Input and output of values of simple types is achieved by the standard procedures READ and WRITE.
The procedure READ takes one actual parameter which must be a variable of a simple type. It reads a value of the corresponding type from the standard input and assigns it to that variable.
The procedure WRITE takes one actual parameter which must be an expression with a simple type. It writes the value of that expression onto the standard output.
Example:
(* read integers and write *) (* until a nonpositive number is read *) READ (i); WHILE 0 < i DO WRITE (i); READ (i) END