To:      SCSI-2 CAM Committee
	From:    Judy Polonsky
	Subject: Physical to/from Logical Block Address Conversion

        Fixed  disk  boot  devices are described physically in  terms  of 
        CYLINDER/HEAD/SECTORS  per TRACK through BIOS INT  13h,  function 
        08h  and by drive geometry table pointers INT 41h (drive  0)  and 
        INT  46h  (drive 1).   Operating  systems  (DOS,  Xenix,  Novell, 
        OS/2,etc.)  use  the INT 13h call and/or examine the INT  41h/46h 
        tables  directly  to determine device capacity and to  issue  I/O 
        requests  before O/S-specific ram-resident drivers are  installed 
        (which in some cases, continue to use the INT 13h protocol).  The 
        standard partition table structure,  the means by which each  O/S 
        determines  which  portion  of the device it  "owns",  also  uses 
        CYLINDER/HEAD/SECTOR   to  specify  beginning/end  of   a   given 
        partition.   Both  the INT 13h interface and the partition  table 
        define  10  bits for cylinder,  8 bits for head,  and 6 bits  for 
        sector.  Since  boot  code  uses  the  physical  address  in  the 
        partition  table  to load in a given  OS's  partition,  a  common 
        algorithm by which to convert SCSI logical block capacity to this 
        physical  interface  is  essential  for  devices  such  as  fixed 
        removable media to maintain bootability across different host bus 
        adapters. 

        A  standard  conversion  algorithm could be based on  fake  fixed 
        physical  parameters  which bear no relation  to  real  geometry, 
        based on actual mode sense geometry within the bit constraints of 
        INT 13h interface, or based on some combination of the above.

        Options for converting logical to physical capacity would be  the 
        following ( comments /criticisms needed!):


        I.  Ignoring mode sense geometry

             a)  Heads  and Secs/trk fixed at maximum values (256 and  64 
        repectively)  - 

             This  has advantage of being straightforward but  can  waste 
        alot  of disk space on the remainder of the disk which can't  fit 
        into  an integral "cylinder" - up to 8MB (256 hds * 64 spt *  512 
        bytes). 

             b)   Heads and Secs/trk fixed initially at small  values  (4 
        and  17  respectively)  - 

             Calculate  "cyls" as a function of fixed heads and  spt.  If 
        they  don't fit (> 1024),  heads or spt can be bumped til logical 
        capacity is fully described. 

        Starting with 17 secs per track has the advantage of  maintaining 
        compatibility  some O/S 's - old versions of DOS, Novell -  which 
        assume 17 secs/trk.  However,  it can result in greater number of 
        smaller  I/O  requests under DOS versions which don't cross  head 
        boundaries  than an algorithm which created larger  "tracks"  and 
        fewer heads.   Don't care under Unix/Xenix,Novell,etc.  since the 
        requests are fixed block sizes in those environments.

         
        II.  Using Mode Sense Geometry

        If  actual geometry parameters returned by MODE sense fit  within 
        the  imposed  bit  constraints  ,   use  them.  This  allows  for 
        performance tuning.  (NOTE:  actual capacity reported to O/S does 
        not include areas reserved by drive for defect management  - READ 
        CAPACITY  results  is probably the actual logical capacity to  be 
        described physically.  Also,  the last "cylinder" is reserved  by 
        INT 13h bios drivers for diagnostics ).

        If actual geometry does not fit,  there are several options:

        1) Truncation of cyls or spt ( whatever doesn't fit) - not a good 
        option as much capacity can be lost.

        2) Default to one of the algorithms in I.  above (no relation  to 
        actual geometry).

        3)  Adjust  remaining two physical parameter til the  overflowing 
        parameter  is  within its maximum allowable value  (1024  cyls,64 
        spt)  starting  from their actual mode sense values  rather  than 
        some small or large fixed defaults. 

        Example.  Cylinders too large -
          
             a)  Bump  heads exclusively till cylinders fit or  max  head 
        value is reached.  If max head is reached, bump secs/trk til cyls 
        fit  or  max secs/trk is reached.  If max secs/trk is  reached  , 
        cylinders should be set to max cylinders (256 hds * 64 spt * 1024 
        cyls = 8 gigabytes).

             b) Bump spt exclusively till cylinders fit or max spt  value 
        is  reached.   If max spt is reached,  bump heads til cyls fit or 
        max heads is reached.  If max heads is reached , cylinders should 
        be  set  to  max  cylinders (256 hds * 64 spt *  1024  cyls  =  8 
        gigabytes).

        Example.  Sectors per Track too large -

             Leaving  cylinders fixed,  set sectors per track to  maximum 
        allowable  value  and  calculate heads as a function of  these  2 
        values. 


        Example.  Sectors per track and Cylinders too large -

             Set cylinders and secs per track to their maximum  allowable 
        values and calculate heads as a function of these two values.