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SCSI Software Interface Protocols (ASPI and CAM)
In order for a device to be used on the SCSI bus, it is necessary for it to have a driver that interfaces it to the bus. (The exception is hard disks, which are normally usable directly by the host adapter, since it is designed for that purpose). The problem with this is making sure the drivers are available for all the devices you want to use and that they work properly. A further problem is making sure that operating systems and applications can work with all of the drivers that are written for various devices.
To help make the situation with drivers more standardized, Adaptec--the biggest name in SCSI host adapters--developed the Advanced (originally Adaptec) SCSI Programming Interface, or ASPI. This driver acts as an abstraction layer that hides the details of the host adapter from the operating system or application and makes device support more universal.
Most of the popular operating systems can make use of ASPI. DOS requires an ASPI driver to be loaded, while the protected mode operating systems such as Windows 95 and Windows NT have native support for ASPI. One problem with using SCSI under DOS or Windows 3.x is that the lack of protected-mode drivers means yet another real-mode drive that uses up precious conventional memory.
An alternative interface protocol is called Common Access Method or CAM. This is a more sophisticated and complex software interface protocol, which is defined as one of the SCSI-3 standards. CAM and ASPI both do basically the same thing; which is used depends on the particular system.