written by Matthew Reed

Title page of the TRS-80 Editor/Assembler manual

Title page of the TRS-80 Editor/Assembler manual

The TRS-80 Editor/Assembler (catalog number 26-2002), better known as EDTASM, was an assembly language programming tool for the cassette TRS-80 Model I. It was introduced in 1978 and cost $29.95. EDTASM was such a common tool that its name (and associated file format) became a generic term for TRS-80 assembler1.

EDTASM was written by Mark Chamberlin of Microsoft and licensed to Radio Shack. A Radio Shack catalog stated:

Microsoft, an industry leader in systems software, has developed this program … so you can expect the ultimate in editing features.

Like the T-BUG debugger that it was frequently paired with, EDTASM worked on Model I computers with either Level I and Level II BASIC. It managed this by including two cassettes in the package. One contained the EDTASM program and could be loaded directly into Level II BASIC. The other was a cassette named “SYSTEM” that contained a short Level I program that could load the EDTASM cassette itself. EDTASM did require 16K, which was a fairly unusual Level I configuration.

The EDTASM manual stated:

In brief the Editor/Assembler is designed for a user to type in source assembler code. The source code is assembled and the resulting object code may be recorded onto tape.

One of the best features of EDTASM was its manual. In addition to an overview of the EDTASM program, it also included the complete documentation for the Zilog Z80 processor used in the Model I.

Radio Shack never offered a disk version of the original EDTASM. They did sell the unrelated Disk Editor/Assembler (catalog number 26-2202) for $99.95. Although its name would suggest that it was a disk version of cassette EDTASM, it was actually a TRS-80 version of Microsoft’s M80 macro assembler package.

A patched (and unlicensed2) version of EDTASM was included in the NEWDOS operating system. Since NEWDOS was the most popular of the early TRS-80 disk operating systems, this meant that the NEWDOS version of EDTASM was possibly the most widely used.


The name EDTASM was used by several other programs for the TRS-80:

  • Microsoft Consumer Products sold Microsoft Editor/Assembler-Plus, also known as EDTASM-PLUS or EDTASM+, which was an enhanced version of EDTASM. It was written by Mark Chamberlin and William Yates. Among many other improvements, EDTASM-PLUS added the Z-BUG debugger. It required a 16K Level II Model I. Radio Shack never sold EDTASM+.
  • Radio Shack sold another Editor/Assembler for the Color Computer in multiple versions: EDTASM on cartridge (catalog number 26-3250) for $49.95 and Disk EDTASM (catalog number 26-3254) for $59.95. This was also written by Microsoft but would have shared no code with the Model I version.
Cover of the EDTASM-PLUS manual

Cover of the EDTASM-PLUS manual

In 1981, Radio Shack released DEBUG (catalog number 26-2000) to replace T-BUG, and the Series 1 Editor/Assembler to replace EDTASM. The cassette version of the Series 1 Editor/Assembler (catalog number 26-2011) cost $29.95 and the disk version (catalog number 26-2013) cost $34.95. Unlike EDTASM and T-BUG, the Series 1 Editor/Assembler and DEBUG would run on both the Model I and the Model III.

The Series 1 Editor/Assembler was an upgrade to EDTASM, described in the manual as being “derived from original Tape Editor/Assembler.” But its filename remained EDTASM and I suspect many people using it never realized they weren’t using the original EDTASM.

  1. Anthony Wood, the technology billionaire who founded Roku, has his personal website at edtasm.com. His first company, AW Software, sold software for the TRS-80. ↩︎

  2. There are many reasons why Radio Shack might not have wanted to sue Apparat (the creators of NEWDOS) for copying TRSDOS code, but I have never understood why they (or Microsoft) didn’t sue over copying the EDTASM program. ↩︎

Categories: Programming, Software