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5-16 SCSI SCRIPTS Instruction Set
When the SACK/ or SATN/ bits are cleared, the
corresponding bits are cleared in the
SCSI Output Control Latch (SOCL) register. Do not set
SACK/ or SATN/ except for testing purposes. When the
target bit is cleared, the corresponding bit in the SCSI
Control Zero (SCNTL0) register is cleared. When the
carry bit is cleared, the corresponding bit in the ALU is
cleared.
Note:
None of the signals are cleared on the SCSI bus in the
Target mode.
Initiator Mode
Select 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 select
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.
This way the SCRIPTS can move to the next instruction
before the selection completes. It continues executing
SCRIPTS until a SCRIPT that requires a response from
the Target is encountered.
If the LSI53C875A 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 DMA Next Address (DNAD) register. Manually set
OPC2 OPC1 OPC0 Instruction Defined
0 0 0 Select
0 0 1 Wait Disconnect
0 1 0 Wait Reselect
011Set
100Clear