![]() ![]() While not enthusiastically embraced by the Linux kernel developers, it offers some advantages over legacy BIOS and is becoming more widely used, especially for 64-bit x86 systems. The Extensible Firmware Interface is a BIOS replacement originally developed by Intel® for its Itanium® systems and later expanded to include x86 and other architectures. Virtualized systems (vmware, qemu, etc.) typically have their own legacy BIOS implementations and will use this method to boot ChromiumOS images. We have a tool and scripts that can change the boot partition GUID in the MBR when we need to select an alternate boot path. The second-stage syslinux bootloader is installed on that partition, along with its corresponding config file (/syslinux/syslinux.cfg). That code can specify one GPT partition to boot, indentified by a matching UniquePartitionGUID field in the Partition Entry Array. The ChromiumOS build process places GPT-aware boot sector code from syslinux in the MBR. Legacy BIOSes will continue to boot ChromiumOS from the MBR. The first part of the kernel then switches to protected mode and jumps to the kernel's 32-bit entry point, passing along the pointer to the zeropage table. At this point, the CPU is still running in Real Mode. After the zeropage table is filled in, a pointer to it is placed in the ESI register and execution continues with the first part of the kernel. It also creates a special table in memory called the “zeropage table.” The bootloader initializes fields in that table by making calls to the BIOS via interrupts. Eventually, the bootloader code identifies the kernel.That code then continues the boot process in some unspecified way - typically that VBR code (as well as the MBR code) is generated by and installed by grub, lilo, syslinux, or some similar bootloader. ![]() It searches the four partition table entries, finds a partition flagged as bootable, copies the first 512 bytes from that partition (the so-called Volume Boot Record or VBR) into RAM, and jumps there.
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