Windows 8 Bible

Understanding Program Types

A couple types of programs could be considered old in the context of this chapter:

• DOS programs: These programs were developed to run under various versions of the Disk Operating System (DOS) that was the precursor to Windows.
• 16-bit Windows programs: These Windows applications were written for Windows 98 and earlier versions of Windows.

What does “16-bit” mean? Three classes of Windows applications exist: 16-bit, 32-bit, and 64-bit. The number of bits indicates the maximum amount of addressable memory supported by the program. Table 39.1 indicates the differences.

TABLE 39.1 Processor Technology and Directly Addressable Memory

Technology	Meaning	Memory Addresses
16-bit	216	65,536
32-bit	232	4,294,967,296
64-bit	264	18,446,744,073,709,600,000

Windows NT, Windows 2000, and Windows XP were all originally 32-bit operating systems. Windows XP was also offered in a 64-bit edition. Windows 98 and earlier were 16-bit operating systems. Windows Vista and Windows 7 were offered in two versions, 32-bit and 64-bit. Likewise, Windows 8 is available in 32-bit and 64-bit versions. Suffice it to say, the higher the bits, the more capable the operating system. For the purposes of this chapter, the key point is that you can run a program on the OS it was designed for or (possibly) on a later version, but you can’t go backwards. For example, you can run a 32-bit application on a 64-bit OS, but you can’t run a 64-bit program on a 32-bit OS.

DOS programs ...