Retro-1620: A Digital Resurrection of the IBM 1620

This project, hosted on GitHub by pkimpel, provides a comprehensive web-based emulation environment for the IBM 1620 Model-2 computer system.

Project Mission: The primary objectives are to develop a browser-accessible emulator for the Model 2 variant and to recover as much legacy software for this system as possible.

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🖥️ Overview of the IBM 1620

The IBM 1620 was a transistor-based computer from the 1960s. It was specifically engineered as a cost-effective tool for engineering and scientific tasks. Unlike modern binary systems, it was a decimal machine featuring variable field lengths and magnetic-core memory.

Production History

Approximately 2,000 units were manufactured, split roughly equally between two iterations:

1. Model 1 (1959): The original release.
2. Model 2 (1962): A significant redesign that maintained object-code compatibility with the first model. Model 1 was the standard $\rightarrow$ Model 2 became the refined successor.

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⚙️ Technical Architecture

Memory and Data Structure

The system's memory ranged from a base of $20,000$ digits up to a maximum of $60,000$ digits. Data was handled digit-sequentially.

Digit Composition: Each single digit was composed of:

• A 4-bit Binary-Coded Decimal (BCD) value.
• A "flag" bit (used for field delimitation and arithmetic signs).
• An odd-parity "check" bit.

Instruction Format

Instructions were standardized at a length of 12 digits: OO PPPPP QQQQQ

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📊 Model Comparison: 1 vs. 2

The transition from Model 1 to Model 2 brought significant performance gains and hardware integration.

Model 2 Exclusive Options:

• Index Register Modification: Address modification using registers stored at 00300-00399 (replacing the Model 1 add table).
• Binary/Octal Support: Bit-wise operations, octal-to-decimal conversion, and binary paper tape I/O.

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🔌 Peripherals and Specializations

Hardware I/O

The 1620 supported various devices. Alphanumeric data was stored in memory as pairs of adjacent digits.

• Basic I/O: Typewriters and paper tape.
• Card Handling: The 1622 card reader/punch (adapted from the IBM 1401's 1402 unit).
• Advanced Peripherals:
  • 1443 Line Printer.
  • 1311 Disk Drive (utilizing removable packs).
  • 1627 Plotter.

Process Control

The 1620 served as the brain for the IBM 1710 and 1720 process-control systems. In these configurations, the machine gained:

• A multi-level interrupt system.
• A specialized set of additional instructions.

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💾 Software Ecosystem

The 1620 boasted a wide array of user-created and IBM-provided software.

• Languages: Most development occurred via FORTRAN II or the SPS assembler.
• Operating System: Systems equipped with the 1311 disk drive utilized a basic batch OS called "Monitor."

Project Status

• Create web-based Model 2 emulator
• Recover all available legacy software
• Expand documentation

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🔗 Resources & Links

License: This project is distributed under the MIT License.

{
  "repository": "pkimpel/retro-1620",
  "platform": "GitHub",
  "target_system": "IBM 1620 Model-2",
  "language_focus": "Decimal"
}