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

108 APPLE II COMPUTER GRAPHICSRemember to let the tape run for a moment on the record setting betweentyping the second "W" and pressing the return key.You may use the Monitor READ command to reload the table, butAPPLESOFT has a much better method using the SHLOAD command.SHLOAD loads the shape table from tape and stores it just below HIMEM.HIMEM is set below the table to protect it, and the starting address of thetable is placed in $E8 and $E9. The beauty of this process is that all thedetails are taken care of automatically. The only command you give isSHLOAD, and, unlike the Monitor command, SHLOAD may be used fromwithin a BASIC program. For our example, rewind your shape table tape,push the play button, typeSHLOADand everything is done for you. TA DA!!The table may also be loaded directly as part of the program by using thePOKE statement. Listing 10-2 POKEs the data for our shape table, asks youto specify the scaling factor (1-255), and then draws the saucer on thescreen with ten different rotation values.Learning by ROTROT = Ill causes the shape to be oriented in the same way as it wasdefined-no rotation. When ROT = 16, DRAW will show the shape rotated90 degrees clockwise from the original, ROT = 32 rotates 180 degrees, andROT = 48 rotates it 270 degrees clockwise (90 degrees counter-clockwise).ROT = 64 is back where it started, and so fo rth in increments of 16 all theway through 256.The ROTation parameter is partially dependent on the setting of SCALE.According to the APPLESOFT manual, at SCALE = 1 , only the four rotationvalues [Ill, 16,32,48] are recognized. However, as you can see by usingListing 10-2, the values [8,24,40,56] also work at SCALE = 1 . At SCALE = 2,sixteen rotations are available, with [4, 12,2©,28,36,44,52,60] being the additionalvalues. The larger the scale, the more rotations available, up to apoint. If you set ROT to a value it does not recognize, it will rotate theshape to a value it does recognize.Hopefu lly, this program will demonstrate and clarify all of those rulesconcerning ROT. Please run the program and watch the values of ROT atthe bottom of the screen, and the effects they have on the figure. You willhave to press reset to end the program.1 REM DEMONSTRATE2 REM POKING TABLE3 REM4 REM POKE ADDRESS5 REM OF TABLE