Amiga Elysian Archive/AmigaElysianArchive.iso/comm/term33in.lha/Documentation/XPR_Libs/xprzmodem.doc

Text File | 1993-02-13 | 33KB | 628 lines

                    Documentation for XPRZModem.library
              Version 2.10, 12 February 1991, by Rick Huebner

|---
|           Update comments for XprZmodem.Library
|    Version 2.50 Update, 15 November 1991, by William M. Perkins
|    Version 2.51 bug fix, 29 January 1992, by John Tillema
|    Version 2.52 recompiled, 6 March 1992, by William M. Perkins
|      Proto additions for the SAS 5.10B Compiler by Jim Cooper
|---

|---
|    All of my additions to this documentation are indicated by
|  <--  this left margin braketing.   -WMP-
|---

1.  Introduction (or "What is this thing, anyway?")
---------------------------------------------------

     XPRZModem.library is an Amiga shared library (with full Lattice C
source code) which provides ZModem file transfer capability to any
XPR-compatible communications program.  The XPR external protocol standard
describes an interface method which allows various file transfer protocols
to be implemented as Amiga shared libraries which may then be used
interchangeably in any compatible communications program.  This takes a
heavy load off of the comm program author, who no longer has to support
scads of different file transfer protocols (many of which are quite tricky
to code) in their program in order to make it widely useful and popular.
The comm program is also smaller and more efficient as a result, since all
those obscure protocols (you know, the ones *you* don't need) are no longer
taking up space.  The XPR standard also helps users, who can mix and match
their favorite file transfer protocols (and implementations thereof) with
their favorite comm programs.  And when new protocols are invented, the
user simply plugs in a new library, and voila!, it's ready to use.
Hopefully, making protocols easy to support will allow more and better comm
programs to be written, as authors can concentrate on their programs
instead of constantly re-inventing the wheel.

     Of course, for all of this wonderful stuff to happen, there needs to
be a good selection of these XPR protocol libraries available to the
public.  It's the classic chicken-and-egg problem; comm program authors
won't be motivated to support the XPR standard unless there are a goodly
number of protocols available for it.  And other programmers won't be
motivated to write XPR protocol libraries until there are a goodly number
of comm programs which can use them.  In an effort to do my bit [ B^) ] for
the Amiga community, which has given me so many neat toys to play with over
the past few years, I decided to try and help get the ball rolling.

     Hopefully, the early availability of a ZModem library will help
stimulate interest in the XPR standard, resulting in better Amiga telecomms
for all of us.  And by making my source code PD, I hope to help others
interested in writing XPR protocol libraries by giving them some serious
example code.  Also, having ZModem library code readily available to John
Q. Hacker should help ensure a steady stream of new lemon-fresh enhanced
ZModem libraries (with enzymes) for all of us to use in the future.

     Of course, no discussion of the XPR standard would be complete without
giving proper credit to the author, Willy Langeveld of the Stanford Linear
Accelerator Center.  Many thanks are due him for this effort.  If you have
any further questions about the XPR standard, be sure and download the spec;
it should be available on BIX (since he's a sysop there), or on most other
major systems.  And I'll try to keep the current version available on my
BBS, as well.

     All files in this archive which are not copyrighted are hereby
released to the public domain (which they were anyway, by way of not being
copyrighted, but I want to make sure YOU realize that).  Do as you like
with them.  Please make lots of copies and distribute them all over the
place, and make lots of derivative works, and everything!  Heck, you can
even publicly perform and/or display this code if you can figure out how...



2.  Installation (OK, enough chatter; let's get to work)
--------------------------------------------------------

     Couldn't be easier.  Just copy the xprzmodem.library file into your
LIBS: directory.  All XPR-compatible comm programs should provide a way for
you to select which XPR protocol you wish to use, either by giving you a
file requester showing LIBS:xpr*.library, or by automatically detecting
these libraries and adding them into their menus.

WARNING:  Versions of VLT prior to revision 4.107 had a bug in the
xpr_sflush() routine which caused random Guru 3 crashes on some systems.  If
you're using a version of VLT older than 4.107, it would be a good idea to
upgrade to the latest rev.  Besides, there's a bunch of new features you're
missing, anyway...

