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:

-> Minor devices 0 through 3 identify the partitions on drive 0.

-> Minor devices 4 through 7 identify the partitions on drive 1.

-> Minor device 128 allows access to all of drive 0.

-> 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

/dev/at* -- Block-special files
/dev/rat* -- Character-special files

See Also

device drivers,
fdisk,
hai,
ideinfo

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.

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.

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

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.

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.

© 2012 by Stephen A. Ness