Bootloader

The bootloader loads after the BIOS sets up the computer and tries to find an operating system to start. A bootloader can start different kinds of operating systems.

On a real mac the bootloader 'boot-132' is used.

Currently, Chameleon or PC EFI are used most. The GNU GRUB 2 should also be able to boot XNU from a GUID disk.

Bootloaders for OSX86 mainly emulate an EFI, but can also load up additional kexts or change the DSDT.

The whole boot-process can be seen in this PDF.

OS X boot process

A Darwin bootloader passes arguments to the XNU kernel in the struct boot_args. This structure is defined in the BootX project in the file boot_args.h .

struct boot_args {

unsigned short Revision; /* Revision of boot_args structure */

unsigned short Version; /* Version of boot_args structure */

char CommandLine[BOOT_LINE_LENGTH]; /* Passed in command line */

DRAMBank PhysicalDRAM[kMaxDRAMBanks]; /* base/range pairs for the 26 DRAM banks */

Boot_Video Video; /* Video Information */

unsigned long machineType; /* Machine Type (gestalt) */

void *deviceTreeP; /* Base of flattened device tree */

unsigned long deviceTreeLength; /* Length of flattened tree */

unsigned long topOfKernelData; /* Last address of kernel data area*/

};

OS X bootflags

The bootloader loads the kernel (default filename is mach_kernel), but also command line arguments (bootflags) can be posted to the kernel which change the way the kernel loads up.

• CPUS=1

only one CPU is used

• -s

OSX boots in single user mode, ie commandline for rescue usage

• -v

OSX boots in verbose mode, which shows the kernel loading process

• arch=i386

OSX boots in 32bit mode

• -f

forces rebuilding of extensions cache mkext

• "Graphics Mode"="1024x768x32"

change display resolution. For a permanent change see Technical_FAQ#How_to_change_display_resolution.3F

• "rd=disk0s1"

boot from a certain partition specified in BSD format. Disk 0 specifies first HDD and s1 specifies first partition as 0 is the MBR.

Chameleon

• Website
• Documentation

AsereBLN Booter

• AsereBLN Booter - Based on Chameleon RC4 / PCEFI10.5

PC EFI

• Website

Weblinks

• Wikipedia article