[obarthel]

The Amiga default file system was not quite that sophisticated. As used on floppy disks, it did well by offering more than one option to reconstruct the layout of the file system data structures. For example, the disk data blocks were chained, so that you could follow this chain and find every block belonging to the same file. But you could also follow the chain of the associated file list blocks and obtain the same information.

Which disk blocks were still available for allocation was tracked by the bitmap (never documented in the original AmigaDOS Manuals), a series of blocks in which each bit stood for an individual block which was either allocated or not. There is just one bitmap for the entire volume and the Amiga default file system did not care for redundancy. The bitmap does not even use block checksums because you could reconstruct it at any time by following the directory structures (unless the bitmap was corrupted and spread its troubles, which you would never suspect or know). This is what, as a by-product, the Disk-Validator accomplished in the Kickstart 1.x days.

The floppy disk version of the Amiga default file system made use of checksums for each of its data structures, with the exception of the bitmap. This made it slow going, but then you quickly learned of defects which the file system reported.

Fun fact: the Amiga default file system in the Kickstart 1.x days was particularly and likely needlessly slow during directory scanning. The metadata produced by the scanning API would include the number of data blocks which a file would consist of. The file system could have easily calculated that figure from the file size, but it did something else instead: It visited every single data block, counting their number one at a time. If you ever wondered why it took Workbench so incredibly long to read the contents of a volume or drawer until at last one single icon appeared, this is why. On the other hand, scanning a directory automatically verified that all of the file data in this directory was sound.

To the best of my knowledge, the Amiga default file system never paid any attention to the disk head position. The file system was designed for larger mass storage devices (up to 54 MBytes in the Kickstart 1.x version, with 512 bytes per block), not so much for floppy disks. The Amiga disk drive, however, could end up recalibrating the disk head position as needed, which could be mistaken for a return to a parking position, if you will.

The Amiga default file system would use write caching, with the lit floppy disk LED indicating that the cache had not yet been pushed to disk. The disk would keep on spinning for some three more seconds after the last write command had been executed. But if you used a third party floppy disk drive which had the LED indicator tied to the read head actuator instead of the spindle motor, you were likely to remove the disk when the buffer had not been flushed yet.

[mrandish]

Thanks for the fascinating detail and trip down memory lane!

It's been a few decades since I used Amiga OS as a daily driver, so I must have been misremembering, or more likely, conflating my recollections with aspects of other disk operating systems which struck me as new or interesting around the same time.