Section 8.1. BIOS and Open Firmware

8.1. BIOS and Open Firmware

Upon power-on the processor first accesses an address that usually resides in read-only memory. This read-only memory is often referred to as Flash ROM (or just Flash). This is where the first code that runs on the system resides. This code is responsible for enabling enough of the system to handle the loading of the kernel.

For x86 systems, this is where the system BIOS resides. The Basic Input Output System (BIOS) is a block of hardware-specific system initialization code that boots the system. In x86 systems, the boot loader and, in turn Linux, depend on BIOS to bring the system to a known state. The interface to BIOS is a uniform set of functions known as interrupts. At load time, Linux uses these interrupts to query available system resources. After BIOS completes its initialization, it copies the first 512 bytes from the boot device (which is discussed in the next section) to address 0x7c00 and jumps to it. Although in some installations, BIOS loads the operating system over a network connection, this discussion is confined to the process when loading Linux from the hard drive. When Linux is loaded, BIOS is still in memory and its functions are accessible and called by way of interrupts.

For PowerPC, the type of initialization code depends on the age of the specific PowerPC architecture. Older IBM systems use PowerPC Reference Platform (PreP) whereas more recent IBM systems use Common Hardware Reference Platform (CHRP). G4 systems and later have been called "True New World" and use Open Firmware (OF) bound to a particular architecture implementation. (For more information on this processor and system-independent boot firmware and how it is bound to one of these formats, see the Open Firmware home page at www.openfirmware.org.)