Commodore 128D - Program Tracing-The TRON and TROFF Commands

I 1
n
Program
Tkraclng—The
TRONand
TROFF
Commands
Notice the RESUME in line 1100. This tells the
computer to return to the line mentioned (in this
case, 120) and continue. Depending on the error
that was trapped, resuming execution may or
may not be possible.
For additional information on error trapping, see
the error functions ERRS, EL and ER, described
in Chapter V, BASIC 7.0 Encyclopedia.
When a problem in a program occurs, or you do
not get the results you expect, it can be useful to
methodically work through the program and do
exactly what the computer would do. This pro
cess is called tracing. Draw variable boxes and
update the values according to the program
statements. Perform calculations and print
results following each instruction.
Tracing may show you, for example, that you
have used a GOTO with an incorrect line num
ber/or calculated a result but never stored it in a
variable. Many program errors can be located
by pretending to be the computer, and following
only one instruction at a time. Your C128 can
perform a type of trace using the special com
mands TRON and TROFF (short for TRace ON
and TRace OFF). When the program is run, with
TRACE ON the computer prints the line num
bers in the order they are executed, as well as
any results. In this way, you may be able to see
why your program is not giving the results you
expected.
Type any short program we have used so far, or
use one of your own design. To activate trace
mode, type TRON in DIRECT mode. When you
run the program, notice how line numbers
appear in brackets before any results are dis
played. Try to follow the line numbers and see
how many steps the computer needed to arrive
at a certain point. TRON will be more interesting
if you pick a program with many branches, such
as GOTO, GOSUB and IF-THEN-line number.
Type TROFF to turn trace mode off before con
tinuing.
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USING C128 MODE—Some BASIC Commands and Keyboard Operations Unique to
C128Mode