Name Real CPUs Built on Microcode Rom and the Boundary Against the 4001
# Name Real CPUs Built on Microcode ROM and the Boundary Against the 4001
## 1. Name Historically Documented Microcoded Machines
IBM's System/360 family, announced in 1964, is a widely documented example of a computer architecture whose different models, despite presenting one common programmer-visible instruction set across the whole line, implemented that instruction set internally through model-specific microcode control stores, letting IBM build cheaper, slower models and more expensive, faster models that still ran the same software, because the microcode (not the exposed instruction set) was where each model's own hardware-specific execution sequence actually lived. Many minicomputer and early microprocessor designs through the 1970s and 1980s likewise relied on microcode ROM internally to implement a rich or variable-length instruction set without requiring a correspondingly large amount of fixed random logic for every instruction.
## 2. State the Boundary Against the 4001 Precisely, One Final Time
The 4001, covered fully in `microprocessor_1971_step06`, is a program ROM: the programmer decides what sequence of 4004 instructions it holds, and the 4004 fetches that sequence one instruction at a time using an address the programmer's own program counter advances. A microcode ROM inside a microcoded CPU is a control store: the CPU designer decides what sequence of internal control signals it holds for each possible opcode, the programmer never sees or addresses it, and it executes entirely underneath whatever instruction the programmer's own program, stored wherever that program lives, happens to have fetched. Both are mask-fixed bit patterns read by row-select addressing, exactly as covered in Steps 2–4 — but they sit at two different, non-overlapping levels of a computing system, and conflating them would understate how differently mask ROM was actually used across this era.
Step 14 closes this series by naming, without retelling, how its addressing-architecture and applications coverage completes the mask-ROM picture alongside `microprocessor_1971` and `eprom_eeprom_history`.