This is stolen from the manual for MCX11 which I had the pleasure of porting to different micros and different languages (the original source is in assembly). The emphasis is mine:

"A real-time executive provides a software framework within
which different processes can operate and gain access to
various system resources. Real-time systems usually consist
of several processes, or tasks, which need to have control
of the system resources at varying times due to the
occurrence of external events. These tasks are at various
times competing for system resources such as memory,
execution time, or peripheral devices. They range from being
compute bound to I/O bound. Tasks which are I/O or compute
bound cannot be allowed to monopolize a system resource if
a more important function requires the same resource. There
must be a way of interrupting the operation of the task of
lesser importance and granting the needed resource to the
more important task.

A multitasking real-time executive promotes an orderly
transfer of control from one task to another such that
efficient usage of the computer's resources is achieved.
Orderly transfers require that the executive keep track of
the needed resources and the execution state of each task so
that they can be granted to each task in a timely manner.
The key word there is timely. A real-time system which does
not perform a required operation at the correct time has
failed. That failure can have consequences which range from
the benign to the catastrophic. Response time to a need for
executive services and the execution time of such services
must be sufficiently fast enough so that no need goes
undetected.

One way to achieve timeliness is the assignment of a
priority to each task. The priority of a task is then used
to determine its place within the sequence of execution of
all tasks. Tasks of low priority may have their execution
pre-empted by a task of higher priority so that the latter
can perform some time critical function.

An event can be any stimulus which requires a reaction from
the executive or a task. Examples of an event would include
a timer interrupt, an alarm condition, or a keyboard input.
Events may originate externally to the processor or
internally from within the software. An executive which
responds to these events as the stimuli for allocating
resources is said to be event driven. If response time to
any event occurs within a period of time which can be
accurately defined and guaranteed, the executive can be said
to be deterministic.

By these definitions, MCX11 is a deterministic, event
driven, multitasking, real-time executive."


ESP Mag did a series on this a long time ago (Feb. 1990). I don't know if the articles are available online, but certainly on CD. I think today the term "multitasking OS" also implies some kind of memory protection, but the above uses "executive" as a replacement term anyway.

FWIW it worked brilliantly on the HC11 and the only thing the HC11 has going for it that the 8051 does not, is that the stack pointer is not limited to a puny memory space.

GB