1 wide scsi send bit, 2 wide scsi receive bit, 3 swide register – Avago Technologies LSI53C876E User Manual
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SCSI Functional Description
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2.2.11.1 Wide SCSI Send Bit
The WSS bit is set whenever the SCSI controller is sending data
(Data-Out for initiator or Data-In for target), and the controller detects a
partial transfer at the end of a chained Block Move SCRIPTS instruction
(this flag is not set if a normal Block Move instruction is used). Under this
condition, the SCSI controller 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
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
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. Also, the
microprocessor or SCRIPTS can use this bit for error detection and
recovery purposes.
2.2.11.2 Wide SCSI Receive Bit
The WSR bit is set whenever the SCSI controller is receiving data
(Data-In for initiator or Data-Out for target) and the controller 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
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 bit can alternatively be
cleared by the microprocessor or through SCRIPTS. Also, the
microprocessor or SCRIPTS can use this bit for error detection and
recovery purposes.
2.2.11.3 SWIDE Register
This register stores data for partial byte data transfers. For receive data,
the
register holds the high-order byte of a
partial SCSI transfer that has not yet been transferred to memory. This