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4-30
SYM53C876/876E Data Manual
Registers
SCSI Registers
Register 08h
SCSI First Byte Received (SFBR)
Read/Write
This register contains the first byte received in any
asynchronous information transfer phase. For
example, when a SYM53C876 SCSI function is
operating in initiator mode, this register contains
the first byte received in the Message-In, Status,
and Data-In phases.
When a Block Move instruction is executed for a
particular phase, the first byte received is stored in
this register— even if the present phase is the
same as the last phase. The first byte received
value for a particular input phase is not valid until
after a MOVE instruction is executed.
This register is also the accumulator for register
read-modify-writes with the SFBR as the destina-
tion. This allows bit testing after an operation.
The SFBR is not writable via the CPU, and there-
fore not by a Memory Move. However, it can be
loaded via SCRIPTS Read/Write operations. To
load the SFBR with a byte stored in system mem-
ory, the byte must first be moved to an intermedi-
ate SYM53C876 SCSI function register (such as
the SCRATCH register), and then to the SFBR.
This register also contains the state of the lower
eight bits of the SCSI data bus during the Selec-
tion phase if the COM bit in the DCNTL register
is clear.
If the COM bit is cleared, do not access this regis-
ter via SCRIPTS operation, as non-determinate
operations may occur. (This includes SCRIPTS
Read/Write operations and conditional transfer
control instructions that initialize the SFBR regis-
ter.)
Register 09h
SCSI Output Control Latch (SOCL)
Read/Write
Bit 7
REQ(Assert SCSI REQ/ Signal)
Bit 6
ACK(Assert SCSI ACK/ Signal)
Bit 5
BSY(Assert SCSI BSY/ Signal)
Bit 4
SEL(Assert SCSI SEL/ Signal)
Bit 3
ATN(Assert SCSI ATN/ Signal)
Bit 2
MSG(Assert SCSI MSG/ Signal)
Bit 1
C/D(Assert SCSI C_D/ Signal)
Bit 0
I/O(Assert SCSI I_O/ Signal)
This register is used primarily for diagnostic test-
ing or programmed I/O operation. It is controlled
by the SCRIPTS processor when executing SCSI
SCRIPTS. SOCL is used only when transferring
data via programmed I/O. Some bits are set (1) or
reset (0) when executing SCSI SCRIPTS. Do not
write to the register once the SYM53C876 SCSI
function starts executing normal SCSI SCRIPTS.
1B7
7
Default >>>
0
1B6
6
1B5
5
1B4
4
1B3
3
1B2
2
1B1
1
1B0
0
0
0
0
0
0
0
0
REQ
7
Default >>>
0
ACK
6
BSY
5
SEL
4
ATN
3
MSG
2
C/D
1
I/O
0
0
0
0
0
0
0
0