LSI LSI53C896, Technical Manual

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2-20 Functional Description
The LSI53C896 offers low level register access or a high-level control
interface. Like first generation SCSI devices, the LSI53C896 is accessed
as a register-oriented device. The ability to sample and/or assert any
signal on the SCSI bus is used in error recovery and diagnostic
procedures. In support of SCSI loopback diagnostics, each SCSI function
may perform a self-selection and operate as both an initiator and a
target.
The LSI53C896 is controlled by the integrated SCRIPTS processor
through a high-level logical interface. Commands controlling the SCSI
functions are fetched out of the main host memory or local memory.
These commands instruct the SCSI functions 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 also handle conditions such as Phase
Mismatch.
2.2.1.1 Phase Mismatch Handling in SCRIPTS
The LSI53C896 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.
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