Here are some examples of conventional operating system
components that have been naturally incorporated into
the Smalltalk language:
[...]
Display handling -- The display is simply an instance of
class Form, which is continually visible, and the
graphical manipulation messages defined in that class are
used to change the visible image.
[...]
Debugger -- The state of the Smalltalk processor is
accessible as an instance of class Process that owns a
chain of stack frames. The debugger is just a Smalltalk
subsystem that has access to manipulate the state of a
suspended process. It should be noted that nearly the
only run-time error that can occur in Smalltalk is for
a message not to be recognized by its receiver.
Smalltalk has no "operation system" as such. The
necessary primitive operations, such as reading a page
from the disk, are incorporated as primitive methods in
response to otherwise normal Smalltalk messages.
Some relevant excerpts:
This is from 1981, BTW.