Command Processing
Furthermore, this field may be used with a different meaning, or it may not have any meaning at all, depending on the type of command. There are also some commands which allocate 3 or more bytes as the transfer data length field. Detailed specifications of these commands are described in the individual command specifications in Chapter 3.
| a. | Transfer block count |
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| When the “Transfer Data Length” is specified as the “Transfer Block Count,” this field specifies | ||||||||||||
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| the number of logical data blocks to be transferred between INIT and the IDD. | ||||||||||||
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| In commands where this field is 1 byte in length, if the field’s specified value is 0, it is regarded | ||||||||||||
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| as specifying 256 blocks, and it is possible to specify a block count ranging from 1 to 256 blocks. | ||||||||||||
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| On the other hand, in commands where this field is 2 bytes in length, if the field’s specified value | ||||||||||||
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| is 0, no data transfer is executed. It is possible to specify a block count ranging from 0 to 65,535 | ||||||||||||
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| b. | Transfer byte length or parameter list length |
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| When this field is specified as the “Transfer Byte Length” or “Parameter List Length,” that | ||||||||||||
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| command specifies data length to be transferred between the INIT and the IDD, expressed as the | ||||||||||||
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| number of bytes. When 0 is specified in this field, data transfer is not executed, except in cases | ||||||||||||
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| where it is expressly stated in the individual command specifications in Chapter 3. | ||||||||||||
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| In commands which send parameters necessary for executing a command from the INIT to the | ||||||||||||
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| IDD, this field is called the “Parameter List Length,” and it specifies the total number of types in | ||||||||||||
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| the parameter list which the INIT is sending. |
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| On the other hand, in commands for receiving information from the IDD (REQUEST SENSE, | ||||||||||||
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| INQUIRY, etc.), this field is called the “Transfer Byte Length,” and specifies the maximum | ||||||||||||
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| number of bytes which the INIT can | receive (the number of bytes of area secured within the | |||||||||||
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| INIT for receiving information). The IDD transfers either the number of effective bytes of the | ||||||||||||
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| type of information specified in the command, or the value specified in the “Transfer Byte | ||||||||||||
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| Length” field, whichever is the smallest number of bytes, and only that number, to the INIT. | ||||||||||||
(5) | Control byte |
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| Bit 7 |
| 6 |
| 5 | 4 |
| 3 | 2 |
| 1 | 0 |
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| 0 |
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| 0 | 0 |
| 0 | 0 |
| 0 | Link | |
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| a. | Link |
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| Command link is specified by this bit is “1.” Details of the operation of the command link are | ||||||||||||
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| described in Section 1.3.2. |
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| b. | Bit 7, 6 | (vendor unique) |
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| bits has no meaning, and the IDD disregards the specified values. |
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