LSI LSI53C1000, Technical Manual

Page: 253 / 384

I/O Instructions 5-19
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. It is allowable
to set bits 25 and 26 individually or in combination:
Direct
Uses the device ID and physical address in the
instruction.
Table Indirect
Uses the physical jump address, but fetches data using
the table indirect method.
Config ID Offset/period 00
Bit 25 Bit 26
Direct 0 0
Table Indirect 0 1
Relative 1 0
Table Relative 1 1
Command ID Not Used Not Used
Absolute Alternate Address