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CHAPTER 4
ERROR
HANDLING
If
you enter improper instructions, syntax, or formats, BASIC-80 issues an error
message. This chapter explains what these errors mean, how they may be trapped
with the ON ERROR statement and pinpointed with the TRON, TROFF, ERR, and
ERL instructions, and how errors may be simulated with the ERROR statement.
BASIC-80 Error Messages
When BASIC-80 encounters an error in a program, it displays an error message.
Appendix A lists the error messages and error codes, and describes their meaning.
These errors stop program execution. You can use the
BASIC-80 editor at this point
to correct obvious errors, add error-trapping instructions, or use the trace facility.
Syntax
Error
Messages
If
BASIC-80 detects a syntax error,
it
stops program execution, displays an error
message, and enters the Edit Mode:
10
LET
A=
20
PRINT A
RUN
SYNTAX ERROR
IN
10
10
•
You can now change line 10, as described in Chapter
3.
To leave the Edit Mode
without making any changes, and to preserve the variable values in the program,
type
Q.
If
you enter any other editing subcommands (including Carriage Return)
variable values are lost.
Overflow, Underflow, and Divide-by-Zero
In BASIC-80, the single- and double-precision floating-point overflow, underflow,
and divide-by-zero errors do not halt program execution.
Overflow. Single-precision floating-point overflow occurs when the magnitude of a
single-precision numeric value exceeds
(±)
3.4 x
10
38
(3.4E38). Double-precision
floating-point overflow occurs when the magnitude
of
a double-precision numeric
value exceeds
(±)
1.79 x
10
308
(1.79D308). When a value of this magnitude
is
generated, the message OVERFLOW
is
displayed, a value equal to the largest possi-
ble magnitude for the given representation with the appropriate sign
is
assigned, and
ex~cution
proceeds.
The following examples show single- and double-precision overflow:
10
A = 1
E20*1
E20
20
D#
= 1
D200*1
D200
30
PRINT A;D#
RUN
OVERFLOW
OVERFLOW
3.402824E
+
38
1.79769313486231
D +
308
4-1