Exercise 2: Why Evaluate Address / Fetch Operands Get Skipped — Possible Solution

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WHY ADD R1, R0, R2 SKIPS THEM
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ADD in register mode gets both of its operands (R0 and R2) directly
from the register file -- it never needs to compute a memory address
or read anything from memory at all. There's simply nothing for
Evaluate Address to compute, and nothing for Fetch Operands to fetch
FROM MEMORY, so both phases have no work to do and are skipped.

WHY LD R1, VALUE DOES NOT SKIP THEM
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LD's whole job is to load a value FROM MEMORY into a register. Before
it can do that, the CPU first has to figure out the actual memory
address that the label VALUE refers to -- that's exactly what
Evaluate Address computes. Only once that address is known can Fetch
Operands actually go read the value sitting at that address into MDR,
ready to be stored into R1 during Store Result.

THE GENERAL RULE
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Whether Evaluate Address and Fetch Operands run depends entirely on
whether the instruction needs to touch MEMORY for its operands, not
just on how "complicated" the instruction looks. Register-only
instructions (like ADD, AND, NOT in register mode) already have their
operands sitting in the register file, so both phases are skipped.
Any instruction that reads from or writes to memory (like LD, ST, and
the other memory-referencing instructions covered in assembly1-3) needs
Evaluate Address to compute WHERE in memory to look, and Fetch
Operands to actually retrieve the value from there.

WHY THIS WORKS AS AN ANSWER
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It explains both instructions using the chapter's own trace and its
LD contrast, then states the general rule in terms of the actual
mechanism (does this instruction need a memory address at all) rather
than a surface-level guess like "simple vs. complex instructions."