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6-20 Instruction Set of the I/O Processor
• 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:
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.
Relative
Uses the device ID in the instruction, but treats the
alternate address as a relative jump.
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
Command Table Offset
Absolute Jump Offset
Command ID Not Used Not Used
Absolute Jump Offset