Subject:  GLOSSARY: DBA - Data Block Address 
Creation Date:  22-SEP-1997 

Oracle Data Block Address (DBA):
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	Each block in an Oracle database is identified by a unique
	file and block number combination. These are represented internally
	in Oracle 7 as a 'DBA' or 'Data Block Address' using 4 bytes.
	Below we show how a file & block number translates into a 4 byte
	DBA.

	For historical reasons we will show a version 6 DBA first then extend
	this to show an Oracle7 DBA. The exact representation is PORT SPECIFIC
	so we will take an example here of Sequent. This uses a 6 bit file
	number in Oracle 6.

    Oracle 6:
    ~~~~~~~~~
	Assume we are looking at file 8, block 56892, this will be represented
	(in binary) as:

			26 bit block number == 56892
		      vv  vvvvvvvv  vvvvvvvv  vvvvvvvv
		00100000  00000000  11011110  00111100
		^^^^^^
		6 bit file number == 8

	Ie: the top 6 bits (on Sequent) represent the file number, while the
	    bottom 26 bits represent a block number within a file.

    Oracle 7:
    ~~~~~~~~~
	In Oracle 6 the number of bits used for the file portion of the DBA
	is quite small (as low as 5 bits on some platforms) and hence limits
	the maximum number of files in a database. 
	Eg: 2^6 = 64 files maximum. But all bits = ZERO and all bits = ONE
				    are special cases so the actual maximum
				    files was 62 (Sequent).

	    2^5 = 32 files maximum !!

	In Oracle 7 extra bits were needed for the file number to allow more 
	files. These extra bits are 'stolen' from the top of the BLOCK number
	and the FILE number 'wraps' into these bits to allow backwards
	compatibility thus (in binary on Sequent):
 
		FFFFFFHH  HHBBBBBB BBBBBBBB BBBBBBBB

		FFFFFF = low order 6 bits of the FILE number
		HHHH   = high order 4 bits of the FILE number
		BBBBBB BBBBBBBB BBBBBBBB = BLOCK number (22 bits)

	This means that file 8, block 56892 is still represented EXACTLY the
	same as in Oracle6, but now we have 10 bits for the FILE number.

	Eg: File 255, block 56892 is represented as:

		11111100 11000000  11011110  00111100

		 F   C    C   0     D   E     3  C
	       \_____/\___/\_______________________/
	          |     |	     |
		  |     |	Block = 0xDE3C = 56892
   		  \_____________	
   			|	\
			V	V
   		      0011   111111 = 0xFF = 255
	
	
	This strange 'wrapping' allows more datafiles AND backwards 
	compatibility to version 6 block addressing for migrated databases.


Port Specifics:
~~~~~~~~~~~~~~~
	To add confusion different ports use different number of bits for
	each portion of the DBA. There are basically 2 numbers used to 
	determine the 'split' points of the DBA. These are shown below with
	example settings.

			<-File Bits->
			Low	
	Platform	Order	Total	DBA Bit Layout
	~~~~~~~~	~~~~	~~~~~	~~~~~~~~~~~~~~
	Sequent V6	 6	 6	FFFFFFbb bbbbbbbb bbbbbbbb bbbbbbbb
	Sequent V7	 6	10	FFFFFFHH HHbbbbbb bbbbbbbb bbbbbbbb
	AIX V7		 8 	10	FFFFFFFF HHbbbbbb bbbbbbbb bbbbbbbb
	MVS		12	12	FFFFFFFF FFFFbbbb bbbbbbbb bbbbbbbb


ODBA:	See [NOTE:30322.1] for instructions on using 'odba'.
~~~~~
	There is a support tool called ODBA to do all the conversions for
	you. All you need to know for a given platform is the bit split as
	shown above. The main port splits are shown in the 'odba' output.


Determining the Bit Split:
~~~~~~~~~~~~~~~~~~~~~~~~~~
	If you dont know the bit split for a platform simply issue the 
	following statements:

		 select dump(chartorowid('00000000.0000.0001')) from dual;
		 select dump(chartorowid('00000000.0000.ffff')) from dual;

	These will allow you to work out the split by displaying the internal
	representation of the DBA for file 1 and file N (where N is the maximum
	file number allowed)

	Eg: 	On Sequent:

	    SQLDBA> select dump(chartorowid('00000000.0000.0001')) from dual;
	    Typ=69 Len=6: 4,0,0,0,0,0
			  ^^^^^^^
				|
	    	Convert these decimal bytes to BINARY:

			00000100 00000000 00000000 00000000
			^^^^^^-- There are 6 LOW ORDER file bits


	    SQLDBA> select dump(chartorowid('00000000.0000.ffff')) from dual;
	    Typ=69 Len=6: 255,192,0,0,0,0
			  ^^^^^^^^^^^
				|
	    	Convert these decimal bytes to BINARY:

			11111111 11000000 00000000 00000000
			^^^^^^^^^^^-- There are 10 file bits in total

	So we have a DBA that uses a 6,10 split.


Oracle 7.2 / 7.3
~~~~~~~~~~~~~~~~~
        In Oracle 7.2 onwards you can use DBMS_UTILITY to do conversions for 
        you. 
        Eg: In Server Manager:

	  DBA -> FILE , BLOCK
	  ~~~~~~~~~~~~~~~~~~~
            variable file# number
	    variable block# number
	    execute :file#:=dbms_utility.data_block_address_file(335596957);
	    execute :block#:=dbms_utility.data_block_address_block(335596957);
	    print file#
	    print block#

	  FILE , BLOCK -> DBA
	  ~~~~~~~~~~~~~~~~~~~
	    variable DBA number
	    execute :DBA:=dbms_utility.make_data_block_address(5,52637);
	    print DBA