Linux Bootable Image

After the vmlinux kernel ELF has been built, it is further processed to strip off the redundant sections (comments and notes). The output is an object file called Image which is then compressed using gzip

cat Image | gzip -f -o > piggy.gz

Next the bootstrap loader is built. Bootstrap loader is the 2nd stage loader which prepare the context for the Linux kernel to run in. It is different from the bootloader (1st stage loader) of which control will be passed over to once the hardware is power on. Bootloader performs low level initialization and diagnosis utilities.

Bootstrap loader perform the following functions:
(1) head.o and head-"arch".o - low level assembly language processor initialization, which include enabling the processor's internal instruction and data cache, disabling interrupts and setting up a C runtime.
(2) misc.o - decompress and relocate linux kernel
(3) other initialization

Bootstrap loader contains an assembly program called piggy.s. The program contains a include (inclib) for piggy.gz. In other words, when piggy.s is assembled, the compressed kernel is piggyback into the bootstrap loader. The bootstrap also include other object code like head.o to form the boatable kernel image called zImage.

In summary, the bootable kernel image contains the following codes:
(1) piggy.o - asm wrapper around piggy.gz which is a compressed vmlinux without notes and comments sections
(2) head.o
(3) head-"arch".o - architecture specific intialization
(4) misc.o
(5) others