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      ahc - Adaptec VL/EISA/PCI SCSI host adapter driver


      For one or more VL/EISA cards:
      device eisa
      device ahc
      For one or more PCI cards:
      device pci
      device ahc
      To allow PCI adapters to use memory mapped I/O if enabled:
      options AHC_ALLOW_MEMIO
      To configure one or more controllers to assume the target role:
      options AHC_TMODE_ENABLE <bitmask of units>
      For one or more SCSI busses:
      device scbus


      This driver provides access to the SCSI bus(es) connected to the Adaptec
      AIC77xx and AIC78xx host adapter chips.
      Driver features include support for twin and wide busses, fast, ultra or
      ultra2 synchronous transfers depending on controller type, tagged queue‐
      ing, SCB paging, and target mode.
      Memory mapped I/O can be enabled for PCI devices with the
      “AHC_ALLOW_MEMIO” configuration option.  Memory mapped I/O is more effi‐
      cient than the alternative, programmed I/O.  Most PCI BIOSes will map
      devices so that either technique for communicating with the card is
      available.  In some cases, usually when the PCI device is sitting behind
      a PCI->PCI bridge, the BIOS may fail to properly initialize the chip for
      memory mapped I/O.  The typical symptom of this problem is a system hang
      if memory mapped I/O is attempted.  Most modern motherboards perform the
      initialization correctly and work fine with this option enabled.
      Individual controllers may be configured to operate in the target role
      through the “AHC_TMODE_ENABLE” configuration option.  The value assigned
      to this option should be a bitmap of all units where target mode is
      desired.  For example, a value of 0x25, would enable target mode on units
      0, 2, and 5.  A value of 0x8a enables it for units 1, 3, and 7.
      Per target configuration performed in the SCSI-Select menu, accessible at
      boot in non-EISA models, or through an EISA configuration utility for
      EISA models, is honored by this driver.  This includes synchronous/asyn‐
      chronous transfers, maximum synchronous negotiation rate, wide transfers,
      disconnection, the host adapter’s SCSI ID, and, in the case of EISA Twin
      Channel controllers, the primary channel selection.  For systems that
      store non-volatile settings in a system specific manner rather than a
      serial eeprom directly connected to the aic7xxx controller, the BIOS must
      be enabled for the driver to access this information.  This restriction
      applies to all EISA and many motherboard configurations.
      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.  The jumpers setting the I/O area for the
      284x should match the EISA slot into which the card is inserted to pre‐
      vent conflicts with other EISA cards.
      Performance and feature sets vary throughout the aic7xxx product line.
      The following table provides a comparison of the different chips sup‐
      ported by the ahc driver.  Note that wide and twin channel features,
      although always supported by a particular chip, may be disabled in a par‐
      ticular motherboard or card design.
            Chip       MIPS    Bus      MaxSync   MaxWidth  SCBs  Features
            aic7770     10    EISA/VL    10MHz     16Bit     4    1
            aic7850     10    PCI/32     10MHz      8Bit     3
            aic7860     10    PCI/32     20MHz      8Bit     3
            aic7870     10    PCI/32     10MHz     16Bit    16
            aic7880     10    PCI/32     20MHz     16Bit    16
            aic7890     20    PCI/32     40MHz     16Bit    16        3 4 5 6 7 8
            aic7891     20    PCI/64     40MHz     16Bit    16        3 4 5 6 7 8
            aic7892     20    PCI/64     80MHz     16Bit    16        3 4 5 6 7 8
            aic7895     15    PCI/32     20MHz     16Bit    16      2 3 4 5
            aic7895C    15    PCI/32     20MHz     16Bit    16      2 3 4 5     8
            aic7896     20    PCI/32     40MHz     16Bit    16      2 3 4 5 6 7 8
            aic7897     20    PCI/64     40MHz     16Bit    16      2 3 4 5 6 7 8
            aic7899     20    PCI/64     80MHz     16Bit    16      2 3 4 5 6 7 8
            1.   Multiplexed Twin Channel Device - One controller servicing two
            2.   Multi-function Twin Channel Device - Two controllers on one
            3.   Command Channel Secondary DMA Engine - Allows scatter gather
                 list and SCB prefetch.
            4.   64 Byte SCB Support - SCSI CDB is embedded in the SCB to elim‐
                 inate an extra DMA.
            5.   Block Move Instruction Support - Doubles the speed of certain
                 sequencer operations.
            6.   ‘Bayonet’ style Scatter Gather Engine - Improves S/G prefetch
            7.   Queuing Registers - Allows queueing of new transactions with‐
                 out pausing the sequencer.
            8.   Multiple Target IDs - Allows the controller to respond to
                 selection as a target on multiple SCSI IDs.


