Machine Language Disk I/O and Other Mysteries

written by Matthew Reed

Low-level floppy disk access was one of the most advanced programming topics on the TRS-80 Model I and III. The floppy disk controller was documented in technical reference manuals, as was the TRS-80 disk interface itself. But translating that information into reliable disk access routines wasn’t easy. All disk operating systems and low-level disk utilities did it, but the difference between working and excellent implementations was great. Kim Watt (with Super Utility) and Vernon Hester (with MULTIDOS) proved that it was possible to write fast and bullet-proof disk routines, but the methods they used weren’t documented anywhere.

Machine Language Disk I/O and Other Mysteries by Michael J. Wagner, volume 5 in IJG’s TRS-80 Information Series, was one of the few sources of reliable information about low-level TRS-80 floppy disk access. It cost $29.95 when it was released in 1982. A 1983 review by John B. Harrell, III in 80 Micro described it as an “expertly assembled, compact, and fact-filled book” and “a perfect reference source for any programmer who would like to try his hand at disk I/O programming.”

The book contains nine chapters and two appendices:

• Chapter 1 serves as an introduction to terms and principles.
• Chapter 2 shows how to select a floppy drive for access.
• Chapter 3 shows how to move the drive head.
• Chapter 4 shows how to read and write from the floppy.
• Chapter 5 ties together the concepts for a complete disk I/O routine.
• Chapter 6 shows how to use higher level TRSDOS disk routines.
• Chapter 7 shows how to use disk interrupts.
• Chapter 8 contains some “handy routines.”
• Chapter 9 contains miscellaneous information and the source code for S/OS, a “Small Disk Operating System.”
• Appendix I: “The Best Term Program in This Book,” a complete terminal program for the Model I.
• Appendix II: datasheets for the Western Digital disk controllers used by the TRS-80 Model I and III.

The first seven chapters contain information specific to the Model I, but include supplements showing differences with the Model III. Unlike some books that split Model III support into separate sections, the supplements here stand on their own. In particular, Chapter 7, describing floppy interrupts (rarely used on the Model I but common on the Model III), has a supplement longer than the main chapter.

The only major omission is discussion of double-density on the Model I. Programming the Percom Doubler or Radio Shack Doubler isn’t covered.

The preface mentions the author wrote his own disk operating system “to be released as EZ/DOS in the coming months.” The TRS-80 DOS market was already saturated, and no other mention of EZ/DOS has been found.