Commodore Plus 4 - Page 107

BASIC Version 3.S Commands, Functions, and System Variables 95
TRON Abbr. trO
Turns on built-in error-tracing functions. The tracing function displays the line
number on each line in the program as the program executes. This function helps
you locate a line that is causing program error.
UNTIL Abbr.uN
Sets a closing condition in a DO . . . LOOP sequence. When you include an
UNTIL clause in a DO loop, the loop executes until a condition is met. See DO.
USR Abbr. uS
USR (number^
This is a BASIC function (i.e., it is invoked by setting some variable equal to it).
USR goes immediately to a user-callable machine language subroutine whose
starting address is contained in memory locations 1281 and 1282. Before you can
use the USR function to access a machine-language subroutine, you must POKE
the subroutine address to memory locations 1281 and 1282. If you do not, the
execution of a USR function aborts the program and displays an ILLEGAL
QUANTITY ERROR message.
The number in parentheses is a variable or formula you are sending to be used
in the machine-language subroutine. It is stored in floating-point accumulator 1.
At the end of the subroutine, the value in floating-point accumulator 1 is returned
to the BASIC program as the value of USR. Using a machine-language subrou
tine is much like using a BASIC user-defined function: you send a value to be
used, and it sends back to the main program the resultant value. USR is a window
between BASIC and machine language. See Chapter 5 for more information.
VAL Abbr.vA
VAL (string)
Converts a text-string number to a numeric value when you have a number in a
text-string variable instead of a numeric variable. For example, you might input a
phone number as a text-string variable, and then want to use it as a number to dial
your automodem.
If the text=string value contains characters that are not numbers or acceptable
parts of numbers (e.g., minus signs, decimal points, or E, which connotes scien
tific notation), the rest of the string is not converted. If the first character is not an
acceptable character, VAL returns a zero.
Example: 10 GETKEY A$ One key is gotten from the
20 IP VAL(A$)<>0 THEN G0SUB 80 keyboard. If it is a digit from
30 END 1 to 9, the subroutine is
80 PRINT"THE DIGIT IS ";A$ executed.
90 RETURN