LSI LSI53C1000, Technical Manual

Page: 146 / 384

4-36 Registers
Register: 0x08
SCSI First Byte Received (SFBR)
Read/Write
SFBR SCSI First Byte Received [7:0]
This register contains the first byte received in any
asynchronous information transfer phase. For example,
when a LSI53C1000 is operating in the 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
SCSI First Byte Received
(SFBR) as the destination. This allows bit testing after an
operation.
The SCSI First Byte Received (SFBR) is not writable
using the CPU, and therefore not by a Memory Move.
However, it can be loaded using SCRIPTS Read/Write
operations. To load the SFBR with a byte stored in
system memory, the byte must first be moved to an
intermediate LSI53C1000 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 Selection phase if the
COM bit in the
DMA Control (DCNTL) register is clear.
If the COM bit is cleared, do not access this register
using SCRIPTS operations, as indeterminate operations
may occur. (This includes SCRIPTS Read/Write
operations and conditional transfer control instructions
that initialize the
SCSI First Byte Received (SFBR)
register.)
7 0
SFBR
0 0 0 0 0 0 0 0