*********************************************
                    *                                           *
                    *    SCSI01.SYS                             *
                    *    OS/2 Disk Driver with SCSI Support     *
                    *    Functional Overview                    *
                    *    Tom Shea   1/10/89                     *
                    *    OS/2 Version 1.1                       *
                    *                                           *
                    *********************************************
            
            Introduction:
            
            DISK01.SYS, the disk driver shipped with the OS/2 SDK and
            BAK, provides support for standard floppy and hard drives on
            AT machines.  SCSI01.SYS provides all the functionality of
            DISK01.SYS, and in addition, supports the Adaptec AHA-154x
            line of multitasking SCSI host adapters. As with DISK01.SYS,
            OEMs wishing to use SCSI01.SYS and the AHA-154x may want to
            modify the driver to best support their particular hardware
            configurations.
            
            
            The SCSI portion of SCSI01.SYS is made up of two different
            sections:  The SCSI Manager, and the SCSI Disk Driver.
            
            The SCSI Manager:
            
            The SCSI manager is implemented as a character device which
            loads before the disk portion of DISK01.  The SCSI manager
            provides a generic multitasking interface to the AHA-154x
            host adapter.  Any device driver can access the SM through
            the ATTACH Device Help function in OS/2. For more
            information on the specification for interfacing to the SCSI
            manager, refer to the Adaptec OS/2 SCSI programming
            interface specification.  Additional drivers for devices
            such as Tape, etc.  will be available in the future
            from Adaptec to take advantage of this interface.  Note that
            the SCSI manager is also available as an installable driver
            without the disk support.  If this driver is installed, the
            SCSI manager in the disk module is de-installed and the 
            installable driver takes over the SCSI manager duties.     
            
            SCSI Disk Driver:
            
            The SCSI Disk Driver uses the SM interface to provide
            multitasking Disk I/O for SCSI Hard Drives. The
            functionality of SCSI01.SYS for Hard  Drives is intended to
            mirror DISK01.SYS.  Basically, SCSI01.SYS will support all
            hard drive configurations supported by the AHA-154x BIOS.
            These include:
            
                (1) Both Drive C: and Drive D: are standard Hard drives.
            
                (2) Drive C: is standard, Drive D: is SCSI Target 0
            LUN_0.
            
                (3) Drive C: is SCSI Target 0 LUN 0, Drive D:, if
            installed, is  SCSI Target 0 LUN 1 or Target 1 LUN 0.
            
            Like  DISK01, SCSI01 is presently limited to 2 Hard drives.
            As future versions of OS/2 supply DISK01 and FDISK code
            which supports more than 2 hard drives, SCSI01 will
            obviously support as many SCSI drives as possible.
            
            SCSI01.SYS does not use the ROM BIOS for Interrupt 13H
            requests from the DOS compatibility box. Instead, interrupt
            13H functionality is emulated by the SDD through the
            multitasking SCSI manager. 
            
            SCSI01 Uses the OS/2 seek optomization routines when
            queueing disk requests.
            
            The SCSI Disk Driver presently assumes that all SCSI hard
            drives on the system support the extended verify command.
            In the future, SCSI01.SYS will be modified to work with
            drives without this support as well as to take advantage of
            drives which support the WRITE w/ VERIFY command.
            
            At the present time, SCSI requests do not go through the
            cache used for requests to standard hard drives.  It is
            currently being studied how to handle caching for SCSI
            drives inside the driver.
            
            Support for LAN Manager 1.1 high performance disk caching and
            Fault Tolerance is not yet implemented.
            
            Although support for the AHA-154x High performance host
            adapter is included in DISK01.SYS, the SCSI support routines
            are in no way related to the single tasking state machine of
            the standard AT controller support. The driver completely
            supports multitasking between multiple drives and other
            devices on the bus, and makes full use of the low overhead
            provided by the host adapters BUS MASTER DMA and Intelligent
            I/O processing.
            
            The Only testing performed on the SCSI manager so far is
            through the SCSI disk support in the same code module.
            Although additional drivers should be able to ATTACH to this
            version of code, this has not been tested.  In addition,
            the ABORT function of the SCSI Programming interface is not
            yet implemented.
            
            SCSI01.SYS does not support the AHA-1640 Microchannel
            adapter at this time.
            
            To use SCSI01.SYS simply rename it to DISK01.SYS and copy it
            to an OS/2 1.1 install diskette.