Lexikon's History of Computing

TRS DOS - Tandy-Radio Shack Disk Operating System

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The following article is courtesy of Kirk L. Thompson, reprinted here with permission and slight editing. Copyright (C) 1995 Kirk L. Thompson.

Background and Genesis of TRSDOS

TRSDOS was originally written for Tandy-Radio Shack's TRS-80 Model I in 1978 when a 5-1/4 inch floppy disk subsystem was added as a peripheral to the model. Like many microcomputers of the period, the Model I and its successors has read-only memory (ROM) at the bottom of addressable memory. This is composed of an interpreter for the BASIC language, core functions, the keyboard matrix, and memory-mapped video. The size of the ROM is 16K, hence TRSDOS can handle up to 48K of free memory.

Versions of the operating system were also developed for the TRS-80 Models II, III, and 4 when those systems were introduced. But due to the changes in the hardware and system software over the years, earlier versions weren't entirely compatible with later ones. However, newer versions included utility programs to copy and adapt older files and programs to the new version. The description below is based on that for the Model III.

The operating system is organized into permanently resident and overlay modules and device drivers. The first is composed of the command-line interpreter, input/output routines, tables and other core routines located just above the ROM at the bottom of free memory. This permanently-resident module occupies 4K. The overlay modules are swapped in and out of memory as needed, using the floppy disk as virtual memory.

Device driver sub-programs are loaded into memory as needed and are used to control non-system peripherals. These are mainly parallel- and serial interfaced printers.

The standard floppy disk drive for TRS-80 systems is single-sided, double-density 5-1/4 inch. Each diskette has some 130K of storage available. TRSDOS can handle up to four drives. The business-oriented Model II uses single-sided, double-density 8-inch drives.

TRSDOS is designed with business applications in mind. Diskettes and individual files can be password protected to control access to them. A legal file reference under TRSDOS could be: A1234567/TXT.PASSWORD:1 The file name is one to eight characters long, composed of only alphanumeric characters, and begins with an alpha. The extension (or file type) is separated from the name by a slash, and is three characters long, can be any alphanumeric, and is optional. The extension is separated from the optional password by a period. The password may be up to eight characters, is alphanumeric, and begins with an alpha.

A colon separates the file name/extension.password from the disk drive number. This number may range from 0 to 3, depending on the number of drives installed. Executable programs always have the extension "CMD," such as FORMAT/CMD, a system utility.

For program development, the system also provides a BASIC interpreter, loaded like an application from disk. For most of its operation, this interpreter uses the built-in ROM BASIC, thus reducing the amount of memory it occupies. Disk-oriented enhancements bring it up to the standard of Microsoft's BASIC dialect of the period. Tandy-Radio Shack and third-party vendors provided other languages, such as a Zilog Z-80 assembler and compilers for Basic, COBOL, FORTRAN, and Pascal.




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