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AHC(4)			  OpenBSD Programmer's Manual			AHC(4)

     ahc - Adaptec VL/EISA/PCI SCSI interface

     ahc0 at isa?    (VL)
     ahc* at eisa?   (EISA)
     ahc* at pci?    (PCI)
     scsibus* at ahc?
     option AHC_ALLOW_MEMIO
     option AHC_TMODE_ENABLE

     This driver provides access to the SCSI bus(es) connected to Adaptec
     AIC7770, AIC7850, AIC7860, AIC7870, AIC7880, AIC7890, AIC7891, AIC7892,
     AIC7895, AIC7896, AIC7897 and AIC7899 host adapter chips.	These chips
     are found on many motherboards as well as the following Adaptec SCSI
     controller cards: 274X(W), 274X(T), 284X, 2910, 2915, 2920, 2930C,
     2930U2, 2940, 2940J, 2940N, 2940U, 2940AU, 2940UW, 2940UW Dual, 2940UW
     Pro, 2940U2W, 2940U2B, 2950U2W, 2950U2B, 19160B, 29160B, 29160N, 3940,
     3940U, 3940AU, 3940UW, 3940AUW, 3940U2W, 3950U2, 3960, 39160, 3985, and

     Driver features include support for twin and wide buses, fast, ultra,
     ultra2 and ultra160 synchronous transfers depending on controller type,
     tagged queuing, and 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
     efficient 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.  This is
     the default mode of operation on every architecture except i386.

     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/asynchronous 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
     routines, 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
     prevent 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
     supported by the ahc driver.  Note that wide and twin channel features,
     although always supported by a particular chip, may be disabled in a
     particular 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 chip.
     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 eliminate
	  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 queuing of new transactions without
	  pausing the sequencer.
     8.	  Ultra160 support.
     9.	  Multiple Target IDs - Allows the controller to respond to selection
	  as a target on multiple SCSI IDs.

     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
     handle as many concurrent transactions as there are physical SCBs.	 To
     fully utilize the SCSI bus and the devices on it, requires much more
     concurrency.  The solution to this problem is SCB Paging, a concept
     similar to memory paging.	SCB paging takes advantage of the fact that
     devices usually disconnect from the SCSI bus for long periods of time
     without talking 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 disconnected) 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.

     ahd(4), cd(4), ch(4), eisa(4), intro(4), isa(4), pci(4), scsi(4), sd(4),
     st(4), uk(4)

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

     The OpenBSD platform dependent code was written by Steve P. Murphree, Jr
     and Kenneth R. Westerback.

     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
     voltage differential boards as shipped by Adaptec.	 Information on how to
     modify HVD board to work correctly in target mode is available from

OpenBSD 4.9			 July 3, 2010			   OpenBSD 4.9

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