[pdevine]

I understand that significant parts of the OS code were burned into ROM in order to free up RAM, which was more expensive. Code for things like the window manager, etc. Later versions of the OS patched bugs in the ROM by loading the patches into RAM. A rather inventive design at the time.

[Someone]

Not only later versions. The original ROM already was patched: http://www.folklore.org/StoryView.py?project=Macintosh&story.... Mac OS was the only OS I know of that had a system call for replacing system calls.

[pornel]

AmigaOS had "library vectors" (see Exec.library SetFunction) for patching ROM libraries too.

[leoc]

The 128KiB of RAM was painfully insufficient even with so much shifted to ROM: text files couldn't be more than a few pages in length. And there was no backing store you could reliably swap OS components into either, because the machine had a single floppy drive and no hard disk, and the floppy disk had to come out to allow disk copying.