Simtel MSDOS 1996 September/Simtel-MSDOS-Sep1996-CD2.iso/disc2/ada/smallada.doc

Text File | 1993-06-05 | 12KB | 268 lines

S M A L L A d a   1 9 9 1
IBM PC-family version

Department of Electrical Engineering and Computer Science
The George Washington University
Washington, DC 20052 USA

This is a compiler/interpreter for a part of the Ada language, namely
the "Pascal subset" plus the Ada tasking support. It is not intended
ever to be a full Ada compiler, rather a vehicle for teaching, learning,
and experimenting with concurrent programming. The compiler is quite
fast, producing P-code which is then interpreted by the interpreter.

The system will run on an IBM PC (or compatible) with the following
minimum configuration: 640K of memory; one disk drive. A hard disk will
greatly improve the speed. The entire system runs in RAM with no disk
accesses, so expect capacity to be limited. 250 statements or so can
be safely accommodated.

SmallAda executables and demo programs are distributed as a self-
extracting archive created by LHA. To install SmallAda, just create a
directory on your hard disk called, say, smallada, move to that
directory, and execute the self-extracting archive sm-archv directly
from your diskette. If your diskette is in drive A:, just type
A:\SM-ARCHV.

The current version is not particularly robust, rather it is a preliminary
step toward effective window-oriented monitoring of task execution.

This package may be copied and used for educational purposes but not for
profit; please let us know by e-mail or regular mail how you like the idea
of this package and what you are doing with it. The more users we know
we have, the better our case for getting funding to continue the work.

project supervised by
Prof. Michael B. Feldman
Department of Electrical Engineering and Computer Science
The George Washington University
Washington, DC 20052
202-994-5253 (voice)
202-994-5294 (fax)
[e-mail]

authors:
1986 Charles Schoening             DOS version of CoPascal
1987 Frederick C. Hathorn          conversion of CoPascal to Ada
1988 Stuart Cramer and Jay Kurtz   refinement of tasking model
1989 Arthur V. Lopes               window-oriented monitoring for IBM-PC
1990 Manuel A. Perez               Macintosh version
1990 Arthur V. Lopes               integrated environment for IBM-PC
1991 Amr El-Kadi                   user-selectable task scheduling

HOW TO RUN THE PC PROGRAM
=========================

To run SMALLAda PC you will need the following files:

SMALLADA.EXE    -- Small Ada integrated environment
SMADA-IN.AVL    -- Small Ada interpreter pseudo code; it can be tunned
                -- to suit any level of interpreter monitoring

Type SMALLADA RACE <enter>. You will be placed in the editor mode. To compile
and run the race program follow these steps: press F10. A menu will appear
on the top of your screen. Press C, for compiler. Then a sub menu will be
displayed. Press E (or <enter>. This will start the compilation process.
If the program is correctly compiled then it will run thereafter. If an
error is found an error message will be issued and after pressing any key
the editor will be activated. The cursor will be at the line where the
error was found.

The program will run much faster if you disable the window-oriented
execution monitoring by pressing <INS>, which toggles monitoring on and
off. If monitoring is enabled, pressing <DEL> toggles step-by-step
execution on and off. With stepped execution on, pressing the space
bar moves through each source line.

You can create and compile your own Ada programs, but we make no promises
about which parts of the language will be handled correctly. We assure you
that there is no support for packages, generics, exceptions, access types
and dynamic data structures. Some of this may change in future versions,
but we make no commitment to support full Ada. And don't be surprised if
the compiler hangs here and there: error repair is in a rather primitive state.
If the compiler actually finds your errors, it will display the messsages in
a window.


THE SmallAda IBM-PC RUNTIME ENVIRONMENT
=======================================

The runtime monitoring of tasking is really the whole point of this
project.

Each task is given a window. If there are more than five tasks some tasks
will share a physical window.

To obtain help on the runtime environment press F1 while a program is running.
In summary, you can control the overall execution speed of the interpreter
(to "slow down the action"), and exert some influence on the open windows.
There is no magic in all this runtime monitoring: it all takes machine cycles.
If you wish your program to run faster, remove all the monitoring windows
(by pressing INS while they are open). You can also step through execution,
source line by source line, by pressing INS while windows are open. Then
pressing the space bar steps through one source line.


THE SmallAda EDITOR
===================

The editor was adapted from the Turbo Pascal Editor Toobox Version 4.0.
The editor works much like the Wordstar editor. Next some of its basic
options are shown.


