accessed as a register-oriented device. Error recovery and/or diagnostic procedures use the ability to sample and/or assert any signal on the SCSI bus. In support of SCSI loopback diagnostics, the SCSI core may perform a self-selection and operate as both an initiator and a target.

The LSI53C875A is controlled by the integrated SCRIPTS processor through a high-level logical interface. Commands controlling the SCSI core are fetched out of the main host memory or local memory. These commands instruct the SCSI core to Select, Reselect, Disconnect, Wait for a Disconnect, Transfer Information, Change Bus Phases and, in general, implement all aspects of the SCSI protocol. The SCRIPTS processor is a special high-speed processor optimized for SCSI protocol.

2.2.1 SCRIPTS Processor

The SCSI SCRIPTS processor allows both DMA and SCSI commands to be fetched from host memory or internal SCRIPTS RAM. Algorithms written in SCSI SCRIPTS control the actions of the SCSI and DMA cores. The SCRIPTS processor executes complex SCSI bus sequences independently of the host CPU.

Algorithms may be designed to tune SCSI bus performance, to adjust to new bus device types (such as scanners, communication gateways, etc.), or to incorporate changes in the SCSI-2 or SCSI-3 logical bus definitions without sacrificing I/O performance. SCSI SCRIPTS are hardware independent, so they can be used interchangeably on any host or CPU system bus. SCSI SCRIPTS handle conditions like Phase Mismatch.

2.2.1.1 Phase Mismatch Handling in SCRIPTS

The LSI53C875A can handle phase mismatches due to drive disconnects without needing to interrupt the processor. The primary goal of this logic is to completely eliminate the need for CPU intervention during an I/O disconnect/reselect sequence.

Storing the appropriate information to later restart the I/O can be done through SCRIPTS, eliminating the need for processor intervention during an I/O disconnect/reselect sequence. Calculations are performed such that the appropriate information is available to SCRIPTS so that an I/O state can be properly stored for restart later.

SCSI Functional Description

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