ANOTHER WARNING:  Versions of VLT prior to 5.034 had a bug in the
xpr_sread() function which caused them to (at least sometimes) fail to
return any bytes received so far when an xpr_sread() call timed out.  This
version of XPRZModem.library requires VLT version 5.034 or later.  (Yes,
Willy, it really was broken B-))

|---
|    XprZmodem.Library version 2.50 and 2.52 tested with WTerm version
|    0.82 and Willy's term program, VLT version 5.045.  No problems
|    occurred.  It should work fine with any term program that was able
|    to work with the version 2.10 of XprZmodem.Library.    -WMP-
|---

3.  Options
-----------

     The XPR standard lays out two ways for the comm program user to
specify options for the XPR protocol.  The more primitive option is for the
comm program to provide a method of passing an option string to the XPR
library before transferring files.  The precise format and usage of this
option string is left up to the XPR protocol author; the comm program just
sends it verbatim.  If an environment variable is found with the same name
as the XPR protocol (i.e. there's a file in the ENV: directory called
"xprzmodem"), the comm program is supposed to use this string (contents of
file) to initialize the protocol options.  Also, a menu option or some such
should normally be provided which will allow the user to be prompted for
the option string interactively.

     Version 2.0 of the XPR standard created a new, more sophisticated way
for the comm program user to specify XPR options.  If the comm program
supports it, the XPR library can give the comm program a list of option
prompts, and the comm program can then let the user interactively set the
various options individually, possibly even using nice gadgets and stuff.

     In any case, no matter how your particular comm program feels like
handling it, these are the options supported by this implementation of
ZModem:

          T{Y|N|?|C}   Text translation mode:
             TY = Text Yes; if receiving, translate CR/LF pairs or solo
                  CR chars to normal Amiga LF chars.  Ignore data past ^Z.
                  If sending, suggests to receiver that they should receive
                  this file in text mode.
             TN = Text No; receive file verbatim, without changes.  If
                  sending, suggest to receiver that they receive this
                  file verbatim, without translations.
             T? = Text status unknown; if receiving, use sender's
                  suggestion as to whether to do EOL translations or not.
                  If sending, tell receiver to use default mode, 'cause we
                  don't know either.
             TC = Text mode set by Comm program; the library asks the comm
                  program whether or not to use Text mode for each file.
                  If the comm program doesn't support the necessary
                  xpr_finfo() call, or if the call fails, this option acts
                  like T?.  From the user's point of view, what this option
                  normally does is set the Text mode to match the comm
                  program's built-in text/binary/end-of-line/translation
                  mode, if any.

          O{Y|N|R|S}  Overwrite mode:
             OY = Overwrite Yes; if about to receive file with same name as
                  one which already exists, delete the old file and receive
                  the new file in its place.
             ON = Overwrite No; if about to receive file with same name as
                  one which already exists, append ".dup" onto the name of
                  the new file to keep them separate.
             OR = Overwrite Resume; if about to receive file with same name
                  as one which already exists, resume receiving file data
                  from the current end of the existing file.
             OS = Overwrite Skip; if (etc.), tell sender never mind, skip
                  this file, we don't want it.  Batch transfers will move
                  on to the next file in the set, if any.

          Bnnn   Buffer size:
             XPRZModem.library adds a layer of file I/O buffering in
             addition to whatever the comm program may or may not provide.
             This option sets the size of XPRZModem's file I/O buffer in
             kilobytes.  The minimum value is 1 KB, for those using RAM
             drives or fast hard drives, or those whose comm programs
             already provide sufficient buffering.  The maximum value is
             as much contiguous RAM as you have available in your Amiga.
             If you specify more than is actually available, XPRZModem will
             keep decrementing the buffer size requested by 1 KB until the
             memory allocation works.  That way, if your RAM is too
             fragmented to use the amount you request, XPRZModem simply
             uses the largest block available.  Buffering is especially
             helpful for floppy drive users; it keeps your drive from
             continuously gronking and slowing things down all through the
             transfer.  NOTE: Versions of VLT prior to 5.034 couldn't handle
             buffer sizes >= 32 KB.  If you wanted to use larger buffers
             before and couldn't, try again now.

          Fnnn   Frame size:
             Although normally avoided, ZModem has the ability to require
             an ACK to be sent from the receiver to the sender every X-many
             data bytes.  Normally you don't want to use this feature,
             because not waiting for ACKs is part of how ZModem works so
             fast.  However, this feature can be very useful in conjunction
             with file I/O buffering on slow devices (namely those floppy
             drives).  If you set up a large I/O buffer to avoid gronking
             your floppy so often, you'll find that when the buffer finally
             *does* get around to being flushed that it can take a looonng
             time; so long, in fact, that the delay can cause timeouts and
             errors.  But if you set your ZModem to require the sender to
             wait for an ACK every buffer's-worth of data, the sender will
             politely wait for you to flush your buffer to the slow floppy
             and send it an ACK saying it's OK to continue now.  This value
             should be set to 0 to disable ACKs (normal mode), or set it to
             the actual number of data bytes allowed between ACKs.  For
             example, if you set B64 because of your floppy, you should
             also set F65536.

