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

CHAPTER 6-TEXT ANO LOW-RES 51BSAVE APPLE, A$400,L$400This statement causes the computer to save $400 bytes of memory beginning with location $400 . The A parameter indicates the starting address,and the L parameter gives the length of the memory range to be BSA VE .The Low-Res screen memory begins at $400 and is $400 bytes long. If youdo not care to work in hex, you may typeBSAVE APPLE, A1024, L1024and do the same thing since $400 equals 1024 .With the screen safe ly saved on the disk, you may recall it at any time.To demonstrate, type:BLOAD APPLEand the apple will reappear on the screen. The A and L parameters may beused with BLOAD, but they are not necessary.You may BLOAD the picture from within a BASIC program by inserting aline in the program similar to line 15 below. Load the Low-Res editor andtype the fo llowing:15 PRINT CHR$ (4) "BLOAD APPLE"When you run the editor it will now load the apple picture automatically.This is handy if you wish to re-edit an existing picture.If you do not have a disk drive, you can save the screen memory oncassette tape by using the Monitor. Type:CALL -151400 .7FFWto tell Monitor to write (hence the W) the range of memory from $400through $7FF onto the tape. To insure an accu rate record ing, it is a wiseidea to put the tape on "record" and let it run fo r a few seconds betweenpressing the W key and the return key. After the Low-Res screen is savedon the tape, you can rewind the tape and load it from Monitor by typing400 .7FFRThe format is the same as the Write command, except the Wis replaced bythe R or READ command.If you have saved and loaded the screen as suggested, you may havenoticed that when the screen is loaded, it brings with it some garbage inthe text window. The BSAVE command saves the whole screen, includingwhatever text was in the text window at the time. That may be circumventedby writing a short BASIC program to fi rst clear the text window andthen save the screen. That program is left to the reader as an end of thechapter exercise.·