Avago Technologies LSI53C895A - Manual (Page 240)

5-18 SCSI SCRIPTS Instruction Set
Data Structure Address (DSA) register, and used as an
offset relative to the value in the Data Structure Address
(DSA) register. The SCSI Control Three (SCNTL3) value,
SCSI ID, synchronous offset and synchronous period are
loaded from this address. Prior to the start of an I/O, load
the
Data Structure Address (DSA) with the base address
of the I/O data structure. Any address on a Dword
boundary is allowed. After a Table Indirect opcode is
fetched, the
Data Structure Address (DSA) is added to
the 24-bit signed offset value from the opcode to
generate the address of the required data. Both positive
and negative offsets are allowed. A subsequent fetch
from that address brings the data values into the chip.
SCRIPTS can directly execute operating system I/O data
structures, saving time at the beginning of an I/O
operation. The I/O data structure can begin on any Dword
boundary and may cross system segment boundaries.
There are two restrictions on the placement of data in
system memory:
•
The I/O data structure must lie within the 8 Mbytes
above or below the base address.
•
An I/O command structure must have all four bytes
contiguous in system memory, as shown below. The
offset/period bits are ordered as in the
SCSI Transfer
(SXFER) register. The configuration bits are ordered
as in the SCSI Control Three (SCNTL3) register.
Use this bit only in conjunction with the Select, Reselect,
Wait Select, and Wait Reselect instructions. Use bits
25 and 26 individually or in
=
combination to produce the
following conditions:
Config ID Offset/period 00
Bit 25 Bit 26 Addressing Mode
0 0 Direct
0 1 Table Indirect
1 0 Relative
1 1 Table Relative