Sony SDX-800V/R manual Read Buffer Descriptor Bit Byte Reserved, Buffer Capacity

READ BUFFER 6.Command Specification

Descriptor Mode - In this mode, a maximum of four bytes of READ BUFFER descriptor information is returned. The device server shall return the descriptor information for the buffer specified by the BUFFER ID field (see the description of the buffer ID). If there is no buffer associated with the specified buffer ID, the device server shall return all zeros in the READ BUFFER descriptor. The BUFFER OFFSET field is reserved in this mode. The allocation length should be set to four or greater. The device server shall transfer the lesser of the allocation length or four bytes of READ BUFFER descriptor.

Table 6-72: READ BUFFER Descriptor

	Bit	7	6	5	4		3	2	1	0
	Byte
	0					Reserved
	1	(MSB)			Buffer Capacity
	2
	3									(LSB)
Buffer ID: The supported values are:

Table 6-73:

Buffer ID	Description		Offset
0	Read Group Buffer	0	- 0x01000000
80	Read Group Buffer	0	- 0x01000000
81	Read Group Buffer	0x01000000 - 0x02000000
82	Read Group Buffer	0x02000000 - 0x03000000
83	Read Group Buffer	0x03000000 - 0x04000000

If an unsupported Buffer ID code is requested, the drive returns a CHECK CONDITION status with an ILLEGAL REQUEST sense key set.

Buffer Offset: The Buffer Offset field contains the byte offset within the specified buffer from which data shall be transferred from. The initiator should conform to the offset boundary requirements returned in the READ BUFFER descriptor. If the target is unable to accept the specified buffer offset, it shall return CHECK CONDITION status, shall set the Sense Key to ILLEGAL REQUEST, and set the Additional Sense Code to ILLEGAL FIELD IN CDB.

Allocation Length: The Allocation Length specifies the maximum number of bytes that the initiator has allocated for returned data. An Allocation Length of ZERO indicates that no data will be transferred. Any other value indicates the maximum number of bytes that will be transferred. The drive terminates the DATA IN phase when Allocation Length bytes of data have been transferred or when all available data has been transferred to the initiator, whichever is less. The Allocation Length must be greater than four (except in data only mode), else the drive will return a CHECK CONDITION status with an ILLEGAL REQUEST sense key set.

Read Data from Echo Buffer: Domain Validation is an important feature introduced in Ultra160 SCSI. It provides a mechanism for SCSI devices to perform at maximum possible operational data transfer speed. Domain validation performs a quick test to help ensure an initiator and target can actually transfer data at negotiated speeds. Using Echo Buffer option, the initiator can do a more extensive data transfer validation test. The initiator transmits the data using the WRITE BUFFER command with the echo buffer option and requests it back using the READ BUFFER command with echo buffer option.

Echo Buffer (1010b): In this mode the drive transfer data to the application client from the echo buffer. The echo buffer shall transfer the same data as when the WRITE BUFFER command with the mode field set to echo buffer was issued. The BUFFER ID and BUFFER OFFSET fields are ignored in this mode. The READ BUFFER command shall return the same number of bytes of data as received in the prior echo buffer mode WRITE BUFFER command from the same initiator.

The initiator may send a READ BUFFER command requesting the echo buffer descriptor prior to a WRITE BUFFER command. If an echo buffer mode WRITE BUFFER command is successful then the initiator may send multiple echo buffer mode READ BUFFER commands to read the echo buffer data multiple times.

Echo buffer descriptor (1011b): In this mode, a maximum of four bytes of READ BUFFER descriptor information is returned. The drive will return the descriptor information for the echo buffer. The BUFFER OFFSET field is reserved in this mode. The allocation length should be set to four or greater.

SONY AIT-3Ex drive SDX-800V series Ver.1.1 6-73