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AHC(4) Device Drivers Manual AHC(4)


ahcAdaptec VL/EISA/PCI SCSI interface


ahc0 at isa? (VL)
ahc* at eisa? (EISA)
ahc* at pci? (PCI)


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 4944UW.
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 buses.
  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 performance.
  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 operation.
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 Adaptec.
August 14, 2012 OpenBSD-current