Function                        Keystrokes

BASIC MOVEVMENT COMMANDS:

Character left:                 left arrow or ^S (^ ==> Control)
Character right                 right arrow or ^D
Word left                       ^left arrow or ^A
Word right                      ^right arrow or ^F
Line up                         up arrow or ^E
Line down                       down arrow or ^X
Scroll up                       ^W
Scroll down                     ^Z
Page up                         PgUp or ^R
Page down                       PGDn or ^C
Beginning of Line               Home or ^Q S
End of Line                     End or ^Q D
Tab                             Tab or ^I
Backward tab                    Shift tab

BASIC EDITING COMMANDS:

New line                        Enter or ^M
Insert line                     ^N
Delete current character        Del or ^G
Delete character left           Backspace or ^H
Delete word                     ^T
Delete to end of line           ^Q Y
Activate menus                  F10
Abort command                   ^U



THE SmallAda TASKING MODEL
===========================

We have tried to be reasonably faithful to Ada tasking. The scheduler is
designed to show some of the important issues of concurrent programming,
therefore time-slicing is implemented, and the length of the slice is
randomly determined. Also, at each rescheduling point the next task to
be scheduled is selected randomly. This is to give a reasonable model of
nondeterministic (unpredictable) behavior, and serves to highlight the
need for mutual exclusion. The dining philosophers examples on the
disk, taken together, show this point rather well.

Current incompletenesses in the tasking model: only named tasks (i.e. no
task types), no dynamically created tasks. We also don't support nested
task declarations; all tasks must be declared in MAIN's declarative section.

The "select" statement is functional including the "terminate" alternative;
checking for termination is tricky in Ada and you might find that some
select structures with terminate alternatives deadlock or don't work at all.
In general, be gentle with the select statement; try to avoid tricky or
pathological code, which will surely choke the compiler or interpreter!

You will also discover quickly that though the "delay" statement is functional,
the elapsed time to expiration of the delay is considerably more than was
requested in the delay statement. As it happens, this is correct Ada, as the
standard says only that a task will be suspended for _at least_ the amount
of time requested. This is to take account of the likelihood that contention
will make it impossible for a task to take immediate control when it wakes
up from a nap. Indeed, we think that the sometimes too-long delays
may be due to the interference in the interpreter caused by all the references
to the user interface for monitoring purposes. The relationship of the delay to
reality depends upon your PC's speed as well. We are working on this problem
but do not yet see an obvious solution. So take the time given in a delay
statement with a grain of salt, please.

If you have tasks in finite (counting) loops, if they complete but there are
other tasks with infinite loops that can never terminate, the system may
detect no activity and report a deadlock state.

The current version now supports user-selected task scheduling. Pressing
<alt-F10> during execution opens a window giving scheduler options.


THE SmallAda SUPPORT PACKAGE (SMALL_SP.PKG)
===========================================

A large number of "intrinsic" library subroutines are included in a
pseudo-package called SMALL_SP.PKG. It is not necessary (or proper) to
compile this package; it is included in the environment. It is necessary
to begin each program with a context clause

WITH SMALL_SP; USE SMALL_SP;

to give the effect of writing genuine Ada. A specification for this
package is included in the distribution; it includes I/O routines,
math functions, and the like. This is not really good Ada-like decomposition,
and we will change it in subsequent versions of the system. Meanwhile, use
the package spec as a guide to all the intrinsic routines. Note especially
that the formatting parameters supplied to the I/O procedures are more
Pascal-like than Ada-like. This will change later to look more like
standard TEXT_IO.


THE DEMONSTRATION PROGRAMS
==========================

We have supplied a set of what we hope will be interesting and instructive
programs illustrating both the sequential aspects of the Ada subset and
the tasking model.

TEST1 through TEST6 can be thought of as "validation" programs testing
specific features of the language; reading them can serve a useful purpose
in understanding our subset.

ATEST1 through ATEST4 demonstrate the differences in program behavior
under different scheduling policies. Each program contains a block
comment explaining its purpose in this demonstration.

SHELLSORT is what its name implies: a standard implementation of Shell sort
in which the state of the array is displayed on the screen as it is sorted;
RACE is a tasking program which we have used with success in concurrent
programming courses: three sort algorithms are activated as Ada tasks
and run in parallel.

4TASKSEM shows the use of the semaphore primitives "exported" by the SMALL_SP
pseudo-package. A semaphore SCREEN is declared and used to give mutual
exclusion on the terminal screen.

Attention: semaphores are NOT Ada primitives! SmallAda was originally
constructed as a modification to the CoPascal system of Ben-Ari
(as described in "Principles of Concurrent Programming", Prentice Hall 1982),
and the semaphore primitives are still there from the system's last
incarnation. To show how to do it in "real" Ada, we include the program
4TASKMON, in which a screen monitor task is declared.

Finally, there are three versions of Dijkstra's legendary dining philosophers
problem. SmallAda has a capacity limitation of around ten tasks. We are
limited to four instead of the usual five philosophers, because four diners,
four chopsticks, and MAIN add up to 9 tasks. EATCHAOS is very instructive:
it demonstrates why it's nice to have monitors for mutual exclusion: without
a monitor guarding the screen, the display becomes rather chaotic. EAT-SEM
uses the SmallAda semaphore primitives; EAT-MON does the same thing but
with a "genuine" Ada monitor.

FINALLY
=======

In case you are curious, the system is coded in Turbo Pascal with the
Turbo Toolbox libraries used for the editor and environment.

Please let us know about bugs; tell us what you're doing with
the sytem. Good luck; have fun!


Good luck; have fun!