On Tue, 9 Sep 2003, at 12:17:54, Mark Terribile wrote: >>>> > Well, I've got my 120G disk up and running and > now I'd like to understand something odd. > > I went into /stand/sysinstall to put down the > partition and slice tables and the FS and found that > it determined the geometry (cyl/head/sectors) as > 15107/255/63 . Since it warned that the values must > match the BIOS values, I set them to the 59131/16/255 > that the BIOS reports. It rejected this entry and > went back to its own. I also tried the values > reported > by the ata driver on startup; these too were rejected. > > Since it's working (with about 117G -- that's > Gi_b_abytes) it must be OK (famous last words!) but > what's going on here? Why does it demand the values > reported by the BIOS if it will refuse them, and why > does the driver come up with another set of values, > also unacceptible to the disklabel/fdisk machinery? >>>>
I expect that most people reading this are aware of some of the limitations of "BIOS" disk addressing, including the physical disk addresses in the disk partition table in the MBR (master bootstrap record) of bootable PC disk drives which allocates only 10 bits for the cylinder number, 8 bits for the head number, and 6 bits for the sector number. There is also a Fixed Disk "Drive Parameter Table" available through the traditional BIOS. The table format allocates 16 bits for the number of cylinders, 8 bits for the number of heads, and 8 bits for the number of sectors per track. Since BIOS sector numbers begin at one (not zero), the BIOS addressing limitations are usually given as 1024 cylinders, 255 heads, and 63 sectors per track. (Note: 1024*255*63 sectors is just under 8GB.) The ATA/IDE specifications limit you to 65535 cylinders, 16 heads, and 255 sectors per track (see http://www.t13.org). If you accept both sets of limitations at once, you get a maximum of 1024 cylinders, 16 heads and 63 sectors, resulting in the infamous "528" MB hard disk size limitation. In an effort to work around this unacceptable limitation, your BIOS may pretend that your disk drive has some other geometry, typically using 255 heads and 63 sectors per track though other fake geometries are sometimes used. When a program uses the traditional BIOS system call to do disk I/O (machine instruction "int 13" with a small function code < 0x16 in the AH register), it must use the same disk geometry that is used by the BIOS for interpreting disk addresses. Otherwise the BIOS might translate the specified physical disk address into the wrong actual disk address. (Note: there are also "Enhanced Disk Drive Services" (EDD) using AH register function codes >=0x40 which have bigger limitations and might support Logical Block Addresses (LBA) instead of physical disk addresses. EDD is also described at http://www.t13.org. I don't know which BIOS/bootstrap programs support/use EDD. Modern BIOS presumably provides EDD services. It doesn't do you any good if your BIOS supports EDD but your bootstrap program doesn't use it. The FreeBSD minimal bootstrap that just boots the active partition, /usr/src/sys/boot/i386/mbr/mbr.s, appears to use EDD services when available. The "booteasy" program that allows you to specify a desired partition via a function key, which may be the program .../tools/srcs/bteasy/BOOT.ASM in your FreeBSD distribution, does not seem to use EDD services. (I am not certain that this is the bootstrap installed by /stand/sysinstall.) The GRUB bootstrap program (in /usr/ports/sysutils/grub) appears to use both EDD services and LBA when available.) The only way to discover the geometry expected by the BIOS is to ask the BIOS. This is no problem for bootstrap programs and some Microsoft "operating systems" that use the BIOS disk services for all disk I/O, but the BIOS services cannot be used by a "real" operating system after it takes over the interrupt and virtual memory management hardware and possibly trashes the data structures built by the BIOS during POST (Power On Self Test). A BIOS service routine is likely to do things unacceptable to a "real" OS, such as screwing with system hardware configuration registers or going into indefinite spin loops waiting for device register bits to change. The FreeBSD /stand/sysinstall and fdisk programs need to use the BIOS disk geometry when constructing MBR partition tables. It would be nice if the kernel could get this information from the BIOS early in the bootstrap process and stash it somewhere for further reference, but this seems not to be a FreeBSD kernel feature. The fdisk and /stand/sysinstall programs apparently try to guess the BIOS disk geometry and then ask the user for the "correct" geometry. It would only be fair if the man pages for those programs told you *how* to determine the correct geometry. Some BIOS self-configuration programs (what you boot when holding down the DEL key) may tell you about some ATA/IDE drives, but I am not sure I trust them. One BIOS gives me a choice of three or four different geometries for each drive, some of which are suspect because they specify more than 63 sectors per track. Keep in mind that your BIOS may support EDD services but your bootstrap program might not use them. An old hack for determining BIOS disk geometry is to clear the disk MBR (zero the first sector), boot DOS, create a new partition table with a moderately sized DOS partition using the DOS FDISK program, and then use the FreeBSD fdisk program to examine the result. Each partition in the MBR partition table is described both by beginning and ending physical disk addresses and by a 32 bit starting block number (zero origin) and a 32 bit size in blocks. The DOS partition should begin after the first track. Its starting block number should be the number of sectors per track. From that and the partition size and the physical address of the last block in the partition, you can compute the number of heads. You might try booting a GRUB floppy and use the "geometry" command to display the disk geometry, but I am not sure where GRUB gets its information. When I recently used the FreeBSD 5.1 /stand/sysinstall program on a 120GB serial ATA disk drive, the program program first said that the drive had 16 heads and 255 sectors per track and some ridiculous number of cylinders. Then the program said that this geometry was incorrect and that it was going to use 255 heads and 63 sectors per track. Then it strongly advised me to tell it the actual geometry used by the BIOS. I doubt it got the initial geometry from the BIOS (which ought to know better than to use more than 63 sectors per track). Perhaps it got the geometry from an ATA disk controller function. This seems more likely and it explains the geometry. I would expect the BIOS to use 255 heads and 63 sectors per track. This is what the GRUB geometry command reports for the drive. So I let /stand/sysinstall use that geometry. Since I don't boot off that drive and I don't access it using any operating system that uses BIOS disk I/O, it won't make any difference if the physical disk addresses are wrong. I am pretty sure that FreeBSD uses the partition starting block numbers and sizes in the partition table instead of the physical C/H/S disk addresses. For an alternative discussion of BIOS geometry, see http://www.ata-atapi.com. Dan Strick [EMAIL PROTECTED] _______________________________________________ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "[EMAIL PROTECTED]"