Editing Sinclair QL Microdrive Troubleshooting Guide (section)

== How a Microdrive works ==

[[File:Sinclair QL Microdrive cartridge.jpg|thumb|360px|A QL Microdrive cartridge. Inside is an endless loop of magnetic tape running over a felt pressure pad.]]

A Microdrive cartridge holds a '''continuous loop''' of magnetic tape. The drive's motor turns a rubber '''drive roller''' (a capstan) that presses against the tape via a pinch wheel in the cartridge, pulling the loop past the read/write head. There is no rewind: to reach a given sector the drive simply waits for it to come round again.

Formatting writes '''sector headers''' (26 bytes, containing the sector number and medium name) and '''data blocks''' (512 bytes plus a checksum byte), separated by blank '''gaps'''. The gaps matter: the drive electronics use them to tell real data from tape noise, and the ZX8302 raises an interrupt as each gap passes. Sectors are written in descending order from 255, and because the tape is a loop the higher numbers get overwritten — the highest surviving sector number is what QDOS reports after <code>FORMAT mdv1_</code>. A healthy cartridge formats to a little over 200 sectors.<ref name="froschle">Fröschle, Tobias. ''Microdrives — The Inner Workings'', QL Forum; hosted in the Sinclair QL Homepage archive at the Internet Archive. Source for the sector/header/gap format, the formatting behaviour and the ZX8302 microdrive register map.</ref>

Sector headers are '''never rewritten''' after formatting, which is why a cartridge that formats to a low sector count should be re-formatted or retired rather than patched.<ref name="froschle" />

The drives are controlled through the ZX8302 at registers $18020 (control/status), $18021 (interrupt control) and $18022/$18023 (data, tracks 1 and 2). Drives are selected by clocking a token along a shift chain — set the select bit, then toggle the clock bit once per drive number. Once selected, the motor runs.<ref name="froschle" />