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COU-541 CD-ROM DRIVE SCSI INTERFACE MANUAL
4
.2.
CollUlland
Descriptor Block
The Command Descriptor Block (COB) is a packet of six or ten bytes sent
from the host computer during a command phase that requests the controller to
__ --
perform some operation.
The common parts of the COB are described here.
4.2.1.
Operation Code
The operation code is the first byte of the COB and indicates whether a six
or ten byte is being sent.
Each command has a unique operation code.
4.2.2. Relative Address Bit
The relative address bit is applicable to some ten byte commands.
This
version of the controller does not implement relative addressing.
4.2.3.
Starting
Logical
Address
The starting logical address specifies the logical block at which the
requested read, verify or play operation is to begin.
Not all COB's have
starting logical addresses.
4.2.4.
Transfer
Length
The transfer length specifies the number of logical blocks to be
transferred.
A
transfer
lengt~
value of zero has special meaning in some
commands.
The size of the logical blocks can be set using the MODE SELECT
command.
4.2.5. Parameter
Length
The parameter length specifies the exact number of bytes of sense data that
the host will transfer to the controller.
4.2.6. Allocation
Length
The allocation length specifies the maximum number of bytes that the
initiator has allocated for returned sense data.
An
allocation length of zero
indicates that no sense data will be transferred.
This condition will not be
considered as an error.
The controller will terminate the DATA IN phase when
allocation length bytes have been transferred or when all available sense data
have been transferred to the initiator, whichever is less.
4-4
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