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SYM53C876/876E Data Manual
5-7
SCSI SCRIPTS Instruction Set
I/O Instructions
5. If the SCSI phase bits do not match the value
stored in the SSTAT1 register, the
SYM53C876 generates a phase mismatch
interrupt and the instruction is not executed.
6. During a Message-Out phase, after the
SYM53C876 has performed a select with
Attention (or SATN/ is manually asserted with
a Set ATN instruction), the SYM53C876
deasserts SATN/ during the final SREQ/SACK
handshake.
7. When the SYM53C876 is performing a block
move for Message-In phase, it does not
deassert the SACK/ signal for the last SREQ/
SACK handshake. Clear the SACK signal
using the Clear SACK I/O instruction.
Bits 26-24 SCSI Phase
This 3-bit field defines the desired SCSI infor-
mation transfer phase. When the SYM53C876
operates in initiator mode, these bits are com-
pared with the latched SCSI phase bits in the
SSTAT1 register. When the SYM53C876
operates in target mode, it asserts the phase
defined in this field. The following table
describes the possible combinations and the
corresponding SCSI phase.
Bits 23-0
Transfer Counter
This 24-bit field specifies the number of data
bytes to move between the SYM53C876 and
system memory. The field is stored in the DBC
register. When the SYM53C876 transfers data
to/from memory, the DBC register is decre-
mented by the number of bytes transferred. In
addition, the DNAD register is incremented by
the number of bytes transferred. This process
is repeated until the DBC register is decre-
mented to zero. At this time, the SYM53C876
fetches the next instruction.
If bit 28 is set, indicating table indirect
addressing, this field is not used. The byte
count is instead fetched from a table pointed to
by the DSA register.
Second Dword
Bits 31-0
Start Address
This 32-bit field specifies the starting address
of the data to move to/from memory. This field
is copied to the DNAD register. When the
SYM53C876 transfers data to or from mem-
ory, the DNAD register is incremented by the
number of bytes transferred.
When bit 29 is set, indicating indirect address-
ing, this address is a pointer to an address in
memory that points to the data location. When
bit 28 is set, indicating table indirect address-
ing, the value in this field is an offset into a
table pointed to by the DSA. The table entry
contains byte count and address information.
I/O Instructions
First Dword
Bits 31-30 Instruction Type - I/O Instruction
Bits 29-27 Op Code
The following Op Code bits have different
meanings, depending on whether the
MSG
C/D
I/O
SCSI Phase
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
Data-Out
Data-In
Command
Status
Reserved-Out
Reserved-In
Message-Out
Message-In