Notes on the Turbo Modula-2 "Monitor" Function
by D. McCord, Echelon
1. Overview
Turbo Modula-2 possesses, as per the Modula-2 language definition, the
feature of "coroutines". Coroutines are an important feature of Modula-2, as
they incorporate into the language itself the concepts of concurrency -
multiple software processes executing, apparently or actually, at the same
time. This is a quite useful feature for situations where there may be more
than one central processing unit (CPU) available, or when a situation of
needing multiple programs operating simultaneously occurs.
Of course, in the context of Turbo Modula-2, which operates on a single Z80-
compatible microprocessor, multiple CPU's are not a consideration. But the
capability of using multiple Modula-2 programs simultaneously can be quite
useful. A good example of this is software which utilizes hardware interrupt
functions. Turbo Modula-2 implements generic Modula-2 features, via
coroutine-oriented functions, to provide interrupt handlers which operate in
a clearly defined, yet autonomous manner from whatever the Modula-2 "main"
module may be doing. This is quite unusual among high level languages -
typically, interrupt handlers are done in assembly language and must be
"kludged" into an application written in a high level language such as
Modula-2.
In an interrupt environment, it is sometimes necessary to control the state
of the host CPU's interrupt handling. In the case of Turbo Modula-2, this
control is best thought of as simply enabling or disabling interrupts, and
the generic Modula-2 "monitor" function is implemented in order to provide
this function.
This note is not intended to be a tutorial on usage of interrupts,
coroutines, and interrupt handlers with Turbo Modula-2. For more detailed
information on those topics, see Steve Hirch's informative notes contained in
the file TM2NOT10.LBR, available from Echelon on SUS disk or on most Z-Node
Remote Access Systems.
This note is intended to supplement the information given in the product
documentation regarding the operation of Turbo Modula-2 monitors.
2. What is a monitor?
A monitor under Turbo Modula-2 is a module which locks out (disables)
hardware interrupts. Refer to the Turbo Modula-2 User's Guide (TM2UG), page
192, for the information contained on the topic of monitors (in the
discussion of the IOTRANSFER procedure from the SYSTEM module).
A monitor's capability of disabling interrupts can be useful, and even
necessary in certain situations. Consider a type-ahead keyboard function.
Typically, an interrupt handler for this kind of function will be written in
a simple way which presumes interrupts will be disabled while the interrupt
handler is executing; to write it in such a way that interrupts would not
have to be disabled could be very "expensive" in terms of code speed and
memory usage (it would have to be written to be reentrant).
3. Incomplete information in TM2UG
TM2UG indicates on page 192 how the monitor function may be enabled for a
module. An important bit of information not included there is that the
monitor function is only active during execution of procedures and sub-
modules; thus, interrupts are not disabled during the "initialization code"
section of a module, which is cometimes referred to as the "module body",
defined as the code between the BEGIN and END statements of the module.
The following code example should clarify this.
MODULE Monitor[1]; (* monitor function invoked here *)
(* This procedure disables interrupts when it executes, also any *)
(* procedures called from this procedure will also have interrupts *)
(* disabled *)
PROCEDURE Pr;
BEGIN
WRITELN("Interrupts are disabled here");
END Pr;
(* This is the initialization code. Interrupts are not disabled here *)
BEGIN
WRITELN("Interrupts are enabled here");
Pr;
END Monitor.
4. Summary
The programmer using the monitor function of Turbo Modula should be aware
that, contrary to what is implied in the manual, hardware interrupts are not
disabled at all places in a module declared as a monitor. Only the
procedures and sub-modules of a monitor module have interrupts disabled. The
initialization code or module body will have interrupts enabled, unless it
has been called from a procedure or sub-module of another monitor module.
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