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Program Trace Routine
SNOOPY links to a user-supplied subroutine when it encounters a command that is not
contained in the set of standard commands. (Because one-character abbreviations are
permitted for most SNOOPY commands, the command cannot begin with the same
letter as a SNOOPY command. Therefore, the commands should begin with a $
character.)
A user command routine is identified to SNOOPY by storing its address in H2 of the
externally defined cell SNUCM$. If SNOOPY cannot recognize the command, the user
routine must return to 0,X11; normal return is to 1,X11. The volatile register set can be
used without saving and restoring it.
When SNOOPY enters the user command routine, registers A1 and A2 contain the
command in Fieldata, the command is left-justified and space-filled, register X2 contains
the current program address (simulated P register), and X10 points to an area containing
the program's simulated control registers. These registers are located offset to a
location that corresponds to their absolute addresses; for example, A0 contents at
014,X10, R5 contents at 0105,X10, and so on. The program can change the value in
SNUCM$ at any time, even while SNOOPY is performing a trace. If it is set to zero,
SNOOPY does not interpret any user commands.
The command routine may reference SNOOPY's command reading routine and lexical
scanner by performing an SLJ to SNGTC$. A0 should contain zero or a pointer to a
TREAD$ packet before it calls SNGTC$.
If register A0 = 0, SNOOPY will not read the input; the remainder of the line containing
the user command will be scanned for information. If A0 contains a TREAD$ packet
pointer, the calling program must specify the input buffer SNBUF$. On return from
SNGTC$, register A0 is unchanged; A1 and A2 contain a Fieldata command which is
left-justified and space-filled (unless A3 = 0); A3 contains the nonblank character count
for A1 and A2; and register R1 has the terminated delimiter. If an entirely blank line is
encountered, registers A3 = 0 and R1 = '/'.
SNOOPY commands conform to element name syntax (alphabetics, numerics, “-”,
and “$”).
24.4.1. Contingency Processing
In some cases, because of time constraints, it is undesirable to use SNOOPY to trace an
entire program. Instead, SNOOPY can gain control only when a contingency occurs.
To call a routine that does this, enter the following:
SLJ SNCNT$
After this calling sequence is read, the program proceeds normally until a contingency is
encountered. When a contingency occurs, SNOOPY takes control and the simulated
P register points to the contingency address + 1. A message indicating the nature of the
error is printed. If the contingency is fatal and the program is in batch mode, SNOOPY
exits (EXIT$). For nonfatal contingencies, the remainder of the program is executed.
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