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5-14 SCSI SCRIPTS Instruction Set
5.4.1 First Dword
IT[1:0] Instruction Type - I/O Instruction [31:30]
The IT bit configuration (01) defines an I/O Instruction
Type.
OPC[2:0] OpCode [29:27]
The OpCode bit configurations define the I/O operation
performed but the OpCode bit meanings change in Target
mode compared to Initiator mode. OpCode bit
configurations (101, 110, and 111) are considered
Read/Write instructions, and are described in Section
5.5, “Read/Write Instructions.” This section describes
Target mode operations.
Target Mode
Reselect Instruction
The LSI53C875A arbitrates for the SCSI bus by asserting
the SCSI ID stored in the SCSI Chip ID (SCID) register.
If it loses arbitration, it tries again during the next
available arbitration cycle without reporting any lost
arbitration status.
If the LSI53C875A wins arbitration, it attempts to reselect
the SCSI device whose ID is defined in the destination ID
field of the instruction. Once the LSI53C875A wins
arbitration, it fetches the next instruction from the address
pointed to by the DMA SCRIPTS Pointer (DSP) register.
31 30 29 27 26 25 24 23 20 19 16 15 11 10 9 8 7 6 5 4 3 2 0
DMA Command (DCMD)
Register
DMA Byte Counter (DBC) Register
IT[1:0] OPC[2:0] RA TI Sel
R ENDID[3:0] RCCTMRACKRATN R
01xxx x x x
0 0 0 0xxxx0 0 0 0 0xx0 0x0 0x0 0 0
OPC2 OPC1 OPC0 Instruction Defined
000Reselect
0 0 1 Disconnect
010WaitSelect
011Set
100Clear