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(Contributed by Randall Hopper, <rhh@ct.picker.com>)
This section attempts to give you enough basic information about your hard disks and the disk booting process so that you can troubleshoot most problems you might encounter when getting set up to boot several operating systems. It starts in pretty basic terms, so you may want to skim down in this section until it begins to look unfamiliar and then start reading.
Three fundamental terms are used to describe the location of data on your hard disk: Cylinders, Heads, and Sectors. It is not particularly important to know what these terms relate to except to know that, together, they identify where data is physically on your disk.
Your disk has a particular number of cylinders, number of heads, and number of sectors per cylinder-head (a cylinder-head also known now as a track). Collectively this information defines the “physical disk geometry” for your hard disk. There are typically 512 bytes per sector, and 63 sectors per track, with the number of cylinders and heads varying widely from disk to disk. Thus you can figure the number of bytes of data that will fit on your own disk by calculating:
For example, on my 1.6 Gig Western Digital AC31600 EIDE hard disk, that is:
which is 1,624,670,208 bytes, or around 1.6 Gig.
You can find out the physical disk geometry (number of cylinders, heads, and sectors/track counts) for your hard disks using ATAID or other programs off the net. Your hard disk probably came with this information as well. Be careful though: if you are using BIOS LBA (see Section 7.3), you can not use just any program to get the physical geometry. This is because many programs (e.g. MSD.EXE or FreeBSD fdisk) do not identify the physical disk geometry; they instead report the translated geometry (virtual numbers from using LBA). Stay tuned for what that means.
One other useful thing about these terms. Given 3 numbers—a cylinder number, a head number, and a sector-within-track number—you identify a specific absolute sector (a 512 byte block of data) on your disk. Cylinders and Heads are numbered up from 0, and Sectors are numbered up from 1.
For those that are interested in more technical details, information on disk geometry, boot sectors, BIOSes, etc. can be found all over the net. Query Lycos, Yahoo, etc. for boot sector or master boot record. Among the useful info you will find are Hale Landis's How It Works document pack. See the Section 6 section for a few pointers to this pack.
Ok, enough terminology. We are talking about booting here.
On the first sector of your disk (Cyl 0, Head 0, Sector 1) lives the Master Boot Record (MBR). It contains a map of your disk. It identifies up to 4 partitions, each of which is a contiguous chunk of that disk. FreeBSD calls partitions slices to avoid confusion with its own partitions, but we will not do that here. Each partition can contain its own operating system.
Each partition entry in the MBR has a Partition ID, a Start Cylinder/Head/Sector, and an End Cylinder/Head/Sector. The Partition ID tells what type of partition it is (what OS) and the Start/End tells where it is. Table 1 lists a smattering of some common Partition IDs.
Table 1. Partition IDs
| ID (hex) | Description |
|---|---|
| 01 | Primary DOS12 (12-bit FAT) |
| 04 | Primary DOS16 (16-bit FAT) |
| 05 | Extended DOS |
| 06 | Primary big DOS (> 32MB) |
| 0A | OS/2® |
| 83 | Linux (EXT2FS) |
| A5 | FreeBSD, NetBSD, 386BSD (UFS) |
Note that not all partitions are bootable (e.g. Extended DOS). Some are—some are not. What makes a partition bootable is the configuration of the Partition Boot Sector that exists at the beginning of each partition.
When you configure your favorite boot manager, it looks up the entries in the MBR partition tables of all your hard disks and lets you name the entries in that list. Then when you boot, the boot manager is invoked by special code in the Master Boot Sector of the first probed hard disk on your system. It looks at the MBR partition table entry corresponding to the partition choice you made, uses the Start Cylinder/Head/Sector information for that partition, loads up the Partition Boot Sector for that partition, and gives it control. That Boot Sector for the partition itself contains enough information to start loading the operating system on that partition.
One thing we just brushed past that is important to know. All of your hard disks have MBRs. However, the one that is important is the one on the disk that is first probed by the BIOS. If you have only IDE hard disks, it is the first IDE disk (e.g. primary disk on first controller). Similarly for SCSI only systems. If you have both IDE and SCSI hard disks though, the IDE disk is typically probed first by the BIOS, so the first IDE disk is the first probed disk. The boot manager you will install will be hooked into the MBR on this first probed hard disk that we have just described.
Now the interesting stuff that you need to watch out for.
The first part of the booting process is all done through the BIOS, (if that is a new term to you, the BIOS is a software chip on your system motherboard which provides startup code for your computer). As such, this first part of the process is subject to the limitations of the BIOS interface.
