Goto 'Bootstrap'

The term ‘Bootstrap’, which originally refers to a person who tries to stand up by pulling his own boots, refers to a subroutine used to establish the full routine(its own left part, i think) or another routine in computer science. Today modern computers act as a vital role in our daily life and many of you may wonder what happens to the computer when you have it powered on. The ‘Bootstrap’, which is also called ‘boot’ for short, is the first step to be done by the computer. The process of ‘Bootstrap’, which starts on when the computer is powered on and usu ends off when the kernel of the operating system begins to run, is just what we are gonna describe.

As a matter of convenience, we are gonna try to understand the ‘Bootstrap’ process of linux operating system on platforms compatible with ‘i386′, since linux is source-opened and its source is absolutely free. We can boot linux from any bootable devices, such as hard disk, floopy, or cd-rom. We choose to boot from a hard disk which is more complex than the other two. Now we suppose a linux-installed computer is in front of us. Press the power button and we will go to the ‘Bootstrap’ process.

The normal ‘Bootstrap’ flow can be described as following:
[Hardware initialization] -> [BIOS routine] -> [Bootloader run] -> End (Kernel startup)

1. Hardware initialization
Immediately after the power-up or an assertion of the RESET# pin, the processor performs a hardware initialization and an optional built-in self-test(BIST for short). The hardware initialization sets the processor’s registers to a known state and places the processor in real-address operating mode which we have mentioned in "Inside the ‘i386′". The process state after power-up or reset is vatal, since it decides the address of the code from which the processor is going to execute. Here lists the initial states of some registers:

(1) R[EAX] = R[EBX] = R[ECX] = R[ESI] = R[EDI] = R[ESP] = R[EBP] = 0×00000000 (Note: If the value in the EAX register does not equal to 0H after the BIST, it indicates that a processor fault was detected.)
(2) the EDX register contains component identification and revision information and different values indicate the various members of these Intel Architecture families.
(3) R[CS] = 0xF000 (Note: In its hidden part, ‘Base’ = 0xFFFF0000, Limit = 0xFFFF, AR = Present, R/W, Accessed.)
(4) R[DS] = R[ES] = R[FS] = R[GS] = R[SS] = 0×0000 (Note: In their hidden parts, ‘Base’ = 0×00000000, Limit = 0xFFFF, AR = Present, R/W, Accessed.)
(5) R[EFLAGS] = 0×00000002 (Note: The 10 most-significant bits of this register are undefined following a reset. Software should not depend on the states of any of these bits.)
(6) R[EIP] = 0x0000FFF0

After the hardware initialization, The first instruction that is fetched and executed is located at physical address 0xFFFFFFF0. The BIOS EPROM containing the software initialization code must be located at this address, otherwise the processor can not locate and fetch its first instruction. Here we know that the processor is using ‘Read-Address mode model’, since it is in ‘real-address’ operating mode. but the address 0xFFFFFFF0 is beyond the 1-MByte addressable range of the processor while in real-address mode. How is the processor initialized to this starting address? As mentioned in "Inside the ‘i386′", the CS register has two parts: the visible segment selector part and the hidden base address part. In real-address mode, the base address is normally formed by shifting the 16-bit segment selector value 4 bits to the left to produce a 20-bit base address according to the normal rule. However, during the hardware initialization, the normal rule doesn’t be followd. the segment selector in the CS register is loaded with 0xF000 and the base address is loaded with 0xFFFF0000. The starting address is thus formed by adding the base address to the value in the EIP register (that is, 0xFFFF0000 + 0xFFF0 = 0xFFFFFFF0). The first time the CS register is loaded with a new value after the hardware initialization, the processor will follow the normal rule for address translation in real-address mode (that is, [CS base address = CS segment selector * 16]). To insure that the base address in the CS register remains unchanged until the EPROM based software initialization code is completed, the code must not contain a far jump or far call or allow an interrupt to occur (which would cause the CS selector value to be changed).

2. BIOS routine (software initialization)
Here, the hardware initialization of the processor is over and the first instruction, which is also the first one of BIOS routine, is executed. From now on, the BIOS routine, which is the very first program run by the processor takes control and begins to run. ‘BIOS’(Basic Input/Output System) is the software embedded on a chip (usu EPROM) located on the computer’s main board and it is also called ‘firmware’.

The BIOS routine also uses ‘Real-Address’ mode model and performs the following operations:
[Power-on self-test] -> [Hardware devices initialization] -> [Load boot sector] -> End (the loaded sector takes control)

(1) Power-on self-test (POST)
The BIOS routine executes a series of tests to establish which devices are present and whether they are working properly. It also initializes the standard devices, such as the memory controller, video controller, IDE controller and floppy controller. Using stored parameters, it initializes the motherboard chipset, and sets timing parameters. It also creates an interrupt vector table and provides a set of services, accessible through interrupts, that allow access to the standard I/O devices. During this phase we may get some messages displayed on the screen, such as the BIOS version banner or etc.

(2) Hardware devices initialization
In this phase, the BIOS routine guarantees that all hardware devices operate without conflicts on the IRQ lines and I/O ports and a table of installed PCI devices will be displayed on the screen.

(3) Load boot sector
After the ‘POST’ and the initialization of hardware devices, the BIOS routine call ‘Int19H’ service routine to search for the valid boot sector, which has the signature ’0x55AA’ in its last two bytes. As soon as a valid sector is found, the BIOS routine continues to call ‘Int13H’ service routine to load the valid sector to the address ’0x00007C00′, and then jumps into this address and executes the code just loaded.

3. Bootloader run
The valid sector loaded from hard disk by BIOS routine is usu called ‘Master Boot Sector’, which consists of ‘Master Boot Record(MBR)’, ‘Disk Partition Table(DPT)’ and ‘Boot Record ID(0x55AA)’. Usu the MBR stores a small program which used to load the first sector of the partition containing the operating system to be started. Today a two-stage boot loader such as LILO, GRUB is required to boot a Linux kernel from disk. These bootloaders may be installed either on the MBR (replacing that small program that loads the boot sector of the active partition) or in the boot sector of every disk partition. Whatever, the final result is the same. These bootloaders usu are broken into two parts, since they are too large to fit into one single sector, which size is 512 bytes. The MBR or the p
artition boot sector contains the first part of one of these bootloaders, which is loaded into memory from address 0x00007C00 by the BIOS routine. Then the first part program moves itself to another special address (it is 0x0009A000 for LILO), loads the second part of the bootloader into memory and jumps to execute the just loaded code. the second part of bootloader offers user a chance to choose from a list of bootable operating systems from disk. After the user has chosen the kernel to be loaded, the boot loader may either copy the boot sector of the corresponding partition into memory (the user has chosen the kernel in other partition) and execute it or directly copy the kernel image into memory (the user has chosen linux kernel in current partition). If the linux kernel is gonna loaded, the bootloader calls a BIOS routine to load the first 512 bytes of the kernel image to the address 0×00090000 , load the code of ‘setup.S’ to the address 0×00090200 and load the rest of the kernel image to either low address 0×00010000 (for small kernel images compiled with make zImage) or high address 0×00100000 (for big kernel images compiled with make bzImage). At last the bootloader jumps to execute the ‘setup.S’ code.

Here, the ‘Bootstrap’ process has come to a conclusion.