C64 MPS-801 Printer Audit

Core Workflow

Use this skill to audit whether EMULATORS models a Commodore MPS-801 at the intended fidelity level.

1. Read references/mps801.md before making claims or edits.
2. Identify the intended behavior tier before judging correctness:
   • Basic text sink: validate printable character filtering, uppercase/lowercase mode, line breaks, double width, output file semantics.
   • PostScript glyph model: validate 6x7 glyph data, graphics bytes, reverse mode, dot positioning, tab positioning, line/page geometry, and required PSMPS801MatrixPrinter*.ps files.
   • Serial-printer peripheral: validate KERNAL-trap LISTEN/OPEN/CLOSE/SEND routing, device number behavior, secondary addresses, and active-function lifetime.
   • Hardware/firmware model: only require internal CPU, RAM/ROM, bus timing, buffer, and mechanical behavior if the implementation explicitly claims that tier.
3. Inspect local implementation paths first:
   • include/COMMODORE/SerialPrinterMPS801.hpp, src/COMMODORE/SerialPrinterMPS801.cpp
   • include/CORE/MatrixPrinterEmulation.hpp, src/CORE/MatrixPrinterEmulation.cpp
   • include/COMMODORE/SerialPrinters.hpp, src/COMMODORE/SerialPrinters.cpp
   • include/COMMODORE/SerialIOPeripherals.hpp, src/COMMODORE/SerialIOPeripherals.cpp
   • include/COMMODORE/IOPBuilder.hpp, src/COMMODORE/IOPBuilder.cpp
   • include/C64/StdSerialPrinter.hpp, src/C64/StdSerialPrinter.cpp, and the VIC-20/C264 equivalents when host printer registration matters.
   • PSMPS801MatrixPrinterI.ps and PSMPS801MatrixPrinterII.ps copies under projects/, setups/, emulators/, and monitors/ when PostScript rendering is involved.
4. Compare against .codex/docs/MPS801.pdf first. Use the external URLs in references/mps801.md to verify historical/spec claims when internet access is available or when the user asks for web confirmation.
5. When changing C++ code, also use $emulators-framework-cpp-style.
6. When host-side C64 CIA2 IEC wiring, not just KERNAL traps, is involved, also use $c64-cia-audit.
7. Report whether each issue is a bug inside the current fidelity tier or a missing higher-fidelity feature.

Audit Priorities

Prioritize correctness in this order:

• Scope honesty: distinguish an output-file printer simulation from a firmware, timing, or mechanism-accurate MPS-801.
• Device and channel behavior: device selector positions 4/5/T, default printer device use, secondary address 0 for cursor-up graphics/uppercase mode and 7 for cursor-down upper/lowercase mode, and active function reset on LISTEN/CLOSE.
• Character sets: PETSCII to printer-code handling, cursor-up and cursor-down tables, business mode, graphical characters, reverse field, and use of the MPS801 ROM-derived glyph comments in SerialPrinterMPS801.cpp.
• Control codes: 0x08 graphics mode, 0x0a line feed, 0x0e double width, 0x0f standard width, 0x10 tab/position setup, 0x11 cursor down, 0x12 reverse on, 0x1a repeat graphic data, 0x1b dot address setup, 0x91 cursor up, and 0x92 reverse off.
• Graphics and positioning: 7 vertical printable dots, 480 horizontal dot positions, dot-address limits, tab behavior, graphic-byte interpretation, repeat-graphics behavior, and automatic print behavior at line/buffer limits.
• Page geometry: 80 columns, 6 LPI in character/double-width modes, 9 LPI in graphics mode, 66 lines on 11-inch paper, paper border handling, and PostScript scale constants.
• Basic vs PostScript parity: document any behavior supported only in MPS801PostscriptMatrixPrinterEmulation and not in MPS801BasicMatrixPrinterEmulation.
• Compatibility limits: do not require 8039 firmware execution, 90-byte line-buffer timing, unidirectional head timing, motor/hammer mechanics, or serial-bus electrical timing unless the implementation claims those features.

Validation Hints

Prefer focused probes:

• BASIC/KERNAL snippets that OPEN the printer on device 4 or 5, choose secondary addresses 0 and 7, send PETSCII uppercase/lowercase text, and CLOSE the channel.
• Byte streams that exercise each control code, especially mixed double width, graphics mode, reverse field, tab/dot positioning, and return to standard mode.
• Output-file assertions for basic mode and PostScript token checks for glyph definitions, page setup, drawByteBits, page breaks, and missing template-file logging.
• Tests that separate serial-routing failures from matrix-printer rendering failures.