MVS Jan 2005.p65 - CBT Tape

TRAP2 and RP illustration
This program illustrates the use of TRAP2 and RP (Resume
Program) instructions.
TRAP2 is a simple instruction that gives control to a routine in
the current context of execution (AR/GR/PSW) and returns
control wherever you want (next to TRAP2 or with PSW
alteration), with modified or unmodified AR and GR. You have
to make sure that the CPU is in the primary-space or AR mode,
and bit 47 of DUCT is on.
The following program runs a loop (eight times with BCT) and
each time issues a TRAP2. TRAP2 gives control to a TRAP
program, which prints R0, R1, R2, and PSW contents. And
also, if the TRAP program finds that R2 contains F'2', it
overwrites the instruction pointed to by the PSW (BCT) with a
X'0101' (PR) to end the loop.
This TRAP/RP pair can easily be used to design an on-line
disassembler or debugger. Please read the comments in the
code and refer to the Principles of Operation manual for more
sophisticated code with TRAP and RP.
CODE
MYTRAP2 CSECT
MYTRAP2 AMODE 31
MYTRAP2 RMODE 24
*=================================================================*
* MODULE-NAME : MYTRAP2 *
* DESCRIPTIVE-NAME: Shows TRAP/RP processing *
* REQUIREMENTS : APF *
* FUNCTION : *
* Initialize TRAP environment, and issue an 8-times loop with *
* a coded TRAP2 instruction. *
* The TRAP takes control, displays GRØ-GR2 and PSW contents *
* and resumes the program. *
* The TRAP program also checks the content of GR2, if GR2=2 then *
* it overwrites instruction (BCT) pointed to by PSW with a PR to *
* end the main routine. *
*=================================================================*
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