The BIOS interface used to read the hard disk during this period (INT 13H, Subfunction 2) allocates 10 bits to the Cylinder Number, 8 bits to the Head Number, and 6 bits to the Sector Number. This restricts users of this interface (i.e. boot managers hooked into your disk's MBR as well as OS loaders hooked into the Boot Sectors) to the following limits:
1024 cylinders, max
256 heads, max
64 sectors/track, max (actually 63, 0 is not available)
Now big hard disks have lots of cylinders but not a lot of heads, so invariably with big hard disks the number of cylinders is greater than 1024. Given this and the BIOS interface as is, you can not boot off just anywhere on your hard disk. The boot code (the boot manager and the OS loader hooked into all bootable partitions' Boot Sectors) has to reside below cylinder 1024. In fact, if your hard disk is typical and has 16 heads, this equates to:
which is around the often-mentioned 528MB limit.
This is where BIOS LBA (Logical Block Addressing) comes in. BIOS LBA gives the user of the BIOS API calls access to physical cylinders above 1024 though the BIOS interfaces by redefining a cylinder. That is, it remaps your cylinders and heads, making it appear through the BIOS as though the disk has fewer cylinders and more heads than it actually does. In other words, it takes advantage of the fact that hard disks have relatively few heads and lots of cylinders by shifting the balance between number of cylinders and number of heads so that both numbers lie below the above-mentioned limits (1024 cylinders, 256 heads).
With BIOS LBA, the hard disk size limitation is virtually removed (well, pushed up to 8 Gigabytes anyway). If you have an LBA BIOS, you can put FreeBSD or any OS anywhere you want and not hit the 1024 cylinder limit.
To use my 1.6 Gig Western Digital as an example again, its physical geometry is:
However, my BIOS LBA remaps this to:
giving the same effective size disk, but with cylinder and head counts within the BIOS API's range (Incidentally, I have both Linux and FreeBSD existing on one of my hard disks above the 1024th physical cylinder, and both operating systems boot fine, thanks to BIOS LBA).
Another gotcha to watch out when installing boot managers is allocating space for your boot manager. It is best to be aware of this issue up front to save yourself from having to reinstall one or more of your OSs.
If you followed the discussion in Section 7.2 about the Master Boot Sector (where the MBR is), Partition Boot Sectors, and the booting process, you may have been wondering just exactly where on your hard disk that nifty boot manager is going to live. Well, some boot managers are small enough to fit entirely within the Master Boot Sector (Cylinder 0, Head 0, Sector 0) along with the partition table. Others need a bit more room and actually extend a few sectors past the Master Boot Sector in the Cylinder 0 Head 0 track, since that is typically free…typically.
That is the catch. Some operating systems (FreeBSD included) let you start their partitions right after the Master Boot Sector at Cylinder 0, Head 0, Sector 2 if you want. In fact, if you give FreeBSD's sysinstall a disk with an empty chunk up front or the whole disk empty, that is where it will start the FreeBSD partition by default (at least it did when I fell into this trap). Then when you go to install your boot manager, if it is one that occupies a few extra sectors after the MBR, it will overwrite the front of the first partition's data. In the case of FreeBSD, this overwrites the disk label, and renders your FreeBSD partition unbootable.
The easy way to avoid this problem (and leave yourself the flexibility to try different boot managers later) is just to always leave the first full track on your disk unallocated when you partition your disk. That is, leave the space from Cylinder 0, Head 0, Sector 2 through Cylinder 0, Head 0, Sector 63 unallocated, and start your first partition at Cylinder 0, Head 1, Sector 1. For what it is worth, when you create a DOS partition at the front of your disk, DOS leaves this space open by default (this is why some boot managers assume it is free). So creating a DOS partition up at the front of your disk avoids this problem altogether. I like to do this myself, creating 1 Meg DOS partition up front, because it also avoids my primary DOS drive letters shifting later when I repartition.
For reference, the following boot managers use the Master Boot Sector to store their code and data:
OS-BS 1.35
Boot Easy
LILO
These boot managers use a few additional sectors after the Master Boot Sector:
OS-BS 2.0 Beta 8 (sectors 2-5)
The OS/2 boot manager
At some point when installing boot managers, you might leave the MBR in a state such that your machine will not boot. This is unlikely, but possible when re-FDISKing underneath an already-installed boot manager.
If you have a bootable DOS partition on your disk, you can boot off a DOS floppy, and run:
to put the original, simple DOS boot code back into the system. You can then boot DOS (and DOS only) off the hard drive. Alternatively, just re-run your boot manager installation program off a bootable floppy.