SysBMap:

512 byte header block:
   - "VERSION  1"
   - 8 bytes: # of images.
   - 5 bytes: # of absolute blocks.
   - 8 bytes: # of blocks actually used.

block map entries:
  - each is 28 bytes
  - 4 bytes for SysBData Segment - often 1023.
  - 8 bytes for absolute block index (zero based, within SysBData). 
  - 8 bytes for "image" number (-1 means unallocated)
  - 8 bytes for the next absolute block in this images block chain
    (-1 means end of chain)

each block in block map is 8K on disk (16 512byte blocks).


After the block map is the layer information.  For each layer:

  - 4 bytes: Layer type # : IDBSYSLT_DEAD(1) or IDBSYSLT_IMAGE(2) currently


Virtual tiled images:

128 byte header:
 - 8 bytes: image width
 - 8 bytes: image height
 - 8 bytes: tile width
 - 8 bytes: tile height 
 - 3 bytes: tile pixel data type. 
 - some spaces
 - compression type (normally NONE)
 - some spaces.

Tilemap:
 - 12 bytes per tile containing offset within virtual file. 

Tile Size map:
 - 8 bytes per tile containing size of tile data. 

Tile Data:
 - Absolute data. 
