Classroom Glossary Public page

Week 9: Cousin Mapping, Retro to Modern

The NES exposed its PPU and APU via memory-mapped I/O registers; a modern cable modem like the Motorola SB6141 exposes its DOCSIS modem registers the same way. This week you draw the cousin-mapping diagram between the two.

Reading

  • nesdev.org wiki: PPU registers + APU registers. Free.
  • Altice, I Am Error, the section walking the Famicom memory map. Anchors the NES side of the mapping.
  • Bogost + Montfort, Racing the Beam, the Stella chip walkthrough. A different memory-mapped-I/O retro-anchor for breadth.

Lecture

Roughly three hours across two sessions. Key arc:

  • What memory-mapped I/O is. Why it dominates retro consoles and modern embedded systems both.
  • NES PPU register map at $2000 through $2007. What each register does.
  • NES APU register map at $4000 through $4017. The mirror trick.
  • SB6141 register surface, the practitioner version. RE-101 will dive in.
  • The cousin-mapping discipline. Document the parallels with precision; do not over-claim.

Lab pack

Lab Pack 9 produces the NES to SB6141 cousin-mapping diagram. See Lab Pack 9: NES to SB6141 Cousin Mapping.

Classroom tools

  • SPK-101 classroom: NES debugger R7, the live memory-map watchpoint. Use it to record which registers a real ROM actually touches in a frame.
  • Workbench: Bitplane decomposer for cross-reference.
  • A diagramming tool of your choice (the academy ships an ASCII-art template).

Architecture comparison sidebar

The NES's PPU control register at $2000 and the SB6141's downstream-channel-status register sit at very different scales but solve the same shape of problem: the CPU writes a byte to a fixed address and the peripheral changes its behaviour. The cousin-mapping discipline is not that the two are identical; it is that the family pattern crosses thirty years of silicon design and a student who sees it will reverse-engineer the SB6141 with the right intuition.

Reflection prompts

  1. Pick three NES PPU registers and three SB6141 registers and write the parallel.
  2. Where does the cousin-mapping break down? What does the SB6141 do that the NES PPU could not?
  3. If you wanted a cleaner cousin-mapping, what other retro console would you compare instead?

What is next

Module 10 is capstone delivery. Your homebrew ROM, your modified core, your demo, your report. The five-minute demo session is in the cohort-wide showcase.