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

CHAPTER 7-PRESERVING YOUR PICTURES 55GRBLOAD APPLERUNThis will (1) set the graphics screen, (2) load the picture of the apple backo nto the screen from disk, and (3) run the program in memory (hopefu llyListing 7-1 ). The list of addresses and values will appear on your printer. Ifit does not, check your program against Listing 7-1 and make sure thatyou have the printer slot assigned correctly in line 40.What ta Do with DataThe list of addresses and values printed by the Low-Res scan #1 may bewritten into a program as a data table and POKEd back into memory usinga BASIC subroutine similar to:l(il(il(il FOR I = 1 TO l(il71(il1(il READ L,V: POKE L ,V1(il2(il NEXT Il(il3(il RETURN1(il4(il DATA 1684,4,1685,64,1809,192,Listing 7-2.The data values in line 1040 are those generated from Listing 7-1 . Thismethod has the advantage of not using up disk space, but it has probablyoccurred to you that entering all of those data values by hand is veryinefficient, not to mention boring and prone to error. We have two alternativesfor those of you with a disk drive.The Data FileInstead of printing the addresses and values on paper, you can write themto a text file on the disk. Load Low-Res Scan #1 (Listing 7-1) and then typethe following lines to modify it.1(il REM WRITE DATA FILE35 D$ = CHR$ (13) + CHR$ (4)4(ll INPUT "FILENAME :";FL$45 PRINT D$ "OPEN" FL$5(il PRINT D$ "DELETE" FL$55 PRINT D$ "OPEN" FL$6(il PRINT D$ "WRITE" FL$1l(il IF PEEK (L) < > ill THEN PRINT ,L :PRINT PEEK (L)14(il PRINT "* "15(il PRINT D$ "CLOSE" FL$155 PRINT CHR$ (7) : REM BEEP SPEAKERListing 7-3. Low-Res Scan #2 (modifications only].