For VL cards:
     ahc0 at isa? port ? irq ?

     For EISA cards:
     ahc* at eisa? slot ?

     For PCI cards:
     ahc* at pci? dev ? function ?

     To enable tagged queueing:
     options AHC_TAGENABLE

     To enable SCB paging:
     options AHC_SCBPAGING_ENABLE

     For SCSI busses:
     scsibus* at ahc?

DESCRIPTION

     This driver provides access to the SCSI bus connected to an Adaptec 274x,
     284x, 2940, 3940, or controllers based on the AIC7770, AIC7850, AIC7860,
     AIC7870, or AIC7880 host adapter chips.  Features include support for
     twin and wide busses, ultra SCSI, two active commands at a time per non-
     tagged queueing target, tagged queuing, and SCB paging which allows up to
     255 active commands on all adapters except those using AIC7770 chips pri-
     or to revision E.  Tagged queueing is enabled with the ``AHC_TAGENABLE''
     configuration option.  SCB paging is enabled with the
     ``AHC_SCBPAGING_ENABLE'' configuration option.

     Per target configuration performed in the SCSI-Select menu, accesible at
     boot in non-EISA models or through an EISA configuration utility for EISA
     models, is honored by this driver with the stipulation that the BIOS must
     be enabled for EISA adaptors.  This includes synchronous/asynchronous
     transfers, maximum synchronous negotiation rate, disconnection, and the
     host adapter's SCSI ID.

     Note that I/O addresses are determined automatically by the probe rou-
     tines, but care should be taken when using a 284x (VESA local bus
     controller) in an EISA system.  Ensure that the jumpers setting the I/O
     area for the 284x match the EISA slot into which the card is inserted to
     prevent conflicts with other EISA cards.

BUGS

     Some Quantum drives (at least the Empire 2100 and 1080s) will not run on
     an AIC7870 Rev B in synchronous mode at 10MHz.  Controllers with this
     problem have a 42 MHz clock crystal on them and run slightly above 10MHz,
     causing the drive much confusion.  Setting a maximum synchronous negotia-
     tion rate of 8MHz in the SCSI-Select utility will allow normal function.

     There are four problems known this version of this driver as follows.
     All of these problems relate to MI SCSI system.

     SCSI residual handling is not complete.

SEE ALSO

     aha(4),  ahb(4)

AUTHOR

     The ahc driver was written by Justin Gibbs.  The AIC7xxx sequencer-code
     assembler was written by John Aycock.

BSD Experimental                April 20, 1996                               2