You can use the VIT to record 32, 64, 96, or 128 bytes
of user-generated information in an SNAP trace. An SNAP trace places
additional information into a VTAM® module
that might be useful when you are resolving a VTAM problem. The VIT traces the specified bytes
and enters the data as an SNAP trace entry in the internal trace table
or in the external trace file.
You do not specify the SNAP trace as an option of the
MODIFY command. Instead, follow these steps to write your code:
- Verify that the pointer to the ATCVT (ATCLCPTR) is not 0. ATCLCPTR
is at low-storage location X'408'.
- Verify that ATCRACTR is not 0.
- Call the internal trace recording routine (ISTRACTR) in a VTAM module. Field ATCRACTR in
the ATCVT points to the trace recording routine. For the hex offset,
see z/OS® Communications
Server: SNA Data Areas Volume 1.
Note: You cannot record SNAP during the early stages of VTAM initialization, nor can you
record them during the late stages of termination.
The SNAP trace requires the following information in registers
1, 14, and 15 on entry to the trace routine:
- Register 1
- Points to the 32, 64, 96, or 128 bytes to be recorded.
- Register 14
- Points to a 2–byte input flag field containing:
- hex 141C for a 32-byte SNAP entry
- hex 142C for a 64-byte SNAP entry
- hex 143C for a 96-byte SNAP entry
- hex 144C for a 128-byte SNAP entry
These flags tell the trace recording routine to record an
SNAP trace.
- Two bytes past the address in register 14
- The address to be returned to after recording an SNAP trace.
- Register 15
- Points to the SNAP trace recording routine (ISTRACTR).
When the trace recording routine returns control to the
calling module, the contents of registers 6 and 7 are unpredictable.
The contents of the other registers are not changed.
The SNAP trace record is unformatted. You should format
the first 4 bytes of the SNAP record for a 1- to 4-character EBCDIC
record identifier (for example, C
"ABCD").
Guidelines: - The 1- to 4-character EBCDIC record identifier should not match
any existing VIT record identifier.
- For the VIT snap entry to be used by the VIT analysis tool, the
VIT entry name must be the first 2 to 4 characters of the VIT entry.