williams-et-al-1983-apple-ii-computer-graphics

76 APPLE II COMPUTER GRAPHICSthe cursor at the bottom of the text window. Lines 9© through 15© acceptyour input for the row and column numbers and, after checking to insurethey are valid, converts them to a base address for the box ind icated. Line15© calculates the base address by adding the row address for the box[RA%(R%)] to the column address (which happens to be one less than thecolumn number). Having added the row and column addresses for thebox, it only remains to add the address for the position in the box for eachbyte of the alien (how do aliens taste?). The fi nal address is calculated andthe proper value POKEd into it in lines 16© through 23©, lines 25© and 26©give you a chance to plot another one, and then lines 27© and 28© wrapup the program. The data in lines 29© and 3©© are the row addresses readin line 5©. The plotting speed may be enhanced somewhat by calculatingall of the fi nal addresses before starting to POKE any of them.Example 4Write a simple BASIC program to plot the cannon given in Figure 8-7.Figure 8-7. Hi-Res cannon.(From Cannonball Blitz by Olaf Lubek, with permission.]