          Ennn   Error count:
             This allows you to set the number of sequential errors which
             will be required to convince ZModem to abort the transfer.  The
             normal value is 10, meaning that 10 errors must happen in a row
             with no valid data being transferred in order to cause an abort.
             This setting is provided for those using XPRZModem with a BBS,
             who may wish to use a relaxed setting, or those with really
             lousy phone lines who are desparate and patient enough to want
             the transfer to continue in spite of horrible performance.

          A{Y|N}   Auto-activate mode:
             AY = Auto-activate Yes; if the comm program supports the
                  ability, the library will automatically go into receive
                  mode when the start of a ZModem download is detected.
             AN = Auto-activate No; don't try to automatically start
                  downloading, make the user activate it.

          D{Y|N}   Delete after sending:
             DY = Delete Yes; delete each file after it has been
                  sucessfully sent.
             DN = Delete No; don't delete files after sending them.

          K{Y|N}   Keep partial files:
             KY = Keep Yes; keep the fragment of a file received so far
                  if file reception is aborted.  This allows you to use the
                  Overwrite Resume option above to pick up where you left
                  off on your next attempt.
             KN = Keep No; delete any partially-received file after an
                  aborted transfer.

          S{Y|N}   Send full directory path:
             SY = Send path Yes; send full filenames including directory
                  path to receiver.
             SN = Send path No; send only simple filenames, not including
                  directory path.

          R{Y|N}   Receive full directory path:
             RY = Receive path Yes; use full filename exactly as received,
                  instead of using the P option directory path.
             RN = Receive path No; ignore received directory path (if any),
                  use P option directory path instead.

          P{dir}   Path to use for received files:
             Px = Store all received files in directory "x" if option RN
                  set.  Ignored if option RY set.  "x" can be any valid
                  existing directory, with or without trailing "/"
                  (e.g. "Pdf0:", "PComm:hold", etc.).

     If setting the options via the option string method (either ENV: file
or primitive comm program), note that the setting for each option must
immediately follow the option character with no intervening characters
("TY", not "T Y" or "T=Y").  When setting multiple options at once,
separate the options from each other with commas and/or spaces; for
example, "TN,OR,F0".  You don't have to specify all options every time; the
options you specify will be merged into the current option settings,
replacing their old values.  Upper/lower case is not significant.  The
default option settings if you don't change anything are "TC, ON, B16, F0,
E10, AY, DN, KY, SN, RN, P".

     If the comm program supports the xpr_options() call added in version
2.0 of the XPR spec, you should be prompted for each option with a nice
prompt message such as "Text mode (Y,N,?,C):" and possibly be able to use
Intuition string and/or button gadgets instead of being stuck with the
semi-cryptic option string format described above.



4.  Serial port settings
------------------------

     ZModem (at least this implementation of it) requires that your serial
port be set to 8 data bits, no parity, 1 stop bit.  This allows ZModem to
send full 8-bit binary data bytes without having them munged on the way
through the modem.  If your comm program supports the xpr_setserial()
function, XPRZModem will use it to set your serial port to 8N1 before doing
a transfer, and will set your port back the way it was again after it's
done.  If your comm program doesn't support xpr_setserial(), you'll need to
make sure it's in 8N1 mode yourself.

     ZModem works well in all serial port handshaking modes; none,
XON/XOFF, or 7-wire/RTS/CTS.  Since any or all of those handshaking modes
may be appropriate at different times, with different modems or remote
systems, XPRZModem lets you set the handshaking mode and doesn't mess with
it.



5.  Receiving files
-------------------

     Once you get the ZModem options and your serial port configuration set
up properly, you're ready to actually use this thing (gasp!).  Receiving
files via ZModem is quite simple.  First, get the file sender going by
using whatever command it wants.  ZModem is a batch file transfer protocol,
meaning that it's capable of transferring several files in a single
exchange, so the remote system may allow you to specify multiple files to
be sent to you at one time.  It may also allow you to use wildcard
characters in the filename(s); this is all system dependant.

