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Instruction Set of the I/O Processor
Block Move Instructions
SYM53C875/875E Data Manual
6-7
boundary, the SYM53C875 will store the last
byte in the SCSI Wide Residue Data Register
during a receive operation, or in the SCSI
Output Data Latch Register during a send
operation. T his byte will be combined with the
first byte from the subsequent transfer so that a
wide transfer can be completed.
5. If the SCSI phase bits do not match the value
stored in the SSTAT 1 register, the
SYM53C875 generates a phase mismatch
interrupt and the instruction is not executed.
6. During a Message Out phase, after the
SYM53C875 has performed a select with
Attention (or SAT N/ has been manually
asserted with a Set AT N instruction), the
SYM53C875 will deassert SAT N/ during the
final SREQ/SACK handshake.
7. When the SYM53C875 is performing a block
move for Message In phase, it will not deassert
the SACK / signal for the last SREQ/SACK
handshake. T he SACK signal must be cleared
using the Clear SACK I/O instruction.
Bits 26-24 SCSI Phase
T his 3-bit field defines the desired SCSI infor-
mation transfer phase. When the SYM53C875
operates in initiator mode, these bits are com-
pared with the latched SCSI phase bits in the
SSTAT 1 register. When the SYM53C875
operates in target mode, the SYM53C875
asserts the phase defined in this field. T he fol-
lowing table describes the possible combina-
tions and the corresponding SCSI phase.
Bits 23-0 Transfer Counter
T his 24-bit field specifies the number of data
bytes to be moved between the SYM53C875
and system memory. T he field is stored in the
DBC register. When the SYM53C875 trans-
fers data to/from memory, the DBC register is
decremented by the number of bytes trans-
ferred. In addition, the DNAD register is
incremented by the number of bytes trans-
ferred. T his process is repeated until the DBC
register has been decremented to zero. At that
time, the SYM53C875 fetches the next
instruction.
If bit 28 is set, indicating table indirect
addressing, this field is not used. T he byte
count is instead fetched from a table pointed to
by the DSA register.
Second Dword
Bits 31-0 Start Address
T his 32-bit field specifies the starting address
of the data to be moved to/from memory. T his
field is copied to the DNAD register. When the
SYM53C875 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. T he table entry
contains byte count and address information.
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