Exercise 1: The Six Phases & What MAR/MDR Hold After Fetch — Possible Solution
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THE SIX PHASES, IN ORDER
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1. Fetch
2. Decode
3. Evaluate Address (only if needed)
4. Fetch Operands (only if needed)
5. Execute
6. Store Result

WHAT MAR AND MDR HOLD WHEN FETCH COMPLETES
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Per the chapter's own Fetch trace:

  MAR <- PC
  MDR <- mem[MAR]
  IR  <- MDR
  PC  <- PC + 1

By the moment Fetch finishes:

- MAR holds the memory ADDRESS of the instruction that was just
  fetched -- specifically, whatever the PC's value was at the start
  of this cycle, before it got incremented.

- MDR holds the actual INSTRUCTION BITS that were read out of memory
  at that address -- the same bits that get copied into IR right
  afterward. By the end of the Fetch phase, MDR and IR technically
  hold the same value, since IR <- MDR was the very next step.

So MAR is "where we just looked," and MDR is "what we found there" --
which then gets handed off to IR so the Decode phase has something to
actually examine.

WHY THIS WORKS AS AN ANSWER
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It lists the six phases in the chapter's own stated order, and
explains MAR/MDR's contents by walking through the exact four-step
Fetch sequence the chapter itself gives, rather than describing them
in the abstract.