
SCSI Operating Registers
SYM53C825A/825AE Data Manual
5-1
Chapter 5
SCSI Operating Registers
T his section contains descriptions of all
SYM53C825A operating registers. T able 5-1 sum-
marizes the SYM53C825A operating register set.
Figure 5-1, the register map, lists registers by oper-
ating and configuration addresses. T he terms “set”
and “assert” are used to refer to bits that are pro-
grammed to a binary one. Similarly, the terms
“deassert,” “clear” and “reset” are used to refer to
bits that are programmed to a binary zero. Any bits
marked as reserved should always be written to ze-
ro; mask all information read from them. Reserved
bit functions may be changed at any time. Unless
otherwise indicated, all bits in registers are active
high, that is, the feature is enabled by setting the
bit. T he bottom row of every register diagram
shows the default register values, which are enabled
after the chip is powered on or reset.
Note: T he only register that the host CPU can
access while the SYM53C825A is
executing SCRIPT S is the ISTAT register;
attempts to access other registers will
interfere with the operation of the chip.
However, all operating registers are
accessible with SCRIPT S. All read data is
synchronized and stable when presented to
the PCI bus.
Note: T he SYM53C825A cannot fetch SCRIPT S instructions from the operating register space.
Instructions must be fetched from system memory or the internal SCRIPT S RAM.
Table 5-1: Operating Register Addresses and Descriptions
Memory
or I/O
Address
Config
Memory
or I/O
Read/Write
Label
Description
00
01
02
03
04
05
06
07
08
09
0A
0B
0C
0D
0E
80
81
82
83
84
85
86
87
88
89
8A
8B
8C
8D
8E
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R
R/W
R
R
R
SCNT L0
SCNT L1
SCNT L2
SCNT L3
SCID
SX FER
SDID
GPREG
SFBR
SOCL
SSID
SBCL
DSTAT
SSTAT 0
SSTAT 1
SCSI Control 0
SCSI Control 1
SCSI Control 2
SCSI Control 3
SCSI Chip ID
SCSI Transfer
SCSI Destination ID
General Purpose Bits
SCSI First Byte Received
SCSI Output Control Latch
SCSI Selector ID
SCSI Bus Control Lines
DMA Status
SCSI Status 0
SCSI Status 1