boot2
defines an important structure,
struct bootinfo
. This structure is
initialized by boot2
and passed to the
loader, and then further to the kernel. Some nodes of this
structures are set by boot2
, the rest by the
loader. This structure, among other information, contains the
kernel filename, BIOS harddisk geometry,
BIOS drive number for boot device, physical
memory available, envp
pointer etc. The
definition for it is:
/usr/include/machine/bootinfo.h:
struct bootinfo {
u_int32_t bi_version;
u_int32_t bi_kernelname; /* represents a char * */
u_int32_t bi_nfs_diskless; /* struct nfs_diskless * */
/* End of fields that are always present. */
#define bi_endcommon bi_n_bios_used
u_int32_t bi_n_bios_used;
u_int32_t bi_bios_geom[N_BIOS_GEOM];
u_int32_t bi_size;
u_int8_t bi_memsizes_valid;
u_int8_t bi_bios_dev; /* bootdev BIOS unit number */
u_int8_t bi_pad[2];
u_int32_t bi_basemem;
u_int32_t bi_extmem;
u_int32_t bi_symtab; /* struct symtab * */
u_int32_t bi_esymtab; /* struct symtab * */
/* Items below only from advanced bootloader */
u_int32_t bi_kernend; /* end of kernel space */
u_int32_t bi_envp; /* environment */
u_int32_t bi_modulep; /* preloaded modules */
};
boot2
enters into an infinite loop
waiting for user input, then calls load()
.
If the user does not press anything, the loop breaks by a
timeout, so load()
will load the default
file (/boot/loader
). Functions
ino_t lookup(char *filename)
and
int xfsread(ino_t inode, void *buf, size_t
nbyte)
are used to read the content of a file into
memory. /boot/loader
is an
ELF binary, but where the
ELF header is prepended with
a.out
's struct
exec
structure. load()
scans the
loader's ELF header, loading the content of
/boot/loader
into memory, and passing the
execution to the loader's entry:
sys/boot/i386/boot2/boot2.c:
__exec((caddr_t)addr, RB_BOOTINFO | (opts & RBX_MASK),
MAKEBOOTDEV(dev_maj[dsk.type], 0, dsk.slice, dsk.unit, dsk.part),
0, 0, 0, VTOP(&bootinfo));
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