at" -- Device Driver" "

Drivers for hard-disk partitions
/dev/at* are the COHERENT system's AT devices for the hard-disk's partitions. Each device is assigned major-device number 11, and may be accessed as a block- or character-special device.

at handles two drives with up to four partitions each:

+o
Minor devices 0 through 3 identify the partitions on drive 0.

+o
Minor devices 4 through 7 identify the partitions on drive 1.

+o
Minor device 128 allows access to all of drive 0.

+o
Minor device 129 allows access to all of drive 1.

To modify the offsets and sizes of the partitions, use the command fdisk on the special device for each drive (minor devices 128 and 129).

To access a disk partition through COHERENT, directory /dev must contain a device file that has the appropriate type, major and minor device numbers, and permissions. To create a special file for this device, invoke the command mknod as follows: 

     /etc/mknod /dev/at0a b 11   0 ; : drive 0, partition 0
     /etc/mknod /dev/at0b b 11   1 ; : drive 0, partition 1
     /etc/mknod /dev/at0c b 11   2 ; : drive 0, partition 2
     /etc/mknod /dev/at0d b 11   3 ; : drive 0, partition 3
     /etc/mknod /dev/at0x b 11 128 ; : drive 0, partition table

Drive Characteristics

When processing BIOS I/O requests prior to booting COHERENT, many IDE drives use translation-mode drive parameters: number of heads, cylinders, and sectors per track. These numbers are called ``translation-mode'' parameters because they do not reflect true physical drive geometry. The translation-mode parameters used by the BIOS code present on your host adapter can be obtained using the command info from within the tertiary-boot routine tboot. (For details on info, see the Lexicon entry for tboot.) It is often necessary to patch the at driver with BIOS values of translation-mode parameters in order to boot COHERENT on IDE hard drives. In COHERENT versions 3.1.0 and later, drive parameters are stored in table atparm in the driver. For the first hard drive, number of cylinders is a short (two-byte) value at atparm+0, number of heads is a single byte at atparm+2, and number of sectors per track is a single byte at atparm+14. For the second hard drive, number of cylinders is a short value at atparm+16, number of heads is a single byte at atparm+18, and number of sectors per track is a single byte at atparm+30. For example, if testcoh is a kernel linked with the at driver and you want to patch it for a second hard drive with 829 cylinders, 10 heads, and 26 sectors per track, you can do: 
     /conf/patch testcoh atparm+16=829:s atparm+18=10:c atparm+30=26:c

To read the characteristics of a hard disk once the at driver is running, use the call to ioctl of the following form: 

     #include <sys/hdioctl.h>
     hdparm_t hdparms;
          . . .
     ioctl(fd, HDGETA, (char *)&hdparms);

where fd is a file descriptor for the hard-disk device and hdparms receives the disk characteristics.

Non-Standard and Unsupported Types of Drives

Prior releases of the the COHERENT at hard- disk driver would not support disk drives whose geometry was not supported by the BIOS disk parameter tables. COHERENT adds support for these drives during installation by ``patching'' the disk parameters into the bootstrap and the /coherent image on the hard disk.

Files

//ddeevv//aatt** -- Block-special files //ddeevv//rraatt** -- Character-special files

See Also

Notes

The driver at offers two varieties of polling: normal and alternate. Normal, as its name implies, is used with most varieties of AT controllers. Alternate polling is for Perstor controllers and some other older equipment. Using the wrong type of polling causes frequent controller timeouts and bad-track messages.

A at also lets you specify the number of seconds to wait for a response from the drive after an I/O request. The default value is six. Some IDE drives occasionally become unresponsive for long intervals (several seconds) while control firmware makes adjustments to drive operation.

A To set either the type of polling or the default waiting period, su to the superuser root; then cd to directory /etc/conf and run the script at/mkdev. This script will walk you through describing your AT controller to COHERENT. Once you have run this script, execute the command 

     /etc/conf/bin/idmkcoh -o cohtest

A to create a test kernel that incorporates your changes; then reboot your system and invoke the new kernel, as described in the Lexicon entry booting. Note that the changes you make to the driver will not be seen by your COHERENT system until you boot the new kernel.

A The at driver lets you have up to two AT hard disks on your system. Note, however, that in our experience, it is very difficult to combine different brands of AT hard disks and have both run successfully. This is especially true with Conner drives, which apparently do not cooperate with other IDE drives as master and slave. Caveat utilitor.