Browser-based emulator revives the IBM 1620 Model 2 and its 1960s software

A 1962 scientific computer in a browser tab

An open-source project on GitHub, which reached the front page of Hacker News, delivers a browser-based emulator for the IBM 1620 Model 2, a transistorized scientific computer from the early 1960s. According to the project's repository, the effort has two stated goals: building a web-based emulator for the Model 2 variant of the system, and recovering as much surviving software for the machine as the team can find. The code is released under the MIT license, and the project points to a separate hosting site where the emulator can be run without installing anything. The Hacker News headline frames the offering around the emulator and IPL-V, a 1963 operating environment for the 1620.

An architecture with no registers

The IBM 1620 was a decimal, variable-field-length machine with magnetic-core memory, designed as an inexpensive system for scientific and engineering work. IBM released the original Model 1 in 1959 and the object-code-compatible but significantly redesigned Model 2 in 1962. About 2,000 systems were built in total, roughly split between the two models, and IBM supported the line until 1970.

Its design has little in common with modern hardware. The system used a two-address, memory-to-memory architecture with no software-accessible registers. Every instruction was a fixed 12 digits long: a two-digit operation code, a five-digit P address that usually held the destination operand, and a five-digit Q address or literal that usually held the source. Memory stored 20,000 digits by default and could grow to 60,000, with each digit made up of a four-bit binary-coded decimal value, a fifth flag bit used for the arithmetic sign and as a field delimiter, and an odd-parity check bit. All arithmetic and data movement processed one digit at a time.

Most striking of all, the Model 1 performed addition and multiplication through a lookup table held in memory rather than with dedicated arithmetic logic. Hardware division, floating-point arithmetic and indirect addressing were optional extras on that first model.

What the Model 2 changed

The Model 2 halved the memory cycle time to 10 microseconds per two digits, down from the Model 1's 20, and optimized many instructions to process two Q-operand digits in a single fetch. Addition moved into hardware, though multiplication still relied on the lookup table. Hardware division and indirect addressing became standard, while floating-point remained a paid option. The repository also describes two further Model 2 options: index register address modification, with the index registers stored in memory at addresses 00300-00399 where the Model 1's addition table had lived, and support for binary, in practice octal, bitwise operations, along with octal/decimal conversion and binary paper tape I/O.

Peripherals, process control and software

Early systems supported only paper-tape and typewriter input and output, with the 1622 card reader and punch, derived from the IBM 1401's 1402 unit, arriving soon after launch. Further peripherals included the 1443 line printer, the 1311 disk drive with removable disk packs, and the 1627 plotter. The typewriter, paper tape, punched card and printer devices all handled alphanumeric data as pairs of adjacent digits in memory.

Beyond the machine room, the 1620 also served as the computing and control component of the IBM 1710 and 1720 process-control systems, a role that added extra instructions and a multi-level interrupt system. Software for the platform was extensive; most programming used the SPS assembler or FORTRAN II, and systems equipped with the 1311 disk drive could run Monitor, a simple batch operating system.

Why it matters

The project turns a machine that survives mainly in documentation into something anyone can start up in a browser tab, with no installation and a permissive license to back reuse. The software recovery goal matters as much as the emulation itself: much of what was written for the 1620 exists only on decaying media or in scattered archives, and a working emulator gives that material a way to run again. For engineers, the 1620 is also a useful contrast to current designs, a fully working system with no registers, digit-sequential decimal arithmetic and table-driven addition. The project links out to scanned manuals and software archives at bitsavers for anyone who wants to explore the original material in depth.