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

CHAPTER 2-COMPUTER PHYSIOLOGY 7ber is called the "address" of that byte. A memory address is much like thenumber on a post office box- one can determine what is in any byte ofmemory by going to the box with the proper address and looking inside.The number of boxes available depends upon two things: The number ofboxes installed (memory size), and the maximum number of digits availablein the address. If you were limited to use only three decimal digits inan add ress, there would be a maximum of H'P (HM'l©) boxes-addressed©©© through 999. The processor within the Apple allows for two bytes(sixteen bits) in the address of any location in memory, and that provides amaximum of 216 addresses. If you have wondered why you can use amaximum of 64K bytes of memory within an Apple II, consider that eachmemory location has to have an address, and that 216, using a littlearithmetic and a good calculator, equals 65,536-the true maximum forthe number of bytes the 65©2 processor can address. Many larger computersuse three or more bytes for addressing; a three-byte address contains24 bits, so by allowing three bytes in the addresses, a computer canaddress 224 memory locations: over SIXTEEN MILLION bytes of memory!Binary BlindnessYour computer is very happy to work in binary. The early machines did soexclusively, but most people fi nd it trying-looking at binary numbers allday makes your eyes cross! To demonstrate, compare the two columns ofbinary numbers below, and try to determine where they differ.1©©1©1 11 1 ©©1 ©1 11©11Zl1 1Zl1 1©llllllllll©lllllllIll 1 Ill 1 Ill 11 Illlllllllll 1 lllllll Ill110 11©1© 11©1 11Zl11Zlllllll 1 llllll 111 lllllll llllll 1 11111 Ill 1 1 llllll 111llllll 1 llllllIll 1 lllllllllllllll 1 Ill 1 lllllllllllllll 111111Zl11Zllll11111Zl1©1ZlIs it any wonder that there were very few programmers in the days whencomputers spoke only binary?Hex ta the RescueThe hexadecimal or hex system of numeration, base sixteen, supplies awelcome solution to the problem of "binary blindness." Hex is advantageousbecause it provides a convenient way to turn the binary ones and