
SYM53C876/876E Data Manual
5-11
SCSI SCRIPTS Instruction Set
I/O Instructions
2. 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 SXFER register. The configuration bits are
ordered as in the 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.
Relative
Uses the device ID in the instruction, but treats the
alternate address as a relative jump.
Table Relative
Treats the alternate jump address as a relative
jump and fetches the device ID, synchronous off-
set, and synchronous period indirectly. Adds the
value in bits 23-0 of the first four bytes of the
SCRIPTS instruction to the data structure base
address to form the fetch address.
Bit 24
Select with ATN/
This bit specifies whether SATN/ is asserted
during the selection phase when the
SYM53C876 is executing a Select instruction.
When operating in initiator mode, set this bit
for the Select instruction. If this bit is set on
any other I/O instruction, an illegal instruction
interrupt is generated.
Bit 23-20
Reserved
Bits 19-16 Encoded SCSI Destination ID
This 4-bit field specifies the destination SCSI
ID for an I/O instruction.
Bits 15-11 Reserved
Bit 10
Set/Clear Carry
This bit is used in conjunction with a Set or
Clear instruction to set or clear the Carry bit.
Setting this bit with a Set instruction asserts
the Carry bit in the ALU. Setting this bit with a
Clear instruction deasserts the Carry bit in the
ALU.
Bit 9
Set/Clear Target Mode
This bit is used in conjunction with a Set or
Config
ID
Offset/
period
(00)
Bit 25
Bit 26
Direct
Table Indirect
Relative
Table Relative
0
0
1
1
0
1
0
1
Command
ID
Not Used
Not Used
Absolute Alternate Address
Command
Table Offset
Absolute Alternate Address
Command
ID
Not Used
Not
Used
Alternate Jump Offset
Command
Table Offset
Alternate Jump Offset