Exercise 2: The Three Jobs DJNZ Combines — Possible Solution
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THE THREE SEPARATE JOBS DJNZ COMBINES
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1. DECREMENT — it subtracts 1 from register B, exactly like a
   standalone "decrement B" instruction would.

2. COMPARE (against zero) — it checks whether that decremented
   result is zero, functioning like a comparison against zero without
   needing a separate CPX-style instruction.

3. CONDITIONAL BRANCH — it jumps back to the loop's target label if
   the decremented result was NOT zero, and falls straight through to
   the next instruction if it WAS zero.

All three of these happen as the effect of executing exactly one
instruction.

THE CONSEQUENCE FOR THE "7 VS. 3" COMPARISON
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On the 6502 side, this chapter's own loop needed three completely
separate instructions to accomplish the same three jobs: INX
(decrement/increment — the counting step), CPX #5 (the comparison
step), and BNE LOOP (the actual conditional branch step). Each of
those instructions has to be individually fetched, decoded, and
executed as its own separate step in the fetch-decode-execute cycle
(assembly1-2/cpu8bit1-2's own shared foundation).

DJNZ collapses exactly those same three logical steps into a single
fetch-decode-execute pass. This is precisely why the loop-management
portion of the Z80's loop body is so much shorter in instruction count
than the 6502's — it isn't that the Z80 is somehow "skipping" any of
the three jobs, it's that one Z80 instruction does the combined work
that took three separate 6502 instructions to express.

WHY THIS WORKS AS AN ANSWER
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It names each of the three jobs DJNZ performs individually rather
than describing it as a vague "shortcut," and explicitly maps each
job back to its own separate 6502 instruction (INX, CPX, BNE) to show
precisely why the instruction-count gap exists.