What BIOS does for us
The BIOS is the firmware in the ROM of a PC. When the PC is powered up, the BIOS is the first program that runs. All other programs must be loaded into RAM first. The BIOS contains the following parts:
* POST (Power On Self Test). The running counter that counts the kilobytes of main memory is the most visible part of the POST.
* The Setup Menu, that lets you set some parameters and lets you adjust the real time clock. Most modern BIOS versions let you set the boot order, the devices that BIOS checks for booting. These can be A (the first floppy disk), C (the first hard disk), CD-ROM and possibly other disks as well. The first device in the list will be tried first. Older BIOS-es have only one boot order: A, C. So the BIOS will try to boot from A first and if there is no diskette in the drive it tries to boot from C.
* The boot sector loader. This loads the first 512-byte sector from the boot disk into RAM and jumps to it. This is where the boot loaders described in this paper start.
* The BIOS interrupts. These are simple device drivers that programs can use to access the screen, the keyboard and disks. Boot loaders rely on them, most operating systems do not (the Linux kernel does not use BIOS interrupts once it has been started). MSDOS does use BIOS interrupts.
Apart from the main BIOS there are extension ROMs, which are started by the main BIOS. Every VGA card has one. Also SCSI host adapters and Ethernet cards can have an extension ROM. It is even possible to put an EPROM on an Ethernet card to boot a machine over the network without any disks.
As far as boot loading facilities are concerned, the PC BIOS is very primitive compared to that of other computer systems. The only thing it knows about disks is how to load the first 512-byte sector.
* The first sector of a diskette can be loaded at address 0000:7C00. The last two bytes of the sector are checked for the values 0x55 and 0xAA, this as a rough sanity check. If these are OK, the BIOS jumps to the address 0000:7C00.
* Booting from a hard disk is very similar to booting from a diskette. The first sector of a hard disk (often called the Master Boot Record) is loaded at 0000:7C00 and next the BIOS jumps to it. The MBR program must move itself to an address that is different from 0000:7C00 as it is supposed to load a different boot sector from a partition to address 0000:7C00 and jump to that.
* Modern BIOS versions can treat a certain file on a CD-ROM as a diskette image. They pretend to boot from a diskette by loading the first 512 bytes of the file to 0000:7C00 and jumping to it. Every attempt to access the same diskette using the BIOS routines, will be redirected to the image file on CD-ROM. Some other ways to boot a CD-ROM may also be supported (with an emulated hard disk or with no disk emulation at all).
When the boot sector is loaded, the CPU is in real mode. For those who are unfamiliar with 80x86 architecture: real mode is very limited compared to 32-bit protected mode (in which Linux runs). For example: data outside a 64K segment can only be accessed if you change a segment register and data outside the first 1MB of address space (which contains 640kB of main memory) cannot be accessed at all. As gcc does not know about real mode, programs compiled with it can only be run in real mode with some tricks and with severe memory size restrictions. This is the reason why most boot loaders (except GRUB) are written in assembly. All boot sector programs, even that of GRUB, are written in assembly1.
In theory a boot loader could do its job by directly accessing the bare metal, but 512 bytes would be a very small space for that. The boot loader has access to BIOS interrupts, which are subroutines that can be invoked by the INT instruction (software interrupts). These work in real mode only. The following routines are used by most boot loaders.
* INT 0x10 for screen output.
* INT 0x16 for keyboard input.
* INT 0x13 for disk I/O. The parameters to specify sectors on disk used to have a very limited range. Originally it was only possible to specify 1024 cylinders on a hard disk, while hard disks can have more cylinders. This imposed limitations on where it was allowed to put the boot loader and any files accessed by it. You were forced to create a small partition near the start of the hard disk and put the boot loader there. There are three categories of BIOS:
o BIOS versions earlier than 1995 could only access IDE disks of around 500MB, as the BIOS sector and head numbers corresponded directly to the register values on the IDE interface.
o BIOS versions between 1995 and 1998 can access IDE disks up to about 8GB. They translate the cylinder, head and sector numbers from the INT 0x13 call to different values that better utilize the allowable ranges on the IDE interface.
o BIOS versions of 1998 or later have a new calling interface using linear block addresses.
In any of the three categories you have BIOS-es that have bugs that cause them to stop at a lower disk size limit (a category 2 BIOS that should work up to 8GB, but stops at 2GB). In those cases it makes sense to upgrade to a new BIOS version.
* INT 0x15 is a catch-all for many BIOS functions, one of which is moving data to extended memory (the BIOS is required to switch to protected mode temporarily to do that). Other functions are for disabling the A20 gate and for determining the memory size.
