Intel 9800758-02 manual Error Trapping

Error Handling BASIC-SO

Underflow. Single-precision floating-point underflow occurs when the magnitude of a single-precision numeric value is less than (±) 1.2 x 10-38(1.2E-38). Double- precision floating-point underflow occurs when the magnitude of a double- precision numeric value is less than (±) 2.2 x 10-308(2.2D-308). When a value of this magnitude is generated, no message is printed, the result is defined as zero, and pro- gram execution continues.

Divide-By-Zero. Divide-by-zero is handled identically to single- and double- precision overflow protocol, except that the message printed is DIVIDE-BY-ZERO.

Integer Operations

Integer overflow and division by zero are treated as normal errors. Program execu- tion halts. Integer overflow occurs when a value greater than 32767.49 or less than -32768.49 is converted to integer representation. Integer underflow cannot occur.

Error Trapping

Error trapping allows you to trap and correct errors. Errors normally halt program execution and return error messages. When given before an error is encountered, the ON ERROR GOTO statement transfers program execution to a specified line number when an error occurs, and suppresses any error messages other than those you specify and write yourself. You can write an error-service routine at the specified location to identify and correct errors. In the following example, when -3 is entered in response to the INPUT statement, an error occurs and the user-written error message displays:

10ON ERROR GOTO 100

20INPUT A

30PRINT SQR{A)

40GOTO 20

100PRINT "ARGUMENT CANNOT BE NEGATIVE."

110PRINT "ENTER NEW VALUE."

120RESUME NEXT RUN

? 3

1.732051

? -3

ARGUMENT CANNOT BE NEGATIVE ENTER NEW VALUE

?

Suppose there is no error routine (made up of lines 10, 100, 110, and 120) in the above example, and you wish to identify this particular error. The ERR and ERL functions return the error code number (see Appendix A) and the line number the error occurs in, respectively. In this way, you can identify the kind of error and the line it occurs in and write individual error handling routines for individual errors. The program below uses ERL and ERR to identify the location and nature of any error.

In the following example, the ON ERROR statement enables the error-service routine at line 100, which identifies one specific error (division by zero) and the line number in which it occurs:

4-2