Emerson MVME147 manual Common Command Set And Disk Operation, Byte Offset, Mode Select Header

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COMMON COMMAND SET AND DISK OPERATION

NOTE:	If all of the parameters match or are not changeable,
	only the mode select header and block descriptor is
	sent, and the parameter list length is be $0C.
	If a parameter is changed based on the current,
	changeable, and the packet parameter, the entire page
	3 including the mode select header and the block
	descriptor is sent, and the parameter list length is $20.

The first twelve bytes that are transferred during the DATA OUT phase for the mode select command make up the mode select header, and the block descriptor which is described below:

Mode Select Header and Block Descriptor

BYTE OFFSET	VALUE	COMMENTS
	Mode Select Header
0	00	Reserved
1	00	Media type
2	00	Reserved
3	08	Block descriptor length
	Block Descriptor
0	00	Reserved
1	00	Number of blocks (MSB)
2	00	Number of blocks
3	00	Number of blocks (LSB)
4	00	Reserved
5	00	Block length (MSB)
6	(Note)	Block length
7	(Note)	Block length (LSB)
NOTE:	The block length is taken from the attach packet,
	byte offset $1C, bytes per block.

The remaining 20 bytes that are transferred during the DATA OUT phase for the mode select command are the page 4 parameters.

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