combine
  with a register's current runtime value the way base+offset does --
  it would require the array's address to already be stored at some
  fixed, predetermined memory location, not held dynamically in R3.

WHY THIS WORKS AS AN ANSWER
------------------------------
It picks base+offset using the chapter's own description of its
purpose, gives the exact instruction with the correct register and
offset, and explains specifically why each of the other three modes
lacks a mechanism to incorporate a value that's only known at
runtime and held in a register -- not just a generic "they don't fit."