DOS Application—Portrait of an Anarchist

The chapter entitled “Multitasking Problems” on page 31 introduced some of the ways in which DOS programs are disruptive in a multitasking environment. They:

• may attempt to access memory belonging to currently-suspended programs,

• may communicate directly with IO ports,

• can call OS code (even routines they shouldn't be able to),

• may disable interrupt recognition when they don't wish to be interrupted,

• frequently call BIOS routines to indirectly communicate with IO devices (thereby bypassing the OS).

In addition, the task assumes that DOS is the OS it is interacting with when it may be a completely different OS (e.g., Windows XP). In this case, all OS calls initiated by the DOS task must be intercepted and passed to the host OS (or another program that substitutes for the DOS OS).