Exercise 1: Which Bits of RAX Each Sub-Register Write Touches — Possible Solution
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USING THE CHAPTER'S OWN BIT DIAGRAMS
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RAX spans the full 64 bits: bits 63 down to 0.
EAX spans the low 32 bits: bits 31 down to 0.
AX spans the low 16 bits: bits 15 down to 0.
AH spans bits 15 down to 8 (the upper half of AX).
AL spans bits 7 down to 0 (the lower half of AX).

WRITING TO AL
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Only bits 7 through 0 of RAX are affected. Every other bit in RAX
(bits 8 through 63, including AH's own bits 15-8) is left completely
unchanged.

WRITING TO AX
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Bits 15 through 0 of RAX are affected — this covers both AH (bits
15-8) and AL (bits 7-0) at once, since AX is defined as those two
bytes combined. Bits 16 through 63 of RAX are left completely
unchanged.

WRITING TO EAX
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Bits 31 through 0 of RAX are affected directly by the value being
written. But per this chapter's own warn-box, writing to a 32-bit
sub-register ALSO zeroes out bits 63 through 32 — so a write to EAX
affects bits 31-0 with the actual new value, AND forces bits 63-32 to
all become 0, even though the instruction's own operand only supplied
32 bits of new data.

WHY THIS WORKS AS AN ANSWER
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It maps each sub-register name to its exact bit range using the
chapter's own diagrams, and correctly distinguishes AL/AX (which only
ever touch their own bit range, leaving everything above untouched)
from EAX (which touches its own 32 bits AND additionally zeroes the
32 bits above it) — the precise asymmetry the chapter's warn-box
describes.