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

CHAPTER 8-Hl-RES GRAPHICS 73The fourth column address is 3 ($3), so the address of the fi rst byte weare after is 8320 + 7168 + 3 = 15491 ($2080 + $1C00 + $3 =$3C83), and the address of the second byte is 15492 ($3C83). The dotpatterns fo r the fourth and fifth byte look like:-----X - - X -----so the bit patterns would be:010 0000 and 000 0010and when you supply the left hand zero you get:0010 0000$20$ 2= 32 2and 0000 0010Hence, to plot the dots, type:HGRPOKE 15491,32POKE 15492,2If you were to supply ones for the left-hand bits you would get:so:1010 0000 and$A0160HGRPOKE 15491,160POKE 15492,1301000 0010$82130Notice that the same dots were turned on with both sets of statements,but that they were diffe rent colors-not very impressive unlessyou have . a color display, but it does illustrate how the eighthbit controls . color in Hi-Res.Monitor may be used to turn on the same dots, and also to clearthe screen without the use of the HGR statement.The Hi-Res screen may be cleared by moving zeros into everybyte, so type:CALL -1512000 : 02001 < 2000 . 3FFEM3C83 :60 03The fi rst command invokes Monitor, the next two clear thescreen (see Chapter 3), and the last places the proper byte in eachof the two addresses.