Not only that, the operating itself was configured by a process known as "sysgen". You relink the entire operating system with options set the way you wanted.

Not only that, the operating itself was configured by a process known as "sysgen". You relink the entire operating system with options set the way you wanted. It was generally a miserable slow process. At least on PDP-11 that's how it went for something like RSX-11. I believe same is true for early IBM mainframe operating systems like DOS 360- I think all programs had to be relinked because one option you had was to move things around in memory, and ancillary programs had to know the memory map. Even later: I wrote a device driver for Xenix: you got a link kit for the OS, you relinked it with your custom driver object file included. On CP/M you patched the running image (perhaps with DDT), then use the CP/M SYSGEN command to install it on a disk.