The BIOS Boot Process

1. Turning on Computers' power causes the processor to enter a reset state, clears all memory locations to zero, perform a parity check of memory and set the CS register to segment address FFFFh and IP register to zero.

2. Hence the first instruction to execute, therefore is at address formed by CS:IP pair, which is FFFF0H, the entry point to BIOS in ROM

3. BIOS routine at FFFF0H checks the various ports to identify and initialize devices taht are attached to the computer and provides services that are used for reading to and for writing from the devices.

4. BIOS then establishes two data areas -

• IVT (Interrupt Vector Table): Begins in low memory at location 0 and contains 256 4-bytes address in the form of segment:offset (Both BIOS and OS uses these IVT for interrupts that occur)
• BIOS data Areas: Beginning at location 400H, largely concerned with the status of attached devices

5. BIOS next determines whether a disk containing the system files is present and, if so, it accesses the bootstrap loader from the disk

6. This BSP (Boot strap program) loads system files from the disk into memory and transfers control to them (System files contains device drivers and other hardware-specific code which initializes internal system tables and the systems' portion of IVT)

NOTE: When a user program requests an IO services of OS, it transfers request to BIOS, which in turn accesses requested device. Sometimes, program makes requests directly to BIOS, such as keyboard and screen services. At other times, a program can bypass both OS and BIOS to access a device directly