Exercise 2: Two Feature Removals, One Shared Category — Possible Solution ==================================================================== WHY THE 2A03 REMOVED DECIMAL MODE ------------------------------ Per this chapter's own explanation, the Ricoh 2A03 (the NES's 6502-derived CPU) is widely reported to have had decimal mode — cpu8bit1-3's own D flag, used for binary-coded decimal arithmetic — deliberately removed specifically to avoid paying a licensing fee tied to that particular piece of circuitry. This wasn't a technical limitation or a design mistake; it was a direct COST decision, made possible because the NES's own use case (games) genuinely didn't depend on decimal-mode arithmetic the way, say, a point-of-sale or accounting application might have. WHY THE LR35902 REMOVED THE SHADOW REGISTER SET ------------------------------ Per this chapter's own explanation, the Game Boy's Sharp LR35902 (a Z80 derivative) removed the ENTIRE shadow register set and IX/IY — exactly the feature cpu8bit1-6 and cpu8bit1-9 established as one of the Z80's own genuinely unique, real advantages (fast interrupt entry via EXX). Removing that much real capability strongly suggests a similar underlying motivation to the 2A03's own decision: cutting something costly (in transistors, and potentially licensing) that the Game Boy's own specific use case didn't strictly require to function. WHAT THE TWO REMOVALS HAVE IN COMMON ------------------------------ Both are the SAME category of decision: a company building a cost-sensitive, mass-market gaming device took an existing, capable CPU design and deliberately stripped out a specific feature — not because the feature didn't work, but because that specific product's requirements didn't need it, and removing it saved real cost (whether transistors, licensing fees, or both). It's the same underlying force this course traced all the way back to cpu8bit1-2's own account of the original 6502's $25 price target — a design constraint driving a feature decision — just showing up a second time, one hardware generation later, applied by a DIFFERENT company to an EXISTING chip rather than during the chip's own original design. WHY THIS WORKS AS AN ANSWER ------------------------------ It states the specific documented reason for each removal individually (licensing avoidance for the 2A03, implied cost/complexity reduction for the LR35902), then names the shared underlying category (a cost-driven feature removal for a specific mass-market product) rather than treating the two facts as an unexplained coincidence.