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

CHAPTER 8-Hl-RES GRAPHICS 71Row AddressColumn AddressPosition Address81922248Final Address1242$282Thus, you at last arrive at the address, but now you need to calculate thevalue to put into it. To do that, again refer to Figure 8-4.The top line of the figure shows the fi rst three bytes of screen memory,taking into account that only seven bits of each byte are actually displayed.Dot number seventeen is the third dot from the left in its box; soto turn it on, you would turn on the third bit from the RIGHT in the byteas shown in the bit pattern on the second line. Remember, the bitpattern is the reverse of the dot pattern.Dot #1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21I I [X] I· . •BYTE # 2 3l0000 0 1 0 0l$4 = 4Figure 8-4. Display conversion.The left-hand bit has been shown in the second line as a zero (off) . Thebinary number in the second line is converted to hex and decimal formin the third line, and the decimal value is the one we are about to POKE.After all that work, you can turn on the desired dot using the fo llowingcommands from BASIC.HGRPOKE 1242,4You could use Monitor by typingHGRCALL -151282 : 4(set graphics)(turn on the dot)Example 2Turn on dots 57, 58, and 59 in line number 116 of the Hi-Res screen.