
Functional Description
SCSI Core
SYM53C875/875E Data Manual
2-1
Chapter 2
Functional Description
T he SYM53C875 is composed of three functional
blocks: the SCSI Core, the DMA Core, and the
SCRIPT S Processor. T he SYM53C875 is fully
supported by the SCSI Device Management Sys-
tem (SDMS ), a complete software package that
supports the Symbios product line of SCSI proces-
sors and controllers. T he PCI Bus Power Manage-
ment support (SYM53C875E) is discussed at the
end of this chapter.
SCSI Core
T he SCSI core supports the 8- or 16-bit data bus.
It supports Ultra SCSI synchronous transfer rates
up to 40 MB/s, SCSI synchronous transfer rates
up to 20 MB/s, and asynchronous transfer rates up
to 10 MB/s on a 16-bit wide SCSI bus. T he SCSI
core can be programmed with SCSI SCRIPT S,
making it easy to “fine tune” the system for specific
mass storage devices or SCSI-3 requirements.
T he SCSI core offers low-level register access or a
high-level control interface. Like first generation
SCSI devices, the SYM53C875 SCSI core can be
accessed as a register-oriented device. T he ability
to sample and/or assert any signal on the SCSI bus
can be used in error recovery and diagnostic proce-
dures. In support of loopback diagnostics, the
SCSI core may perform a self-selection and oper-
ate as both an initiator and a target.
T he SYM53C875 SCSI core is controlled by the
integrated SCRIPT S processor through a high-
level logical interface. Commands controlling the
SCSI core are fetched out of the main host mem-
ory or local memory. T hese 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. T he SCRIPT S processor is a
special high-speed processor optimized for SCSI
protocol.
DMA Core
T he DMA core is a bus master DMA device that
attaches directly to the industry standard PCI Bus.
T he DMA core is tightly coupled to the SCSI core
through the SCRIPT S processor, which supports
uninterrupted scatter/gather memory operations.
T he SYM53C875 supports 32-bit memory and
automatically supports misaligned DMA transfers.
A 536-byte FIFO allows the SYM53C875 to sup-
port 2, 4, 8, 16, 32, 64, or 128 longword bursts
across the PCI bus interface.
SCRIPT S Processor
T he SCSI SCRIPT S processor allows both DMA
and SCSI commands to be fetched from host
memory or internal SCRIPT S RAM. Algorithms
written in SCSI SCRIPT S control the actions of
the SCSI and DMA cores and are executed from
32-bit system RAM. T he SCRIPT S processor exe-
cutes complex SCSI bus sequences independently
of the host
CPU.
T he SCRIPT S processor can begin a SCSI I/O
operation in approximately 500 ns. T his compares
with 2-8 ms required for traditional intelligent host
adapters. 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 SCRIPT S are hardware-
independent, so they can be used interchangeably
on any host or CPU system bus.