Chapter Fourteen

Internal I/O Statements

The READ, DATA, and RESTORE statements work together to allow the BASIC user to pass predetermined information to his or her program. This is, in a sense, internal I/O. XDATA ($A9E7) The information to be passed to the BASIC program is stored in one or more DATA statements. A DATA statement can occur any place in the program, but execution of a DATA statement has no effect. When Execution Control encounters a DATA statement, it expects to process this statement just like any other. Therefore an XDATA routine is called, but XDATA simply returns to Execution Control. XREAD ($B283) The XREAD routine must search the Statement Table to find DATA. It uses Execution Control's subroutines and line parameters to do this. When XREAD is done, it must restore the line parameters to point to the READ statement. In order to mark its place in the Statement Table, XREAD calls a subroutine of XGOSUB to put a GOSUB-type entry on the Runtime Stack. The BASIC program may need to READ some DATA, do some other processing, and then READ more DATA. Therefore, XREAD needs to keep track of just where it is in which DATA statement. There are two parameters that provide for this. DATALN ($B7) contains the line number at which to start the search for the next DATA statement. DATAD ($B6) contains the displacement of the next DATA element in the DATALN line. Both values are set to zero as part of RUN and CLR statement processing. XREAD calls Execution Control's subroutine GETSTMT to get the line whose number is stored in DATALN. If this is the first READ in the program and a RESTORE has not set a 103

Chapter Fourteen different line number, DATALN contains zero, and GETSTMT will get the first line in the program. On subsequent READs, GETSTMT gets the last DATA statement that was processed by the previous READ. After getting its first line, XREAD calls the XRTN routine to restore Execufion Control's line parameters. The current line number is stored in DATALN. XREAD steps through the line, statement by statement, looking for a DATA statement. If the line contains no DATA statement, then subsequent lines and statements are examined until a DATA statement is found. When a DATA statement has been found, XREAD inspects the elements of the DATA statement until it finds the element whose displacement is in DATAD. If no DATA is found, XREAD exits via the ERROOD entry point in the Error Handling Routine. Otherwise, a flag is set to indicate that a READ is being done, and XREAD joins XINPUT at :XINA. XINPUT handles the assignment of the DATA values to the variables. (See Chapter 13.) XREST ($B268) The RESTORE statement allows the BASIC user to re-READ a DATA statement or change the order in which the DATA statements are processed. The XREST routine simulates RESTORE. XREST sets DATALN to the line number given, or to zero if no line number is specified. It sets DATAD to zero, so that the next READ after a RESTORE will start at the first element in the DATA line specified in DATALN. 104

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