Power-On Self-Test (POST) Process

The Power-On Self-Test (POST) process starts when power is applied to the system. Electrical current makes its way from the power lead on the motherboard to the ROM-BIOS chips, and when the current is received by the BIOS chips they immediately begin executing their programs. One of the first checks is the memory (both a count and integrity test). After the memory check, the POST process moves on to find out what ports or I/O devices exist on the system. If the system is equipped with a PnP BIOS (Plug and Play BIOS), as most new systems are, then the BIOS-level PnP configuration takes place. The next thing that happens is a search for bootable disk devices. The order of this search is defined by the settings stored in CMOS memory, but is often: a: (floppy drive), c: (first partition on the first bootable hard drive), and then CD-ROM.

For each device in the list of potential bootable devices, the partition table is checked for the active partition. Floppy disks and the CD-ROM will only check the first partition. For this partition, the first sector is read and checked for a boot loader. For MS-DOS and Windows 9x the boot loader is io.sys, and for Windows 2000 and Windows XP the boot loader is ntldr.

When this file is located, it is executed. If it was not found on the first potential bootable device, then the second and third devices are checked before reporting a boot failure.