The Unabridged Pentium 4 IA32 Processor Genealogy

Solution—Redirect Memory Accesses to Separate Memory Areas

The OS can multitask multiple DOS applications by taking the following precautions:

• Load each DOS application program into a separate 1MB area of memory.

• When a DOS program is executing, it only generates memory accesses within the first 1MB of memory. Since it actually resides in a different 1MB area other than the first MB, the processor must in some manner automatically redirect each of its memory accesses to the area that it really resides in. Figure 12-1 on page 213 illustrates a scenario wherein two DOS application programs have each been placed in a separate 1MB memory area along with a complete copy of what each expects in the 1st MB of memory space.

  Figure 12-1. Paging Redirects ...