
2-40
Functional Description
2.11.1 Wide SCSI Send Bit (WSS)
The WSS bit is set whenever the SCSI core is sending data (Data Out
for initiator or Data In for target), and the core detects a partial transfer
at the end of a chained Block Move SCRIPTS instruction. Note that this
flag is not set if a normal Block Move instruction is used. Under this
condition, the SCSI core does not send the low-order byte of the last
partial memory transfer across the SCSI bus. Instead, the low-order byte
is temporarily stored in the lower byte of the SCSI Output Data Latch
(SODL) register and the WSS flag is set. The hardware uses the WSS
flag to determine what behavior must occur at the start of the next data
send transfer. When the WSS flag is set at the start of the next transfer,
the first byte (the high-order byte) of the next data send transfer is
married with the stored low-order byte in the SODL register; and the two
bytes are sent out across the bus, regardless of the type of Block Move
instruction (normal or chained). The flag is automatically cleared when
the married word is sent. The flag is alternately cleared through
SCRIPTS or by the microprocessor. Additionally, the microprocessor or
SCRIPTS can use this bit for error detection and recovery purposes.
2.11.2 Wide SCSI Receive Bit (WSR)
The WSR bit is set whenever the SCSI core is receiving data (Data In
for initiator or Data Out for target), and the core detects a partial transfer
at the end of a block move or chained block move SCRIPTS instruction.
When WSR is set, the high order byte of the last SCSI bus transfer is
not transferred to memory. Instead, the byte is temporarily stored in the
SWIDE register. The hardware uses the WSR bit to determine what
behavior must occur at the start of the next data receive transfer. The bit
is automatically cleared at the start of the next data receive transfer. The
microprocessor or SCRIPTS can alternatively clear this bit. Additionally,
the microprocessor or SCRIPTS can use this bit for error detection and
recovery purposes.
2.11.3 SCSI Wide Residue (SWIDE) Register
This register stores data for partial byte data transfers. For receive data,
the SWIDE register holds the high-order byte of a partial SCSI transfer
which has not yet been transferred to memory. This stored data may be