Boot Loader Function

Boot loader

Most computer systems can only execute code found in the memory (ROM or RAM). Modern operating systems are stored on hard disks, or occasionally on LiveCDs, USB flash drives, or other non-volatile storage devices. When a computer is first powered on, it doesn't have an operating system in memory. The computer's hardware alone cannot perform complex actions such as loading a program from disk, so an apparent paradox exists: to load the operating system into memory, one appears to need to have an operating system already loaded.

The solution is to use a special small program, called a bootstrap loader, bootstrap or boot loader. This program's only job is to load other software for the operating system to start. Often, multiple-stage boot loaders are used, in which several small programs of increasing complexity summon each other, until the last of them loads the operating system. The name bootstrap loader comes from the image of one pulling oneself up by one's bootstraps (see bootstrapping). It derives from the very earliest days of computers and is possibly one of the oldest pieces of computer terminology in common use.

Early programmable computers had a row of toggle switches on the front panel to allow the operator to manually enter the binary boot instructions into memory before transferring control to the CPU. The boot loader would then read the operating system in from an outside storage medium such as paper tape, punched card, or a disk drive.

Pseudo-assembly code for the bootloader might be as simple as the following eight instructions:

0: set the P register to 8
1: check paper tape reader ready
2: if not ready, jump to 1
3: read a byte from paper tape reader to accumulator
4: if end of tape, jump to 8
5: store accumulator to address in P register
6: increment the P register
7: jump to 1

A related example is based on a loader for a 1970's Nicolet Instrument Corporation minicomputer. Note that the bytes of the second-stage loader are read from paper tape in reverse order.

0: set the P register to 106
1: check paper tape reader ready
2: if not ready, jump to 1
3: read a byte from paper tape reader to accumulator
4: store accumulator to address in P register
5: decrement the P register
6: jump to 1

The length of the second stage loader is such that the final byte overwrites location 6. After the instruction in location 5 executes, location 6 starts the second stage loader executing. The second stage loader then waits for the much longer tape containing the operating system to be placed in the tape reader. The difference between the boot loader and second stage loader is the addition of checking code to trap paper tape read errors, a frequent occurrence with the hardware of the time, which in this case was an ASR-33 teletype.

In modern computers the bootstrapping process begins with the CPU executing software contained in ROM (for example, the BIOS of an IBM PC) at a predefined address (the CPU is designed to execute this software after reset without outside help). This software contains rudimentary functionality to search for devices eligible to participate in booting, and load a small program from a special section (most commonly the boot sector) of the most promising device. It is usually possible to configure the BIOS so that only a certain device can be booted from and/or to give priority to some devices over others (a CD or DVD drive is usually given priority over a hard disk, for instance).

Boot loaders may face peculiar constraints, especially in size; for instance, on the IBM PC and compatibles, the first stage of boot loaders located on hard drives must fit into the first 446 bytes of the Master Boot Record, in order to leave room for the 64-byte partition table and the 2-byte 0xAA55 'signature', which the BIOS requires for a proper boot loader.

Some operating systems, most notably pre-1995 Macintosh systems from Apple, are so closely interwoven with their hardware that it is impossible to natively boot an operating system other than the standard one. A common solution in such situations is to design a bootloader that works as a program belonging to the standard OS that hijacks the system and loads the alternative OS. This technique was used by Apple for its A/UX Unix implementation and copied by various freeware operating systems and BeOS Personal Edition 5.