> At that time, the separation of of portable and non portable parts of an OS (CP/M ran on very different and fairly incompatible machines) was quite remarkable

> At that time, the separation of of portable and non portable parts of an OS (CP/M ran on very different and fairly incompatible machines) was quite remarkable On microcomputers, perhaps, but not on computers in general. Unix is a well-known example of an OS that did this, with the hardware-dependent parts mostly confined to a small amount of assembly language and much of the C source platform-independent. This was the state of Unix by around 1973 or so. And that was near the dawn of minicomputers. Even before Unix and minicomputers, it was starting to become common on mainframe computers as manufacturers started releasing revised and novel models (and dealing with the porting efforts). Notably, the Burroughs B5000 didn't have an assembly language at all, and system software (the Master Control Program, an awesome name that (perhaps unfortunately) lost the jargon skirmish with "Operating System") was written primarily in a dialect of Algol-60. The MCP went on to run on successor systems starting with the B6500 with only a small number of changes to support the new architecture, and Unisys's offerings to this day still run a continuously updated version of that original MCP -- first released in 1961.