8bitfiles.net/archives/archives.tar/archives/genie-commodore-file-library/Information/SURVIVALKIT1.SFX/help.xmodem-pad

Text File | 1993-06-30 | 5.6 KB | 108 lines

Date: 930626

File: Xmodem Padding


XMODEM Padding, How to deal with it!
====================================

So, Bunky, you've plugged into the telecom world.  You've downloaded a slew
of really nifty programs and you're ready to roll!  Well, I really hate to tell
you this, but there could be a GOTCHA in your way.  Who said life was fair?

The GOTCHA!
----------

The GOTCHA involves programs and XMODEM padding.  They sometimes don't get
along at all.  The culprit here is the XMODEM protocol.  Remember, XMODEM was
developed for CP/M systems, and it works fine on MS-DOS systems too. Trouble
is, XMODEM only deals in 128-byte or 1024-byte blocks.  What happens when
someone transmits a file that doesn't happen to be a multiple of 128/1024
bytes?  Simple... XMODEM sticks padding on the end of the last block. This is
the (in)famous XMODEM padding we all know and love.  The padding character is
usually a NULL (a CHR$(0)) or control Z (a CHR$(26)), depending on the phase of
the moon and the biorhythms of the programmer who wrote that particular
implementation of XMODEM.

Most programs you download simply ignore the extra data at the end.  A very few
(some machine language files and BASIC-64 and BASIC-128 compiled programs)
depend on the end of the file being the LAST thing in memory.  With the extra
XMODEM bytes at the end, these programs will go off to never-never land when
RUN.

So how do we deal with padding?  Several ways.

1) Download only ARC, SDA or SFX files.  ARC and SDA save the original file
length as part of the file description and the padding never gets applied to an
individual file.  However, this is the "chicken" way out and you'll end up
missing some WONDERFUL programs!

2) Use a terminal program that automatically strips off Xmodem padding from a
file.  A few terminals will attempt to do this as the program downloads.  If it
is successful, you'll never even SEE padding on a file.  This is a good choice,
but it really limits you to your choices of terminal programs to use.  Most of
the better terms don't attempt to chop off padding on the fly. WIZARD will
perform this stripping automatically.

3) Strip it off yourself, if you find you have to.  It's really pretty easy
and just takes a short while.  After you've done it once or twice, it's no
trouble at all.

There are several methods of stripping off Xmodem padding.  The simplest is to
use a "padding stripper" program.  There are a couple here in the Commodore
software libraries that allow you to easily remove the padding (CNTRL-Z's) from
any file you choose.


Another option is to use program that "edits" a section of memory.  One of the
easiest is built right into Bob's Term Pro 64 and 128.  Simply load the program
into the buffer.  Using the cursor controls, move to the very end of the file.
If there is padding on the file it will usually be CHR$(26) or CHR$(0)
characters.  CAUTION: Don't confuse xmodem padding with the normal end of a
BASIC program.  All BASIC programs end with 3 CHR$(0) characters.  Since 99% of
the padding you will encounter will be CHR$(26) this won't be a problem (just
something to keep in mind SHOULD it pop up!)  Now, you're at the end and have
located the padding.  Simply delete those extra characters and save the file to
disk.  You should save it under a different filename from the original in case
you make a mistake.  That way you won't nuke your only copy!

What happens if the file is too big to fit in your editor's buffer?  If you are
using Bob's Term, just fill the buffer with the file you are checking, then
write it back to disk with a different filename.  Then, load the 2nd half into
the buffer with the FILL FROM BYTE command from the FILL menu.  You'll want to
begin at the byte you left off on (which is the number shown as the size of the
buffer when empty...28500 for the 64 version if I remember correctly).  Now
when the 2nd half loads, you can scroll thru like usual, and delete the padding
bytes at the end.  Then DUMP to DISK with the same filename as the first buffer
save. It will tell you the filename is duplicate, and ask if you want to
Replace or Append.  Just Append!  Your file will be restored to it's original
size, less the padding!  (It's the same procedure that's used to split up great
big SEQ files into something a little more manageable)!

A final option is to use a "sector editor" and edit the file directly on the
disk.  While this is a perfectly valid option, it does require a little
knowledge of disk data formats and a firm grasp of the hexadecimal numbering
system.  Once you have your favorite sector editor fired up, move to the end of
the file you need to strip.  On that last block the second byte in the file is
the number of valid data bytes in this block.  Just locate and count the number
of padding bytes and subtract this from the number of bytes in the block (the
2nd data byte on this disk block).  Change the byte and write the sector back
to the disk.  If you use this method, BE SURE OF WHAT YOU ARE DOING!  Always
work on a backup disk or you could easily destroy valid data in the sector you
are working on or elsewhere on the disk.


One redeeming factor.  Virtually all the files that have problems with Xmodem
padding will be ARCed or in SDA format here in the Commodore RoundTable.  (One
of the jobs of a SysOp is to watch out for these little buggers and head off
problems BEFORE they get to you!)  Since ARC knows the proper size and stores
it as part of the ARChive,padding is NEVER a problem if you have downloaded an
ARC or SDA/SFX file.


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