It uses Acorn's own ADFS filesystem from the BBC Micro, which was released in 1981 if I remember correctly. MS-DOS and the IBM PC were new that year and not any

It uses Acorn's own ADFS filesystem from the BBC Micro, which was released in 1981 if I remember correctly. MS-DOS and the IBM PC were new that year and not any kind of standard yet. The Acorn MOS and PC DOS are contemporaries. It would be a decade before Linux 0.01 existed, 14 years before FAT32. https://en.wikipedia.org/wiki/Advanced_Disc_Filing_System https://en.wikipedia.org/wiki/Advanced_Disc_Filing_System It may look esoteric to you now, but it was a solid pragmatic choice in 1987 when this OS shipped, the same year as OS/2 1.0 and thus rather before HPFS which trailed OS/2 itself by several years as I recall.