MS-DOS—compatible applicationsIf we look at how existing MS-DOS applications use the operating system and hardware, we find that we can assign them to one of four categories:
MS-DOS—compatible applications
ROM BIOS—compatible applications
Hardware-compatible applications
"Ill-behaved" applications
MS-DOS—compatible applications use only the documented MS-DOS function calls and do not call the ROM BIOS or access the hardware directly. They use ANSI escape sequences for screen control, and their input and output is redirectable. An MS-DOS—compatible application will run on any machine that supports MS-DOS, regardless of the machine configuration. Because of the relatively poor performance of MS-DOS's built-in display and serial port drivers, few popular programs other than compilers, assemblers, and linkers fall into this category.
ROM BIOS—compatible applications use the documented MS-DOS and ROM BIOS function calls but do not access the hardware directly. As recently as three years ago, this strategy might have significantly limited a program's potential market. Today, the availability of high-quality IBM-compatible ROM BIOSes from companies such as Phoenix has ensured the dominance of the IBM ROM BIOS standard; virtually no machines are being sold in which a program cannot rely as much on the ROM BIOS interface as it might on the MS-DOS interface. However, as we noted in Chapters 6 and 7, the ROM BIOS display and serial drivers are still not adequate to the needs of high-performance interactive applications, so the popular programs that fall into this category are few.
Hardware-compatible applications generally use MS-DOS functions for mass storage, memory management, and the like, and use a mix of MS-DOS and ROM BIOS function calls and direct hardware access for their user interfaces. The amount of hardware dependence in such programs varies widely. For example, some programs only write characters and attributes into the video controller's regen buffer and use the ROM BIOS to switch modes and position the cursor; others bypass the ROM BIOS video driver altogether and take complete control of the video adapter. As this book is written, the vast majority of the popular MS-DOS "productivity" applications (word processors, databases, telecommunications programs, and so on) can be placed somewhere in this category.
"Ill-behaved" applications are those that rely on undocumented MS-DOS function calls or data structures, interception of MS-DOS or ROM BIOS interrupts, or direct access to mass storage devices (bypassing the MS-DOS file system). These programs tend to be extremely sensitive to their environment and typically must be "adjusted" in order to work with each new MS-DOS version or PC model. Virtually all popular terminate- and-stay-resident (TSR) utilities, network programs, and disk repair/optimization packages are in this category.