The Model I

A Model I from a Radio Shack catalog

The Radio Shack TRS-80 Microcomputer System (later known as the Model I) was introduced by Radio Shack on August 3, 1977. It made computer history as one of the first mass marketed, fully assembled microcomputers. The initial price was $599, which included a typewriter-style (not membrane) keyboard and monitor. It ran a Zilog Z-80 at 1.77 MHz and came with only 4K of RAM, and a 4K ROM of what was later called Level I BASIC. Only uppercase characters could be displayed, and the only means of storage was a cassette recorder. Despite these limitations, the computer was enormously successful. The initial production run sold out in less than a month, and it took almost a year for Radio Shack to catch up with the demand.

Radio Shack sold many Model I upgrades, as did numerous other independent companies. These upgrades included a Microsoft Level II BASIC 12K ROM upgrade, lowercase modification, and a numeric keypad. Radio Shack later sold an external device called an expansion interface. Their version provided up to 48K of RAM and single-density disk drive support, among other things. Several other companies also sold their own expansion interfaces which had different capabilities. There even were hobbyist kit versions. Later on, double-density modifications to the expansion interfaces were produced. The de facto double density standard was created by a company named Percom in 1980, and was used by most companies making double-density boards. In 1982, Radio Shack came out with their own double density board that worked in a completely different manner from the Percom doubler.

The original Model I operating system was TRSDOS, written by Randy Cook. Like the unmodified Model I, it worked only in single density. The original versions of TRSDOS had speed and reliability problems, which were addressed with subsequent updates. The final updated version was TRSDOS 2.3.

Many people developed their own patches to TRSDOS to fix bugs and add new features. This process of patching spawned a thriving third-party market of new but TRSDOS compatible operating systems for the TRS-80. Among the most popular were DOSPLUS, LDOS, NEWDOS, and MULTIDOS. All of these were later modified for double-density support and were rewritten to run on the Model III.

Radio Shack released one other Model I operating system much later. This was TRSDOS 2.7, a double-density operating system designed for the Radio Shack doubler. This version of TRSDOS was not compatible with the other versions, and never achieved much popularity.

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