     This may be all you have to do.  If you specified option AY
(auto-activate on), and your comm program supports it, XPRZModem should
automatically activate at this point and start receiving your files.  If
you specified AN, or your comm program doesn't support auto-activation, you
should now use whatever option your comm program provides to activate file
reception; this will usually be a menu option or button gadget.  Either
way, once XPRZModem starts receiving files, it should automatically receive
all of the files you specified into the proper directory as indicated by
the R and P options.

     Make sure that you have set the ZModem options properly before
starting the transfer; especially, make sure you only use TY if you know
that all of the files being transferred in this batch are printable ASCII
text files.  If you use TY on normal binary files like .ARCs or .ZOOs,
they'll be mangled beyond use.



6.  Sending files
-----------------

     Sending files using ZModem is fairly straightforward.  First, activate
the file receiver with whatever command the remote system requires.  You
may or may not need to specify a filename or directory to the remote
system; this depends on their implementation of ZModem.  Once the remote
system is ready to receive files, activate your comm program's ZModem send
function.  Your comm program will prompt you for which file(s) to send.
Although ZModem is a batch protocol, your comm program may or may not allow
you to specify multiple file names to be sent; also, wildcards may or may
not be supported.  These decisions are up to the comm program author;
ZModem will handle multiple files and wildcards if the comm program allows
them.  Once you've specified the file name(s), the file(s) will be sent to
the remote system.  Note that the T option serves only as a suggestion to
the receiving system when sending files; the receiver makes the final
decision as to whether to take your advice or to force the files to be
received in text or binary mode.

     If errors occur while sending the file(s), you'll probably notice a
small enhancement I made to the normal ZModem error recovery procedures.
Normally, file transfer protocols have to compromise between efficient data
transmission on good, clean phone lines and quick error recovery on bad,
noisy phone lines.  If you pick a large packet size, you get high
throughput on clean lines due to low packet overhead, but you have slow
recovery times and large amounts of retransmitted data on noisy lines.  If
you've used YModem on noisy lines you've seen this problem.  But, if you
use small packets to reduce retransmitted data on noisy lines, you increase
the amount of time the protocol spends sending packet overhead, and your
throughput suffers.  The solution is to vary the block size according to
the experienced error rate during the transfer.  That way you aren't stuck
with a rigid packet length which will sometimes be the wrong size no matter
what.  I came up with this idea back when I first wrote the ZModem code for
Opus, and cleared it for future compatibility with ZModem's designer, Chuck
Forsberg, back then.  Since then the basic concept has been extensively
tested in the Opus BBS system, and has proven quite effective; it has also
been incorporated into various other ZModem implementations over time.  The
actual algorithm for deciding what size packets to use when is pretty much
up to the protocol author; XPRZModem uses a modified version of the Opus
algorithm which prevents locking the packet size at a small value when a
large one-time burst of errors occurs.



7.  Notes for comm program authors
----------------------------------

     That's pretty much everything you need to know in order to use
XPRZModem properly.  Here are some notes for the "other" XPR standard
users, namely the comm program authors:

     Certain XPR callback functions *must* be implemented by the comm
program author in order for XPRZModem to be used.  If any of these
functions are not supported by your comm program, XPRZModem will display an
error message and abort when invoked.  These required functions are:

          xpr_fopen(), xpr_fclose(), xpr_fread(), xpr_fwrite(),
          xpr_fseek(), xpr_sread(), xpr_swrite(), and xpr_update()

     The xpr_update() function provides many data fields for your comm
program to potentially display to the user.  These are the XPR_UPDATE
struct elements which XPRZModem will keep updated during transfers:

          xpru_protocol, xpru_filename, xpru_filesize, xpru_msg,
          xpru_errormsg, xpru_blocks, xpru_blocksize, xpru_bytes,
          xpru_errors, xpru_timeouts, xpru_blockcheck, xpru_expecttime,
          xpru_elapsedtime, and xpru_datarate

     As you can see, XPRZModem tries to provide as many status fields as
possible.  Although all of them are useful, the ones which are most
important to ZModem users are filename, filesize, msg and/or errormsg, and
bytes.  Please try to provide at least these fields in your status display,
plus as many of the rest as you can manage.

