Section 2 - Commodore Computers

May, 1982.I$Ue 24 COMPUTEI 149
May, 1982. Issue 24 COMPUTE! 149
are not in the midst of handling a command. If
are not in the midst of handling a command. If
FLAG FLAC is not 0, then the user r has typed in something
like GOTO GOTO, which is a syntax error. Since
like COTO COTO, which is a syntax erro r. Since
Applesoft sy ntax is very clea r on this point - all
Applesoft syntax is very clear on this point — all
commands must be separated by ends of line e (colon
or carriage return) - we return "? SYNTAX ER­
ROR'1 ROR" if we find a command token while executing
a command. Since BASIC is nol not in direct control of
program execution at this point, this program
must do its own error checking.
g.
4. Our lookup ta ble diffe rs in format fro m the one
4. Our lookup table differs in format from the one
at $1)000. $0000. We also store jump p locations, less 1, I, and
will get to these via an RTS, as the Applesoft inter
will get to th ese vi a an RTS, as the Applesoft interpreter
does. However our table e also holds the
target tokens themse lves. T he first and second
target tokens themselves. The first and second
locations of CMDTBL (CoMmanD TaBLe) hold
the e low loll' and high bytes of the address of the subroutine
which will execute the command whose
routine which will execute the command whose
token is stored in the third location. Similarly y for
the fourth and fifth locations (addresses) and the
the fourth and fifth loca tions (addresses) and the
sixth location (token), (token). and so on. TheT command
table is stored in this ve rsion of the program at
table is stored in this version of the program at
location $0354. ThereT is no need whatever to store
it here. YouY O LI can n put it in any convenient ni ent place in
memory, as long as you change the four places in
memory, as long as )'O U change th e four places in
the program which refer to the starting address of
the program which refer to the starting address of
CMDTBL (before JMPGOT J MPCOT and after r GOTONE).
COTONE).
5. End of CMDT BL is indicated by a zero va lu e for
5. End of CMDTBL is indicated by a zero value for
the target token. We load the token into register Y
at $3 1 ID 0 in order to test for r end of table. If Y is
zero, the command token n is not one we are looking
g
for, r, so we exil.
exit.
The Labelled GOTO/GOSUB Example
1. Comments on the patch.
We \Ale use Smith's routine as an example partially
all y
because itw"s was published in COMPUTE!, so youu may
be familiar with it, and partiallyy because we have
found it quite usefui. useful. The routine has been completely
rewritten in three ways, once for ROM
Applesoft, once forr RAMCARD Applesoft, and
once for r Applesoftl Ila. la, or TAPERAM. . A more
complete discussionn of the logic of the routine is in
Smith's anicle.
article.
The effect of the patch is as follows: Taking X
to mean any arithmetic expression orr variable, (X
may, but need not, be a literal number), then if the
value of X is 1000, COTO GOTO X will be treated by
Applesoft in the same way as COTO GOTO 1000. Similarly,
COSUB GOSUB X will be treated as COSUB GOSUB 1000.
Thus labels cann be defined forr subroutines (as all
rational programming languages, including nearly
all assemblers, allow) and for COT GOTOO statements
(reminiscent of FORTRAN'S RAN's ASSIGN ICN statement). If
X is a Teal real number, it is rounded down to the nearest
integer, so be wary of arithmetic ti c expressions,
expressions.
This patch does not affect the behavior of
ON .. . COT O and ON ... COSUB, so these must still
use li ne numbers rather than labels. However, you
ca n replace these computed COTO's in your code
by computed COTO's of a very different type,
which are reminiscent of PASCAL's CASE handling.
As an example, at the start of your program you
This patch does not affect the behavior of
ON...GOTO and ON...GOSUB. so these must still
use line numbers rather than labels. However, you
can replace these computed GOTO's in your code
by computed GOTO's of a very different type,
which are reminiscent of PASCAL'S CASE handling.
As an example, at the start of your program you
might DIMension a matrix SELECT(20) and assign
the values of 20 different line numbers to the
values of SELECT(I). To GOTO these lines, you
values of SELECT(I). T o COTO these lines, you
can compute the valu e of I, and then COT O
SELECT (I). With decent commenting on the intention
and conditions of each choice of SELECT's
line numbers (which is bes t done at the place in the
program that the line numbers are actually ass igned
to SELECT's elements,) your program will probably
be much more readable than one with an equally
commented ON ... COTO statement. T his is our
experience with these two d iffe rent forms of computed
COTO and COSUB, and we have stopped
using g ON...GOTO . COTO completely.
can compute the value of I, and then GOTO
SELECT(I). With decent commenting on the in
tention and conditions of each choice of SELECT's
line numbers (which is best done at the place in the
program that the line numbers are actually assigned
to SELECT's elements,) your program will probably
be much more readable than one with an equally
commented ON...GOTO statement. This is our
experience with these two different forms of com
puted GOTO and GOSUB, and we have stopped
Spaghetti Structure structure
A different method of implementing a computed
GOTO COT O is ideal for making your program structure
resemble a plate of spaghetti. . If you change the
value of X at various points in the program and
repeatedly repeateclly use X in GOTO COTO X or GOSUB COSUB X statements,
then, if yo u succeed in debuggin g your
and neighbors with your ability to produce unintel­
ments, then, if you succeed in debugging your
program, you will be able to amaze your friends
and neighbors with your ability to produce unintel
ligible, yet functional, code. The labelled GOTO/ COTOI
GOSUB facility as presented here can be used to
COS U B facili ty as presented here can be used to
dramatically increase the readability of your program,
or it can be abused to degrade the structure
of your program. We strongly recommend that
you assign line numbers to specific variables at the
start of the program, use informative names for
those variables, and never change their values once
assigned.
.
2. The actual patches
The ROM Applesoft patch is given in the
appended listing, directlyy after the lookup table.
The program is vinually the same as Smith's, with
The program is virtually the same as Smith's, with
a few more comments. nts, Note that whether er you use
this program or Smith's, IF...THEN X, IF...GOTO IF...COTO
X and IF...THEN .THE COTO GOTO X will not work properly.
We have tried and tried, but cannot fixx this flaw in
a program of reasonable length. A statement of the
a program of reasonable length. A statement of the
form IF...THEN:
E : COTO GOTO X (or COSUB GOSUB X) will
work correctly. There must be a colon between the
THEN and the COTO GOTO or the COSU GOSUB.
The RAM Applesoft patch is much simpler.
For either RAM Applesoft version, you do not
need the subinterpreter. Instead , modify COTO
need the subinterpreter. Instead, modify GOTO
directly. COTO GOTO is a subroutine of CO GOSUB, so this
modifies both. Change the JSR $DA0C (the LINe
modifies both. Change the J SR $DAOC {the LINe