The ASCBASID field of the ASCB is the ASID (2 bytes).
The current task control block (TCB)
The PSATNEW field of the PSA is the pointer to the new TCB. The PSATOLD field
of the PSA is the pointer to the old TCB. If the old TCB pointer,
PSATOLD, is zero, an SRB was dispatched.
If the TCBRBP field of the TCB points to itself, instead of to
a request block (RB), the TCB is the pseudo-wait TCB and is not chained
to any other TCB.
TCB chain (by priority)
The location X'10' points to the communication vector
table (CVT).
The CVTASVT field of the CVT points to the address space vector
table (ASVT).
The ASVTENTY field of the ASVT begins a series of one word entry
that point to address space control blocks (ASCB), one for each active
ASID.
The ASCBASXB field of the ASCB points to the ASXB.
The ASXBFTCB field of the ASXB points to the first TCB in the
TCB queue.
The ASXBLTCB field of the ASXB points to the last TCB in the TCB
queue.
The TCBBACK field of the TCB points to the previous TCB. In the
first TCB on the queue, this field contains a fullword of zeros.
Subtask chains (end of the chain is always zero)
The TCB field TCBOTC points to the TCB that attached this TCB.
The TCB field TCBLTC points to the TCB most recently attached.
The TCB field TCBNTC points to another TCB attached by the TCB.
The region control task (RCT) TCB is the only TCB not created
by an ATTACH.
Dispatching
Dispatchable task flags are in TCB fields TCBFLGS4 and TCBFLGS5.
If any bit in the 2 bytes is set to 1, the TCB is nondispatchable.
If bit 7 of TCBFLGS5 is set to 1, the reason its not dispatchable
is indicated by a flag bit that is set to 1 in TCB field TCBNDSP1,
TCBNDSP2, or TCBNDSP3. For details, see z/OS® MVS™ Data
Areas in http://www.ibm.com/systems/z/os/zos/bkserv/.