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

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:

 

Bit 25

Bit 26

 

 

 

Direct

0

0

Table Indirect

0

1

Relative

1

0

Table Relative

1

1

 

 

 

Direct

Uses the device ID and physical address in the instruction.

Command

ID

Not Used

Not Used

Absolute Alternate Address

Table Indirect

Uses the physical jump address, but fetches data using the table indirect method.

Command

Table Offset

 

 

Absolute Jump Offset

Relative

Uses the device ID in the instruction, but treats the alternate address as a relative jump.

Command

ID

Not Used

Not Used

Absolute Jump Offset

6-20

Instruction Set of the I/O Processor

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LSI 53C810A technical manual Direct, Table Indirect, Relative