     Although only the XPR callback functions listed above are crucial for
XPRZModem, almost all of them are used if they are provided.  Although
XPRZModem will function without any of the other routines, its performance
or capabilities may be degraded somewhat.  Specifically, this is what you
give up if you choose not to supply any of these other XPR callback
functions:

          xpr_sflush(): Used when performing error recovery and resync
               when sending files.  If not provided, extra timeout errors
               and delayed error recovery will be likely.  The files will
               still be transferred properly, but errors will degrade
               overall throughput more than usual.

          xpr_chkabort(): Called between sending or receiving packets.
               If not provided, there's no way for your comm program user
               to abort a transfer in progress except by trying to somehow
               force it to decide to give up and abort on its own, such as
               by turning off the modem and hoping the protocol will abort
               after enough timeouts (it will, eventually...).

           xpr_gets(): Called to prompt the user interactively for options
               when your comm program invokes XProtocolSetup() with a null
               xpr_filename field (if xpr_options() isn't available
               instead).  If not provided, you'll have to prompt
               the user for the options string yourself, and pass this
               string in xpr_filename when invoking XProtocolSetup().

           xpr_setserial(): Called to obtain the current serial port
               settings, and to change the serial port to 8N1 if it's not
               already set that way.  If not provided, XPRZModem will
               assume all transfers are being done at 2400 bps, which
               won't hurt anything, and your users will have to make sure
               that the serial port is set to 8N1 themselves.

           xpr_ffirst() and xpr_fnext(): If either of these routines are
               missing, XPRZModem will lose the ability to send multiple
               files in a batch.  The xpr_filename pointer passed to
               XProtocolSend() will be assumed to point to the actual full
               filename of the single file to be sent in this batch.  If
               both of these routines are provided, XPRZModem will rely
               upon them completely to obtain the names of the files to
               send, and the xpr_filename pointer will not be used for any
               purpose by XPRZModem except to be passed to ffirst/fnext.
               This gives your comm program a way to send not just a single
               filename template's worth of files in a batch, but a list of
               different filenames.  If, for example, you set xpr_filename
               to point to the first node of a linked list of filenames
               and/or templates to be sent, rather than just having it
               point to a string, you can have your ffirst and fnext
               routines traverse this linked list in order to determine the
               next file to be sent.  Or you could have xpr_filename point
               to a buffer containing a list of filenames separated by
               commas, and your ffirst/fnext routines could return each
               filename in this list in turn.  The key here is that if you
               provide these two routines, you're in complete control over
               the series of names fed to XProtocolSend.  If you omit these
               routines, XPRZModem is stuck with single-file mode.  Once
               again, if these two routines are provided, XPRZModem will
               *always* call them; it makes no attempt to use the
               xpr_filename pointer for anything itself.  This is not
               explicitly spelled out in the XPR standard, but it seems the
               only reasonable way to handle batch protocols to me.
               Hopefully other XPR library authors will follow this
               precedent as well, so that comm program authors will be able
               to count on multiple-filename batch sessions being handled
               properly.

           xpr_finfo(): Used to determine the filesize of files being sent,
               in order to tell the receiving system how big they are.
               Also used to determine the size of a file which already
               exists when in Overwrite Resume mode; XPRZModem must be able
               to get the size of the current portion of the file in order
               to be able to tell the sender where to resume sending from.
               If this routine isn't provided, Overwrite Resume mode is
               not allowed.  This routine is also used to check if Text
               mode should be set to Y or N for each file when option TC
               is set.

           xpr_options(): If you don't supply this, users will be stuck
               with setting the library options via the semi-cryptic text
               string method (ENV: and/or xpr_gets()).  This routine and
               xpr_update() have a lot to do with the look and feel of your
               program when using XPR libraries; any skimping on these two
               routines will be painfully obvious to the user.  Conversely,
               doing a nice job on these two routines will really make your
               program shine.

           xpr_unlink(): Required by the DY and KN options, so if you don't
               supply it, those options are not allowed.



8.  The future
--------------

     I don't want or expect this to be the last or only XPR ZModem library
available.  There are a lot of less-commonly-used ZModem features which
have popped up over the past few years, and many people might like to see
some of them made available.  Such as 32-bit CRC (although I consider
CRC-16 perfectly adequate for the max 1K packets sent by ZModem), full
control-character escaping, or maybe 8th bit escaping to allow use of 7-bit
serial channels.  I didn't want to add a bunch of rarely-used bells and
whistles to this version of the library, because I want it to be able to
serve as comprehensible example code.  I just want to provide a good solid
ZModem which reliably handles the majority of people's needs.  Hopefully,
this will serve as a foundation for future enhanced versions, while
providing a safe fallback for people to come back to if that fancy new
enhanced version (with neo-maxi zoomed weebies) turns out to need some more
debugging.

