Catch the Bus!
A bus protocol defines both the hardware (physical) and software (logical) components of the SCSI interface standard. SCSI is just like other connection technologies in that it uses this two-fold approach. The SCSI bus is an intel-ligent I/O subsystem that attaches to the host computer's system bus. The anchor point (interface) for a SCSI bus on the host end is the initiator, which most vendors refer to as the bus controller. The bus controller connects via the SCSI cable to targets. "Targets" are intelligent I/O devices such as disk drives, tape drives, optical drives and scanners. In SCSI vocabulary, the meaning of the term "device" includes everything, including the bus control-ler, and the term "target" is normally used to refer to a device that is managed by the controller. The bus controller may issue multiple concurrent I/O requests to one or more targets on the bus, and send or receive data blocks from these devices as they become available. SCSI targets are nor-mally disconnected from the bus. They may only connect when the initiator allows them to do so. SCSI targets connect to the bus in order to receive commands, report status or transfer data. A device's onboard cache memory is used to accept data bursts from the controller, or to queue outbound data that is waiting to transfer to the controller.
The number of devices that can be attached to a bus is a function of the spe-cific protocol in use, for example, Narrow SCSI (seven targets), Wide SCSI (15 targets) or Fibre Channel-AL SCSI (127 targets). We will cover the vari-ous SCSI protocols in a moment.
The bus controller takes much of the I/O load off the system CPU by using direct memory addressing (DMA), also known as bus mastering. This can re-duce CPU load from about 30% to 5% by having the controller's own internal processor transfer data in and out of memory directly. As a result, the host CPU is free to work on other tasks.
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