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SCSI Functional Description
2-57
The
third option
is to have the system BIOS not report the existence
of the SCSI controller chips when the SCSI BIOS and operating
systems make PCI BIOS calls. This approach requires modifications
to the system BIOS and assumes the operating system uses PCI
BIOS calls when searching for PCI devices.
2.2.18 Chained Block Moves
Since the SYM53C1010 has the capability to transfer 16-bit wide SCSI
data, a unique situation occurs when dealing with odd bytes. The
Chained Move (CHMOV) SCRIPTS instruction along with the Wide SCSI
Send (WSS) and Wide SCSI Receive (WSR) bits in the
SCSI Control
Two (SCNTL2)
register are used to facilitate these situations. The
Chained Block Move instruction is illustrated in
Figure 2.7
.
2.2.18.1 Wide SCSI Send Bit
The WSS bit is set following a wide SCSI send operation (Data-Out for
initiator mode or Data-In for target mode) when the SCSI core is holding
a byte of chain data. The SCSI core holds the byte when 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
SCSI Output
Data Latch (SODL)
register for asynchronous transfers or in the chain
byte holding register for synchronous transfers and the WSS flag is set.
The hardware uses the WSS bit to determine what behavior must occur
at the start of the next data send transfer. If the WSS bit 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 byte of chain data. The two bytes
are sent out across the bus regardless of the type of Block Move
instruction (normal or chained). The WSS bit is automatically cleared
when the “married” word is sent. Performing either a SCSI receive
operation or any narrow transfer also clears the bit. In addition, SCRIPTS
and the microprocessor can clear the WSS bit as well as use it for error
detection and recovery purposes.