AHC(4) FreeBSD Kernel Interfaces Manual AHC(4)
NAME
ahc - Adaptec VL/ISA/PCI SCSI host adapter driver
SYNOPSIS
To compile this driver into the kernel, place the following lines in your
kernel configuration file:
device scbus
device ahc
For one or more PCI cards:
device pci
Alternatively, to load the driver as a module at boot time, place the
following lines in loader.conf(5):
ahc_load="YES"
ahc_isa_load="YES"
ahc_pci_load="YES"
DESCRIPTION
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
queueing, SCB paging, and target mode.
Per target configuration performed in the SCSI-Select menu, accessible at
boot is honored by this driver. This includes synchronous/asynchronous
transfers, maximum synchronous negotiation rate, wide transfers,
disconnection, the host adapter's SCSI ID. 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
many chip-down motherboard configurations.
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 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
busses.
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 queueing of new transactions
without pausing the sequencer.
8. Multiple Target IDs - Allows the controller to respond to
selection as a target on multiple SCSI IDs.
CONFIGURATION OPTIONS
To allow PCI adapters to use memory mapped I/O if enabled:
options AHC_ALLOW_MEMIO=(0 -- disabled, 1 -- enabled)
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 and it is the default. This
option can also be dynamically configured through a device hint
documented below.
To statically configure one or more controllers to assume the target
role:
options AHC_TMODE_ENABLE=<bitmask of units>
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.
Note that controllers can be dynamically configured through a
device hint documented below.
BOOT OPTIONS
The following options are switchable by setting values in
/boot/device.hints.
They are:
hint.ahc.N.tmode_enable
A hint to define whether the SCSI target mode is enabled,
defaults to disabled (0 -- disabled, 1 -- enabled).
hint.ahc.N.allow_memio
A hint to define whether memory mapped io is enabled or disabled
for this adapter, defaults to enabled (0 -- disabled, 1 --
enabled).
HARDWARE
The ahc driver supports the following VL/ISA/PCI parallel SCSI
controllers and cards:
o Adaptec AIC7770 host adapter chip
o Adaptec AIC7850 host adapter chip
o Adaptec AIC7860 host adapter chip
o Adaptec AIC7870 host adapter chip
o Adaptec AIC7880 host adapter chip
o Adaptec AIC7890 host adapter chip
o Adaptec AIC7891 host adapter chip
o Adaptec AIC7892 host adapter chip
o Adaptec AIC7895 host adapter chip
o Adaptec AIC7896 host adapter chip
o Adaptec AIC7897 host adapter chip
o Adaptec AIC7899 host adapter chip
o Adaptec 274X(W)
o Adaptec 274X(T)
o Adaptec 2910
o Adaptec 2915
o Adaptec 2920C
o Adaptec 2930C
o Adaptec 2930U2
o Adaptec 2940
o Adaptec 2940J
o Adaptec 2940N
o Adaptec 2940U
o Adaptec 2940AU
o Adaptec 2940UW
o Adaptec 2940UW Dual
o Adaptec 2940UW Pro
o Adaptec 2940U2W
o Adaptec 2940U2B
o Adaptec 2950U2W
o Adaptec 2950U2B
o Adaptec 19160B
o Adaptec 29160B
o Adaptec 29160N
o Adaptec 3940
o Adaptec 3940U
o Adaptec 3940AU
o Adaptec 3940UW
o Adaptec 3940AUW
o Adaptec 3940U2W
o Adaptec 3950U2
o Adaptec 3960
o Adaptec 39160
o Adaptec 3985
o Adaptec 4944UW
o Many motherboards with on-board SCSI support
SCSI CONTROL BLOCKS (SCBs)
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.
SEE ALSO
ahd(4), cd(4), da(4), sa(4), scsi(4)
HISTORY
The ahc driver appeared in FreeBSD 2.0.
AUTHORS
The ahc driver, the AIC7xxx sequencer-code assembler, and the firmware
running on the aic7xxx chips was written by Justin T. Gibbs.
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.
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.
FreeBSD 15.1-STABLE-HBSD September 29, 2025 AHC(4)