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

22 APPLE II COMPUTER GRAPHICSgates memory from within a BASIC program, without having to enter theMonitor. The command PRINT PEEK (37) , for example, will print the contentsof the thirty-seventh location in memory. PEEK is a decimal-orientedcommand, so in the previous example both the address (37), and the valueprinted are in decimal form.Another application of the statement involves PEEKi ng to see if a key hasbeen pressed. This is usefu l when the computer is required to processcontinuously and still look for a signal to interrupt, such as a change ofdata. Many of the computer-based arcade games provide examples of thissince they must continuously animate the figures on the screen, but stillcheck for keyboard, game paddle, and/or joystick input in order to receivethe signal to fi re, change the player's direction, and so on.To illustrate use of the PEEK statement, key in the program given inListing 4-1 . When you have fin ished keying in the program, run it. Whatdo you see? You should see the letter "L" being continuously printed onthe screen. Press another key. "*ie character which was pressed will nowbe pri nted until yet another key is pressed, and so on. Program executionmay be stopped by pressing the reset key.10 REM DEM0NSTRATE PEEK20 REM30 P$ = "L"40 REM50 REM TOP OF LOOP60 PRINT P$;70 P = PEEK ( 49152) : REM SEE IF KEY PRESSED80 IF P > 127 THEN PRINT CHR$(7);:P$ = CHR$ (P-128): POKE -16368, 090 GOTO 50Listing 4-1.The following line-by-line explanation should clarify the example and itsuse of the PEEK command.30 P$ = "L"Line 30 assigns P$ an initial value of L.60 PRINT P$;Line 60 prints the value of P$. The semicolon suppresses the usually automaticline feed so that the printing will continue across the screen.70 P = PEEK ( 49152) : REM SEE IF KEY PRESSEDLine 70 assigns P the value in location 49152. Location 49152 has twofunctions: the first is to signal when a key has been pressed, and thesecond is to temporari ly hold the numeric code for that key. When a key ispressed, the left-most bit in byte 49152 is turned on, so the byte has a