
SCSI Operating Registers
5-16
SYM53C875/875E Data Manual
Register 08 (88)
SCSI First Byte Received (SFBR)
Read/Write
T his register contains the first byte received in any
asynchronous information transfer phase. For ex-
ample, when the SYM53C875 is operating in initi-
ator 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. T he first byte received value for a
particular input phase is not valid until after a
MOVE instruction is executed.
T his register is also the accumulator for register
read-modify-writes with the SFBR as the destina-
tion. T his allows bit testing after an operation.
T he SFBR is not writable via the CPU, and there-
fore not by a Memory Move. T he Load instruction
may not be used to write to this register. However,
it can be loaded via SCRIPT S Read/Write opera-
tions. T o load the SFBR with a byte stored in sys-
tem memory, the byte must first be moved to an
intermediate SYM53C875 register (such as the
SCRAT CH register), and then to the SFBR.
T his register will also contain the state of the lower
eight bits of the SCSI data bus during the selection
phase if the COM bit in the DCNT L register is
clear.
Register 09 (89)
SCSI Output Control Latch (SOCL)
Read /Write
Bit 7
RE Q(Assert SCSI RE Q/ Signal)
Bit 6
ACK (Assert SCSI ACK / Signal)
Bit 5
BSY (Assert SCSI BSY / Signal)
Bit 4
SE L(Assert SCSI SE L/ Signal)
Bit 3
AT N(Assert SCSI AT N/ 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)
T his register is used primarily for diagnostic
testing or programmed I/O operation. It is con-
trolled by the SCRIPT S processor when exe-
cuting SCSI SCRIPT S. SOCL should only be
used when transferring data via programmed
I/O. Some bits are set (1) or reset (0) when
executing SCSI SCRIPT S. Do not write to the
register once the SYM53C875 starts executing
normal SCSI SCRIPT S.
1B7
7
1B6
6
1B5
5
1B4
4
1B3
3
1B2
2
1B1
1
1B0
0
Default>>>
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