     Bug reports, questions, or comments may be sent to me at:

          BIX: rahuebner
          Compu$erve: 73105,1022

     Or, if you think it's important, and you want an answer in less than a
week or two, call my BBS:

          The Wind Dragon Inn
          1-402-291-8053, 300-9600+ bps, HST/V.32/V.42
       or 1-402-291-3636, 300-2400 bps

     I check the messages on my BBS fairly often, so that's where to
get ahold of me quick.  I'll also try and stock the latest XPR standard
spec and XPR libraries there.

|---
|    Any comments about the version 2.50 to 2.52 update code may be
|    directed to me, William M. Perkins on BIX, with mail sent to
|     wmperkins.
|---


9.  The past (revision history)
-------------------------------
1.0, 29 July 89:  Original release.



1.1, 3 August 89:  Fixed zsdata() to send file data packet in one swrite()
call instead of calling zsendline for every byte, to prevent hammering the
serial.device with single-byte write requests during uploads, and to speed
up effective data transmission rates.



2.0, 28 October 89:  Converted from Manx to Lattice C 5.04.  Created
prototypes and made other tweaks as required.  Designed new library skeleton
for Lattice code, replacing the old Manx library skeleton.  Added new
options TC, A, D, K, S, R, and P.  Added support for xpr_options() callback
routine, and generally brought things up to par with XPR Spec 2.0.



2.10, 12 February 91: Fixed the following generally minor problems:

  No longer munges A6 register (this was potentially serious), and added
callback glue to ensure comm program can't munge OUR registers either.
Decided that the protective glue was much safer than the more elegant direct
invocation used in version 2.0.

  Slightly less transmission overhead (concatenates all output into single
swrites, builds output packets a bit faster).

  Considerably less receive overhead; does a lot more waiting and a lot
fewer sreads, especially at high speed.  WARNING: this change doesn't work
with VLT version 4.846 or earlier (yes, Willy; it really was broken B-)).
This change may or may not actually do you any good, depending upon how
your comm program implements the xpr_sread() function.

  Fixed problems getting synchronized with some systems on uploads.

  No longer closes files twice.

  Now uses fully-reentrant sprintf() from amiga.lib; no more nasty BSS.

  A couple of obscure error messages were > 50 bytes long, causing problems
with some comm program's transfer status windows, e.g. the infamouse VLT
"Incredible Shrinking Status Window."

  Stabilized spastic data rate by computing elapsed time more accurately.

  Fixed sprintf() error which caused invalid filelength to be sent on uploads.

  Aligned all data for optimal performance on 68030++ CPUs (now that I have
my A3000... B-)).  Doesn't really make any noticeable difference, but it
makes us 68030 owners feel better anyway.  I'm also including a version of
the library compiled for the 68020+ CPU, on the same principle.

  Now uses .DUP2 instead of .DUP.DUP, etc.

  Added config option E for number of errors which cause an abort.

  Fixed bogus IO_Torture false alarm concerning timer.device.

  Tried to fix an elusive Enforcer hit on reading location 0, but I'm not
sure I really got it, since I had trouble reproducing the problem.

|---
|    2.52 version, 6 March 1992: Recompile code for 68020 library code.
|    Non-68020 code worked fine but John Tillema was not able to test
|    the 2.51 68020 version.
|
|    2.51 version, 29 January 1992: Rxtimeout changed from 600 to 300
|    for upload timeout problem by John Tillema.
|
|    2.50 version, 15 November 1991:  Added code to support 32 bit CRC
|    (Circular Redundency Check).  CRC-32 adds a little more protection
|    to the data being sent and received than does CRC-16.  Source for
|    the CRC-32 additions came from the Unix version of RZ/SZ by Chuck
|    Forsberg.
|
|    Added code to update_rate() function in utils.c to avoid the
|    Guru # 80000005 if you decide to adjust the system clock during an
|    upload or download from Daylight Saving Time to Standard Time.  :-)
|
|    Proto additions using libinit.o and libent.o, and eliminating all
|    of the assembler code was supplied by Jim Cooper of SAS.  Jim
|    also supplied the mysprintf() code to replace sprintf().  This
|    version of XprZmodem can be compiled with the SAS version 5.10 C
|    Compiler.  I do not know how well it might compile with the Aztec
|    compiler.
|
|    THINGS TO DO
|
|    - Preserve date of file being transfered.
|
|    - Investigate possibility of saving file protection bits.
|
|    - Work out ways to increase the transfer speed.
|
|    - Additional changes as time and others may suggest.  :-)
|
|    -WMP-
|---