X3T9.2/91-184
_________________________________________________________________

                                                  Date: 11/5/91

John Lohmeyer
NCR
Chairman X3T9.2

Subject: Additional Comment on ATA Rev 2.6


     Users  of  the ATA standard may be confused by  the  current 
documentation of the Set Features Command. The title of the table 
makes  it appear that there are numerous Set  Features  Commands. 
Actually  the table is a listing of command modifiers  using  the 
Set  Features  register.  To avoid the confusion  I  suggest  the 
editorial  modifications shown below. In addition a pair  of  the 
modifiers  are identified as vendor unique when in fact they  are 
called  out  elsewhere  in  the standard.  I  have  included  the 
unmodified Write Same section for reference.

9.21  Set Features

This  command is used by the host to establish the following parameters  which 
affect the execution of certain drive features as shown in Table 9-4.

          TABLE 9-4:  SET FEATURE REGISTER DEFINITIONS
    +-----+--------------------------------------------------------------+
    | 01h | Enable 8-bit data transfers                                  |
    | 02h | Enable Write Cache *                                         |
    | 22h | Write Same Specified Block                                   |
    | 33h | Disable Retry *                                              |
    | 44h | Vendor unique length of ECC on Read Long/Write Long commands |
    | 54h | Set cache segments to Sector Count Register value *          |
    | 55h | Disable read look-ahead feature                              |
    | 66h | Disable reverting to power on defaults                       |
    | 77h | Disable ECC *                                                |
    | 81h | Disable 8-bit data transfers                                 |
    | 82h | Disable Write Cache *                                        |
    | 88h | Enable ECC *                                                 |
    | 99h | Enable Retries *                                             |
    | AAh | Enable read look-ahead feature                               |
    | ABh | Set maximum prefetch using Sector Count Register value *     |
    | BBh | 4 bytes of ECC apply on Read Long/Write Long commands        |
    | CCh | Enable reverting to power on defaults                        |
    | DDh | Write Same Entire User Medium                                |
    +-----+--------------------------------------------------------------+
            * These commands are Vendor-specified

See 10.3 for protocol. If the value in the register is not supported or is 
invalid, the drive posts an Aborted Command error. 

At power on, or after a hardware reset, the default mode is the same as that 
represented by values greater than 80h. A setting of 66h allows settings of 
greater than 80h which may have been modified since power on to remain at the 
same setting after a software reset. 

The Set Features register values 22h and DDh are used in conjunction with  the 
Write  Same command (See 9.29.) See vendor specification for the default  mode 
of the commands which are vendor-specified.

9.29  Write Same

This command executes in a similar manner to Write Sectors except that only 
one sector of data is transferred. The contents of the sector are written to 
the medium one or more times.

NOTE: The Write Same command allows for initialization of part or all of the 
medium to the specified data with a single command.

If the Features Register is 22h, the drive shall write that part of the medium 
specified by the sector count, sector number, cylinder and drive/head 
registers. If the Features Register contains DDh, the drive shall initialize 
all the user accessible medium. If the register contains a value other than 
22h or DDh, the command shall be rejected with an aborted command error. 

The drive issues an interrupt to indicate that the command is complete. Any 
error encountered during execution results in the termination of the write 
operation. Status is available in the Error Register if an error occurs. The 
error posting is the same as that of the Write Sectors command. 





                                   G.E. Milligan
                                   Member X3T9


cc:
Dal Allan ENDL