Core block reference words
There is an 8-byte core block reference word (CBRW) for each data
level, each of which the C language treats as a pointer followed by
two unsigned short integers. The CBRW is used to store the main storage
address and control information about main storage blocks used by
the entry. z/TPF programs
format the CBRW whenever a main storage block is attached to or detached
from the ECB. The format consists of the main storage address (4 bytes),
the block type indicator (2 bytes), and the block byte count (2 bytes).
The main storage block address has the same label as the CBRW. The
block type indicator and the block byte count have their own labels.
In the following example, x is
the data level number, a hexadecimal digit from 0 to F. 
The block type indicator (ce1ctx) specifies the size
of the block attached to the ECB at level x.
0001 No block attached
0011 127-byte block attached (L0)
0021 381-byte block attached (L1)
0031 1055-byte block attached (L2)
0051 4095-byte block attached (L4)It is important to note that only the block type indicator accurately reflects the status of the level at any given time. If ce1ctx contains X'0001', then the data in the rest of the CBRW for level x is not meaningful. The z/TPF system does not initialize ce1crx and ce1ccx after a block is released. Only if ce1ctx contains X'0011', X'0021', X'0031' or X'0051' is a currently valid main storage address in the level x CBRW.
The z/TPF system updates the CBRW as a result of an application request to obtain or release a main storage block or to read or write a file record. Application programs must never modify the CBRW, but may refer to it to determine data level status and the address of the acquired block.