From :  Keith Ball
         To   :  John Nguyen
         Date :  31 October 1983

         Re   :  New Drive command


         Test and set/clear byte in 512 byte sector

         Command code value

            xxx (decimal)
            $xx (hexidecimal)


         Bytes sent to drive :

         byte 1  COMMAND CODE
              2  OPERATION CODE
              3  SECTOR
              4    NUMBER
              5    (d,lsb,msb)
              6  BYTE
              7    INDEX  (lsb,msb)
              8  MASK

         This specification uses the same sector addressing as
         specified in John's Bank and Drive command document.

         OPERATION CODE determines the type of operation to be
         performed.  Values range from 0 to 2, inclusive.  0
         means no operation, 1 means clear, 2 means set and all
         other values are invalid.

         BYTE INDEX is the offset from the first byte of the
         specified sector to the desired byte.  Values range from
         0 to 511, inclusive.  An index value of 0 selects the
         first byte in the 512 byte sector, while a value 511
         selects the last byte.

         MASK is a bit mask specifying which bits within the byte
         selected are operated upon.  A mask bit with a value of
         1 means to perform the operation on the corresponding
         bit in the selected byte.  If the mask bit is 0 the bit
         is ignored.  Example, if mask bit 7 is 1 then perform
         the operation on bit 7 of the byte selected.


         Bytes received from drive :

         byte 1 RESULT CODE
              2 test byte before operation
.pg
         RESULT CODE is the standard set of disk result codes
         returned for a disk operation.  Errors specific to this
         command are :

               a) invalid sector number,
               b) invalid byte index,
               c) invalid operation code.

         Disk result codes for any other possible disk operation
         errors are also returned.

         The second byte returned is the value of the byte
         selected before the specified operation is performed.
         This is the test result.


         Example :

         A byte within the 512 byte sector $37 (hex) with a byte
         index of $76 (hex) has a value of $52 (hex).

         The host sends the following drive command :

          byte  value       comment
           1     $xx        command code
           2     $02        operation code = set bits
           3     $01        sector
           4     $37          number
           5     $00          (d,lsb,msb)
           6     $76        byte
           7     $00         index  (lsb,msb)
           8     $0F        mask = set all low bits

         The host receives :

          byte  value       comment
           1     $00        result code (assume it worked)
           2     $52        test byte before operation

         After the command the byte within the 512 byte sector
         $37 (hex) with a byte index of $76 (hex) has a value of
         $5F (hex).