Find all your variables
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If you have ever tried to modify a BASIC program
from a magazine (or even one of your own!) then you know how useful
it is to have a list of which variables have been used and where
they are used. This machine code utility fits into page 6 and does
both tasks at high speed. It was developed from a BASIC program
given in the ATARI BASIC SOURCE BOOK (Compute! Publications).
The BASIC program below POKEs the code for the
utility into page 6 and will work with either Revision A or Revision
C BASIC ROMs. Once the code is in place, any BASIC program can be
loaded and a simple USR call used to generate a cross reference (or
XREF) listing. The utility can operate in three modes and can send
output to the screen, a printer or a disk file.
FULL CROSS REFERENCE MODE
To produce a list of every variable name currently
held by BASIC together with a list of the line numbers where these
names are used enter an immediate mode USR call (i.e. don't use a
line number) like:
X = USR (1536,1)
where 1536 is the start address of the utility and
the '1' selects full XREF mode. This will output the full XREF to
the screen. Use Control-1 to pause and restart the listing otherwise
it will scroll past too fast for it to be read!
NAMES ONLY MODE
To produce just a list of the names of all the
variables use a USR like:
X = USR (1536,0)
or X = USR (1536)
These two commands have the same effect, the names
are listed to the screen, one per line. Each name is followed by its
'variable number' which is allocated by BASIC. This number, which is
in the range 128 to 255, can be used in the following mode.
SINGLE VARIABLE CROSS REFERENCE MODE
This mode is called up by a USR call like:
X = USR (1536,NUMBER)
where NUMBER is the variable number of the
required variable (use 'Names Only' Mode to find the number of the
variable of interest). This command will list on the screen the name
of the specified variable followed by the numbers of all the lines
which refer to this variable. For example, X=USR(1536,128) will list
all references to the first name shown in a 'Names Only' listing.
SENDING OUTPUT TO PRINTER OR DISK
It is quite easy to switch the XREF output to a
printer or a disk file. In order to keep the size of the utility
below 256 bytes (so it could fit into page 6) a channel to the
appropriate device has to be opened before XREF is called. When XREF
is called, it is given an extra parameter which identifies the
channel to be used. For example to use a printer use commands like:
OPEN
#4,8,0,"P:"
X
= USR (1536,0,4)
to send a names only listing to a printer. Once
the channel is open, subsequent XREF calls can be made, e.g.:
OPEN
#4,8,0,"P:"
X
= USR (1536,0,4)
X = USR (1536,128,4)
X
= USR (1536,150,4)
CLOSE
#4
To use a disk file instead of a printer simply
change the OPEN command.
ERROR MESSAGES
XREF has one error message, ERROR 0, which means
too many parameters have been supplied. In addition to this, the
normal I/O error messages may be generated by BASIC, e.g. if the USR
call specifies channel 3 for output when that channel is closed then
ERROR 133 will be produced.
To stop the XREF output, e.g. if a full XREF was
requested by accident, press the BREAK key (there may be a short
delay before the STOPPED message appears).
Note that XREF can show when it is time to clean
up the Variable Name Table. If XREF does not list any line numbers
for a variable then either the lines which referred to it have been
deleted or else some immediate mode commands have used it. These
'unused' variables take up space which can be reclaimed by LISTing
the BASIC program to disk (or tape), typing NEW to clear out the old
program and then ENTERing the program from disk (or tape).
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