First Dword - LSI LSI53C875A Technical Manual

5.4.1 First Dword

31 30 29 27 26 25 24 23
DMA Command (DCMD)
Register
IT[1:0] OPC[2:0] RA TI Sel
0 1 x x x x x
5-14
20 19
R ENDID[3:0]
x 0 0 0 0 x x
IT[1:0] Instruction Type - I/O Instruction
The IT bit configuration (01) defines an I/O Instruction
Type.
OPC[2:0] OpCode
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
5.5, "Read/Write Instructions."
Target mode operations.
Target Mode
OPC2 OPC1 OPC0
0
0
0
0
1
Reselect Instruction
The LSI53C875A arbitrates for the SCSI bus by asserting
the SCSI ID stored in the
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
SCSI SCRIPTS Instruction Set
16 15 11 10 9
DMA Byte Counter (DBC)
R CC TM
x x 0 0 0 0 0 x
Instruction Defined
0 0 Reselect
0 1 Disconnect
1 0 Wait Select
1 1 Set
0 0 Clear
DMA SCRIPTS Pointer (DSP)
8 7 6 5 4
Register
R ACK R ATN
x 0 0 x 0 0
This section describes
SCSI Chip ID (SCID)
3 2 0
R
x 0 0 0
[31:30]
[29:27]
Section
register.
register.