Following a Pointer Chain

Figure 1 gives a recursive function for following a chain of task control blocks (TCB). The function, CheckTCB, invokes Obtain_Data (see Figure 1) to retrieve the TCB pointers from the dump and Put (see Figure 1) to display the addresses.

Figure 1. Example of a REXX EXEC Function to Follow a TCB Structure

CheckTCB: Procedure expose storage.
TCBADDR= ARG(1)
GEN$ = " "
Call Put
Gen$ = "TCB at address "TCBADDR
Call Put
TCBNTC   = Obtain_Data(TCBADDR,128,4)
TCBLTC   = Obtain_Data(TCBADDR,136,4)
TCBRTWA  = Obtain_Data(TCBADDR,224,4)
TCBCMP   = Obtain_Data(TCBADDR,16,4)
Gen$ = " TCBntc...."TCBNTC
Call Put
Gen$ = " TCBltc...."TCBLTC
Call Put
Gen$ = " TCBrtwa..."TCBRTWA
Call Put
Gen$ = " TCBcmp...."TCBCMP
Call Put
/*************************************/
If TCBLTC ^= "00000000"
  Then
    Call CheckTCB TCBLTC
If TCBNTC ^= "00000000"
  Then
    Call CheckTCB TCBNTC
Return

If X'00F78400' is the address of a TCB, you would invoke this function as follows:

CheckTCB 00F78400

This simple function can be coded more efficiently using the RUNCHAIN subcommand to follow a chain of control blocks. A REXX function, however, allows you to customize the analysis of the control block chain. See Customizing Control Block Analysis for an example of customizing the CheckTCB function.