      The ahc driver supports the following SCSI host adapter chips and SCSI
      controller cards:
            Adaptec AIC7770 host adapter chip
            Adaptec AIC7850 host adapter chip
            Adaptec AIC7860 host adapter chip
            Adaptec AIC7870 host adapter chip
            Adaptec AIC7880 host adapter chip
            Adaptec AIC7890 host adapter chip
            Adaptec AIC7891 host adapter chip
            Adaptec AIC7892 host adapter chip
            Adaptec AIC7895 host adapter chip
            Adaptec AIC7896 host adapter chip
            Adaptec AIC7897 host adapter chip
            Adaptec AIC7899 host adapter chip
            Adaptec 274X(W)
            Adaptec 274X(T)
            Adaptec 284X
            Adaptec 2910
            Adaptec 2915
            Adaptec 2920
            Adaptec 2930C
            Adaptec 2930U2
            Adaptec 2940
            Adaptec 2940J
            Adaptec 2940N
            Adaptec 2940U
            Adaptec 2940AU
            Adaptec 2940UW
            Adaptec 2940UW Dual
            Adaptec 2940UW Pro
            Adaptec 2940U2W
            Adaptec 2940U2B
            Adaptec 2950U2W
            Adaptec 2950U2B
            Adaptec 19160B
            Adaptec 29160B
            Adaptec 29160N
            Adaptec 3940
            Adaptec 3940U
            Adaptec 3940AU
            Adaptec 3940UW
            Adaptec 3940AUW
            Adaptec 3940U2W
            Adaptec 3950U2
            Adaptec 3960
            Adaptec 39160
            Adaptec 3985
            Adaptec 4944UW
            NEC PC-9821Xt13 (PC-98)
            NEC RvII26 (PC-98)
            NEC PC-9821X-B02L/B09 (PC-98)
            NEC SV-98/2-B03 (PC-98)
            Many motherboards with on-board SCSI support
      Every transaction sent to a device on the SCSI bus is assigned a ‘SCSI
      Control Block’ (SCB).  The SCB contains all of the information required
      by the controller to process a transaction.  The chip feature table lists
      the number of SCBs that can be stored in on-chip memory.  All chips with
      model numbers greater than or equal to 7870 allow for the on chip SCB
      space to be augmented with external SRAM up to a maximum of 255 SCBs.
      Very few Adaptec controller configurations have external SRAM.
      If external SRAM is not available, SCBs are a limited resource.  Using
      the SCBs in a straight forward manner would only allow the driver to han‐
      dle as many concurrent transactions as there are physical SCBs.  To fully
      utilize the SCSI bus and the devices on it, requires much more concur‐
      rency.  The solution to this problem is SCB Paging, a concept similar to
      memory paging.  SCB paging takes advantage of the fact that devices usu‐
      ally disconnect from the SCSI bus for long periods of time without talk‐
      ing to the controller.  The SCBs for disconnected transactions are only
      of use to the controller when the transfer is resumed.  When the host
      queues another transaction for the controller to execute, the controller
      firmware will use a free SCB if one is available.  Otherwise, the state
      of the most recently disconnected (and therefore most likely to stay dis‐
      connected) SCB is saved, via dma, to host memory, and the local SCB
      reused to start the new transaction.  This allows the controller to queue
      up to 255 transactions regardless of the amount of SCB space.  Since the
      local SCB space serves as a cache for disconnected transactions, the more
      SCB space available, the less host bus traffic consumed saving and
      restoring SCB data.


      The ahc driver appeared in FreeBSD 2.0.


      The ahc driver, the AIC7xxx sequencer-code assembler, and the firmware
      running on the aic7xxx chips was written by Justin T. Gibbs.


      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.
      This confuses the drive and hangs the bus.  Setting a maximum synchronous
      negotiation rate of 8MHz in the SCSI-Select utility will allow normal
      Although the Ultra2 and Ultra160 products have sufficient instruction ram
      space to support both the initiator and target roles concurrently, this
      configuration is disabled in favor of allowing the target role to respond
      on multiple target ids.  A method for configuring dual role mode should
      be provided.
      Tagged Queuing is not supported in target mode.
      Reselection in target mode fails to function correctly on all high volt‐
      age differential boards as shipped by Adaptec.  Information on how to
      modify HVD board to work correctly in target mode is available from


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