GT.M partitions GDS blocks into 512-block groups. The first block of each group contains a local bitmap. A local bitmap reports whether each of the 512 blocks is currently busy or free and whether it ever contained valid data that has since been KILLed.

The two bits for each block have the following meanings:

These two bits are internally represented as:

The interpreted form of the local bitmap is like the following: >

Block 0  Size 90  Level -1  TN 1 V7   Master Status: Free Space 
                  Low order                         High order 
Block        0: |  XXXXX...  ........  ........  ........  | 
Block       20: |  ........  ........  ........  ........  | 
Block       40: |  ........  ........  ........  ........  | 
Block       60: |  ........  ........  ........  ........  | 
Block       80: |  ........  ........  ........  ........  | 
Block       A0: |  ........  ........  ........  ........  | 
Block       C0: |  ........  ........  ........  ........  | 
Block       E0: |  ........  ........  ........  ........  | 
Block      100: |  ........  ........  ........  ........  | 
Block      120: |  ........  ........  ........  ........  | 
Block      140: |  ........  ........  ........  ........  | 
Block      160: |  ........  ........  ........  ........  | 
Block      180: |  ........  ........  ........  ........  | 
Block      1A0: |  ........  ........  ........  ........  | 
Block      1C0: |  ........  ........  ........  ........  | 
Block      1E0: |  ........  ........  ........  ........  | 
'X' == BUSY '.' == FREE ':' == REUSABLE '?' == CORRUPT 

If bitmaps marked as "?", they denote that they are corrupted (not currently in a legal combination) bitmaps. The consequences of corrupted bitmaps are:

Possible loss of data when GT.M overwrites a block that is incorrectly marked as free (malignant).

Reduction in the effective size of the database by the number of blocks incorrectly marked as busy (benign).