LSI 53C875A - Manual (Page 218)

5-18 SCSI SCRIPTS Instruction Set
RA Relative Addressing Mode 26
When this bit is set, the 24-bit signed value in the DMA
Next Address (DNAD) register is used as a relative
displacement from the current DMA SCRIPTS Pointer
(DSP) address. Use this bit only in conjunction with the
Select, Reselect, Wait Select, and Wait Reselect
instructions. The Select and Reselect instructions can
contain an absolute alternate jump address or a relative
transfer address.
TI Table Indirect Mode 25
When this bit is set, the 24-bit signed value in the DMA
Byte Counter (DBC) register is added to the value in the
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.
Config ID Offset/period 00