NAME
	CMD_READ/ETD_READ -- read sectors of data from a disk.

   FUNCTION
	These commands transfer data from the track buffer to a supplied
	buffer. If the desired sector is already in the track buffer, no disk
	activity is initiated. If the desired sector is not in the buffer, the
	track containing that sector is automatically read in. If the data in
	the current track buffer has been modified, it is written out to the
	disk before a new track is read. ETD_READ will read the sector label
	area if the iotd_SecLabel is non-NULL.

   IO REQUEST INPUT
	io_Device	preset by the call to OpenDevice()
	io_Unit		preset by the call to OpenDevice()
	io_Command	CMD_READ or ETD_READ
	io_Flags	0 or IOF_QUICK
	io_Data		pointer to the buffer where the data should be put
	io_Length	number of bytes to read, must be a multiple of
			TD_SECTOR.
	io_Offset	byte offset from the start of the disk describing
			where to read data from, must be a multiple of
			TD_SECTOR.
	iotd_Count	(ETD_READ only) maximum allowable change counter
			value.
	iotd_SecLabel	(ETD_READ only) NULL or sector label buffer pointer.
			If provided, the buffer must be a multiple of
			TD_LABELSIZE.

   IO REQUEST RESULT
	io_Error - 0 for success, or an error code as defined in
	           <devices/trackdisk.h>
   NOTES
	Under versions of Kickstart earlier than V36, the io_Data had to
	point to a buffer in chip memory. This restriction is no longer
	present as of Kickstart V36 and beyond.

   SEE ALSO
	CMD_WRITE