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Program Trace Routine
In each of the following circumstances, SNOOPY ignores all commands on the line after
the last one performed; they can be reentered if desired.
• Trace termination (unless mode bit 16 was set).
• The @@X C sequence interrupts the trace.
• SNOOPY encounters an invalid command.
• A CHANGE, TOFF$, or EXIT$ command is used without a trailing asterisk.
Entering a blank line (carriage return) in reply to the “C--” has the same effect as the
STEP command. A blank line response to a parameter request may be erroneous or may
have a special meaning, depending on the nature of the command.
To enter an Executive control statement, answer a command solicitation by entering a
line beginning with the character %. The remainder of the line should be an Executive
control statement without the initial @ character. The statement must be legal for
submission to the CSF$ Executive Request function. SNOOPY converts the initial %
character to an @ and submits the resulting image to CSF$.
If the CSF request is processed normally, the status bits from register A0 are displayed
and SNOOPY solicits the next command. Otherwise, SNOOPY intercepts the
appropriate contingency and prints the message “CSF$ ERROR” followed by command
solicitation. For example, to assign a temporary file named T35, respond to “C--” by
typing “%ASG,T T35,F2”.
When command mode is entered at trace termination, the trace may be completed only
by using the GO or STEP commands or a command with an equivalent effect. Once
such a command is entered, no further commands are executed, and the trace
terminates. The activity then continues execution or exits, depending on the type of
trace termination.
In all cases where number is called for, octal notation is assumed. Unless indicated
otherwise, a leading zero is not required. In general, SNOOPY uses octal notation
everywhere except in register designations and in the instruction cycle count printed by
SNOOPY.
Certain commands (TOFF$, EXIT$, and CHANGE) clear SNOOPY's command buffer
before reading further commands because of the potentially irreversible nature of the
operation that will be performed. If this is not desired, affix an asterisk to the command
(TOFF*). For example, to terminate a trace and continue an execution without typing in
two lines, use the sequence “TOFF$*GO”.
Refer to 24.4.2 for a list of all SNOOPY commands. All commands except ALTPRT,
TON$, RBK, RLIB, and STEP can be abbreviated to the first character.
In some cases, particularly for complex programs, the set of commands SNOOPY
provides may not meet all debugging requirements. For this reason, there is a method
that allows the command interpreter to be extended as needed.
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