Mac OS X Internals: A Systems Approach

4.13. Launching the Kernel

In Chapter 5, we will discuss system startup from the point where the kernel begins to execute. Let us briefly examine the kernel binary to determine the starting point of the kernelthat is, the point at which BootX transfers control.

The Mac OS X kernel is a Mach-O executable. Recall from Chapter 2 that we can use the otool command-line program to view a Mach-O executable's header and load commands.

$ file /mach_kernel /mach_kernel: Mach-O executable ppc $ otool -hv /mach_kernel /mach_kernel: Mach header magic cputype cpusubtype filetype ncmds sizeofcmds flags MH_MAGIC PPC ALL EXECUTE 9 2360 NOUNDEFS $ otool -l /mach_kernel /mach_kernel: Load command 0 cmd LC_SEGMENT cmdsize 532 segname __TEXT vmaddr 0x0000e000 vmsize 0x0034f000 ... Load command 2 cmd LC_SEGMENT cmdsize 124 segname __VECTORS vmaddr 0x00000000 vmsize 0x00007000 fileoff 3624960 filesize 28672 maxprot 0x00000007 initprot 0x00000003 nsects 1 flags 0x0 Section sectname __interrupts segname __VECTORS addr 0x00000000 size 0x00007000 offset 3624960 align 2^12 (4096) reloff 0 nreloc 0 flags 0x00000000 reserved1 0 reserved2 0 ... Load command 8 cmd LC_UNIXTHREAD cmdsize 176 flavor PPC_THREAD_STATE count PPC_THREAD_STATE_COUNT ... srr0 0x00092340 srr1 0x00000000

The SRR0 register contains the value 0x00092340 in the initial thread state of this particular kernel. The code at this address is the entry point of the kernel. We can use nm to determine the symbol, if any, that has this address.

$ nm /mach_kernel | grep 00092340 00092340 T __start