Every computer with the IBM PC architecture follows approximately the same boot process. We have divided this process into four levels of abstraction (see figure 2), which correspond to phases of the bootstrap operation. The first phase is the Power on Self Test or POST [17]. POST is invoked in one of four ways:
The final step of the POST process calls the BIOS operating system bootstrap interrupt (Int 19h). The bootstrap code first finds a bootable disk by searching the disk search order defined in the CMOS. Once it finds a bootable disk, it loads the primary boot block into memory and passes control to it. The code contained in the boot block proceeds to load the operating system, or a secondary boot block depending on the operating system [10] [7].
Once the BIOS has performed all of its power on tests, it begins searching for expansion card ROMs which are identified in memory by a specific signature. Once a valid ROM signature is found by the BIOS, control is immediately passed to it. When the ROM completes its execution, control is returned to the BIOS.
Ideally, the boot process would proceed in a series of levels with each level passing control to the next until the operating system kernel is running. Unfortunately, the IBM architecture uses a ``star like'' model which is shown in figure 2.
