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Instruction Set of the I/O Processor
I/O Instructions
6-8
SYM53C875/875E Data Manual
I/O Instructions
First Dword
Bits 31-30 Instruction Type - I/O Instruction
Bits 29-27 Op Code
T he following Op Code bits have different
meanings, depending on whether the
SYM53C875 is operating in initiator or target
mode. Note: Op Code selections 101-111 are
considered Read/Write instructions and are
described in that section.
Target Mode
Reselect Instruction
1. T he SYM53C875 arbitrates for the SCSI bus
by asserting the SCSI ID stored in the SCID
register. If the SYM53C875 loses arbitration,
then it tries again during the next available
arbitration cycle without reporting any lost
arbitration status.
2. If the SYM53C875 wins arbitration, it
attempts to reselect the SCSI device whose ID
is defined in the destination ID field of the
instruction. Once the SYM53C875 has won
arbitration, it fetches the next instruction from
the address pointed to by the DSP register.
T herefore, the SCRIPT S can move on to the
next instructions before the reselection has
completed. It will continue executing
SCRIPT S until a SCRIPT that requires a
response from the initiator is encountered.
3. If the SYM53C875 is selected or reselected
before winning arbitration, it fetches the next
instruction from the address pointed to by the
32-bit jump address field stored in the DNAD
register. T he SYM53C875 should manually be
set to initiator mode if it is reselected, or to
target mode if it is selected.
Disconnect Instruction
T he SYM53C875 disconnects from the SCSI bus
by deasserting all SCSI signal outputs.
Wait Select Instruction
1. If the SYM53C875 is selected, it fetches the
next instruction from the address pointed to by
the DSP register.
2. If reselected, the SYM53C875 fetches the next
instruction from the address pointed to by the
32-bit jump address field stored in the DNAD
register. T he SYM53C875 should manually be
set to initiator mode when reselected.
3. If the CPU sets the SIGP bit in the ISTAT
register, the SYM53C875 will abort the Wait
Select instruction and fetch the next
instruction from the address pointed to by the
32-bit jump address field stored in the DNAD
register.
Set Instruction
When the SACK / or SAT N/ bits are set, the corre-
sponding bits in the SOCL register are set. SACK /
or SAT N/ should not be set except for testing pur-
poses. When the target bit is set, the corresponding
bit in the SCNT L0 register is also set. When the
carry bit is set, the corresponding bit in the Arith-
metic Logic Unit (ALU) is set.
Note: None of the signals are set on the SCSI bus
in target mode.
Clear Instruction
When the SACK / or SAT N/ bits are set, the corre-
sponding bits are cleared in the SOCL register.
SACK / or SAT N/ should not be set except for test-
ing purposes. When the target bit is set, the corre-
sponding bit in the SCNT L0 register is cleared.
When the carry bit is set, the corresponding bit in
the ALU is cleared.
Note: None of the signals are reset on the SCSI
bus in target mode.
OPC2
OPC1
OPC0
Instruction Defined
0
0
0
0
1
0
0
1
1
0
0
1
0
1
0
Reselect
Disconnect
Wait Select
Set
Clear