Malicious Mobile Code

DOS Technologies

A DOS PC boots up, places DOS in control, and then runs a myriad of possible files and programs. Booting is the group of processes a PC executes to check itself for basic configuration errors and to load the operating system. A lot is happening during the first minute a PC is turned on.

PC Boot Sequence

Tip

The following explanation is going to assume an Intel PC running MS-DOS with one hard drive.

On every PC, many processes and checks must be made prior to any program or user being able to execute the first command. Much of the initial boot sequence, as shown in Figure 2-1, is dedicated to performing simple hardware self-checks and is the same regardless of the operating system. Once the operating system (OS) begins to boot, the sequence differs according to the particular needs of the OS.

Figure 2-1. Normal PC boot sequence (regardless of operating system)

After you flip the power switch, the power supply does a quick self-check and sends a signal to the CPU to start. The CPU initializes itself and starts executing hardware self-check code located in the read-only memory basic input/output system ( ROM BIOS) chip located on the motherboard. The ROM BIOS chip contains instructions that are “burned into” the chip and aren’t normally changed. Early on, it took special equipment using ultraviolet light to write to the ROM BIOS chip. Today, the “burn-in” process can ...