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

CHAPTER 12-BYTE-MOVE SHAPES 141POKE 8192,32POKE 8192,64The dot will move to the right, and the last statement will leave the dotat the right edge of byte number 8192. To continue the journey, the dotmust move to the first dot of byte number 8193, and then continue acrossthat byte. To accomplish that, you can type the same series of values, butuse 8193 as the address:POKE 8193 ,1POKE 8193 ,2POKE 8193 ,4POKE 8193,8POKE 8193 ,16POKE 8193 ,32POKE 8193 ,64This pattern will repeat until you get through address 8231, or you gettired of it, whichever comes first.As you can see, each position across the byte requires a different value,and there are seven positions across the byte, so seven becomes a veryimportant number. You can also see that the last dot in every byte youtravelled across was left on. As we mentioned befo re, some arrangementmust be made to erase the last value before moving on to the next byte.POKEing a zero as the fi nal value in each series will erase that left-overdot.To move the dot across byte number 16 in that row (the row starts withbyte number (/)), you would fi nd the add ress by adding the offset, 16, to thebase address, 8192 (did everybody get 82(/)8?), and then POKE the series ofvalues using that address.There is a nice relationship between the X-coordinate of a point and theoffset and value needed to refer to it in byte-move. Consider the sevenvalues used to move the dot across the byte as seven versions of the dot,and number them zero through six. If you wish to turn on a particular dotacross the row, you need to determine which version of the dot to POKE,and the offset from the beginning of the row.For example, let' s turn on the dot in the fi rst row which has an X­coordinate of 75. Divide the coordinate by seven to fi nd the quotient (theresult of the division) and remainder. Seven goes into 75 ten times (rememberyour g'zintas?) with five remaining. The quotient (1(/J) is the offset, andthe remainder (5) is the number of the separation (versions of the dot) tobe used. Therefore, the address is 82(/)2 (8192 + 1(/J), and the value is 32.So,POKE 822,32and you will turn on the dot having 75 as its X-coord inate.