TweetFollow Us on Twitter

XModem Transfer
Volume Number:2
Issue Number:12
Column Tag:Forth Forum

Batch Text File Transfer by XMODEM

By Jörg Langowski, MacTutor Editorial Board, Grenoble, France

If you don't happen to have a tape backup unit for your Macintosh, but access to some larger computer installation over a terminal line (with, usually, lots of disk space), there is a slow but secure way to keep backups of your files by uploading them to that machine. There are all sorts of communication utilities that help in doing so, most of them using the XMODEM protocol for up- and downloading files.

The telecommunications programs I am aware of at this moment are excellent tools for transferring single files to and from the Macintosh. If one wants to back up one full disk by such a file transfer, however, the task becomes a little tedious since one has to select one file at a time; particularly time-consuming if your disk has 327 short text files on it. The XMODEM standard does offer a batch transfer mode in which the filename is transferred followed by the file, and if anyone out there knows of a terminal program that has this mode implemented, please let me know.

For the time being, the transfer of one whole volume on a HFS disk from the Mac to another system by XMODEM batch transfer is a neat little project to implement in Forth. Besides being useful, it'll help us gain some insight into HFS file handling.

We will limit ourselves to transferring text files here. Since you all are aware that files on the Macintosh consist of the data and the resource fork, applications and other resource-containing files have to be changed into text files using a utility like BINHEX first. However based on our example, it is rather easy to implement the MacBinary format [a special standard combining both forks together with the Finder information into one structure: see MacTutor V2#1], and transfer files of any type automatically. At the present time, a text file backup system is fine for me, since the documents I most often have to back up, like manuscripts or current versions of Mach 2 and NEON source files, are text files anyway.

The XMODEM Batch Transfer Protocol

How does an XMODEM file transfer proceed? Each file is split up into sectors of 128 bytes each (a relic of old CP/M times), each sector starting at one. A simple file transfer (not batch mode) has the following protocol:

Simple File Transfer Protocol

Sender Receiver

waits for NAK,

80 sec timeout sends NAK at

10 sec intervals

Loop n times for n sectors

send header:

SOH ($01)

sector no. (0-255)

sector no. complement (255-0)

send sector data, 128 bytes

send checksum:

(header bytes + sector data ) mod 255

compute checksum and

compare with checksum


send ACK ($06) if OK and

NAK if not

If ACK received, increment

sector no.

otherwise resend sector

End Loop

send EOT ($04)

send ACK

The transfer may be cancelled at any time by the sender or receiver transmitting a Ctrl-X ($24) to the other end.

In batch mode, there is an additional protocol defined to transmit the filename before the file transfer starts:

Batch Mode Protocol

Sender Receiver

waits for NAK,

80 sec timeout sends NAK at

10 sec intervals

sends ACK

awaits filename characters,

1 sec timeout

on error -> above NAK

Loop 11 times

send filename char

(bit 7=0, upper case)

add to checksum receive filename char

await ACK 1 sec timeout add to checksum


End Loop

send Ctrl-Z

add to checksum receive Ctrl-Z

add to checksum

send checksum

receive checksum +


ACK if OK and "u" if not await ACK

- Normal file transfer starts -








expect NAK to start new

filename NAK

ACK for new file name

or EOT if finished

The routines necessary to implement this protocol are printed in Listing 1. [For the single file transfer protocol, I adapted some routines from an article by Robert Taylor in Dr. Dobbs Journal 83 (9/1983) p.66.]

The program in Listing 1 is best used by having some terminal emulation like Mockterminal running on the Macintosh at the same time. Mach2 users may append this code directly to the "Terminal Emulator" example on the Mach2 demo disk and then start the terminal emulator before doing the transfer. For other Forths, the program should be easily adaptable, you'll have to change the routines for setting the baud rate and for doing the modem input and output.

Look at the definition of the words send and send-filename and the inner loop of send for the implementation of the XMODEM protocol in Forth.

In order to be able to transfer one whole volume automatically, we must have some means for accessing the files on a volume one by one. The example contains some file and directory handling words for this purpose. I have also left in there some words I needed in testing (like $openWD) which are not used in the actual implementation, but could be useful at times.

The Mach2 word $open opens a file on the default volume, given a name string, and returns a file ID number. In order to access files in any arbitrary folder (= HFS volume), we have to get a volume ID number first and set the default volume to this ID. The word promptvolID calls the standard file package to prompt the user to select a file in the folder that is to be transferred. Then the volume ID of this folder is returned. Calling setvol with a volume ID sets the default volume. getidxfile takes as its input a volume ID and an index n and will look for the n-th file in the volume. The filename is returned in a global parameter block, parblock, and can be used to open the file with $open after setting the correct default volume. send-batch simply scans the default volume for all files and transfers them one by one. So far, for text files only. Have fun implementing the MacBinary standard.

Reader Feedback

I got several comments on the floating point routines that we published lately. Ed Moskowitz from Winfield, IL points out correctly that the routines will not work with the new Mach2 since they use D5 and D6. This is correct, and D5 and D6 have to be added to the MOVEM.L save and restore list at the routine entry and exits to fix this bug.

Mike Morton from Cambridge, MA had some useful comments to make about improving the speed of those routines. Excerpts from his letter:

"• If you're doing lots of _Pack4 calls, you can do a GetTrapAddress and call the package directly, saving 30 to 50 microseconds ( )

• Instead of BTST #7, D0, you can do TST.B D0 and use BMI to see if the bit is on.


• when you AND #$7FFFFF to both D3 and D1 in the division routine, it's faster to move the mask to a register just once and use it twice. ( )

• When you're doing multiplications [and divisions, JL], you might optimize for cases where some of the number is zero. So your four MULUs might test whether the operands are zero and handle that case explicitly. I spent some time writing 32-bit fixed-point multiply routines a while ago, and this helped a lot because many numbers don't have any bits on the bottom of the longword. If you expect many of the f.p. numbers to not have many significant bits, this helps some.

( ) "

Thanks, Ed and Mike, for your comments.


One letter came clear from Tasmania, from Phil Barnard:

" I am suffering from the new ROM syndrome at the moment, although the faster and more spacious drives are very pleasant after the original units. Having to replace the interrupt vectors for the interrupt button was a nuisance. As you are no doubt aware the new ROM/System does not present the familiar bomb-box, but an 'empty' dialog.

It may be that the new dialog can be used to resume in the same fashion as the old one, but I don't yet know how. G <return> sends it back into any loop that you may wish to exit. Looking at the register contents by typing A0 through D7 is of limited debugging use. Can it do anything else?"

[Why, I even installed a control panel with flashing LEDs and toggle switches on the top of my Mac. Why do you want anything else? ... Well, G 40F6D8, of course, you probably know by now that that is the address of _Exittoshell and will (sometimes) send you straight back to the Finder. I don't use it for many other things. JL]

"The following routine brings back the old familiar bomb-box on interrupt."

\ installation of custom interrupt vectors; Phil Barnard
\ prints out the seven vectors from the interrupt table
: get.ints ( -- ) cr
 $ 64 -base @ . cr \ VIA
 $ 68 -base @ . cr \ SCC
 $ 6C -base @ . cr \ VIA & SCC
 $ 70 -base @ . cr \ Debugging Button
 $ 74 -base @ . cr \     "
 $ 78 -base @ . cr \     "
 $ 7C -base @ . cr \     "


\ installs RTS in the last four (debugging) vectors in the interrupt 

: put.ints ( -- )
 $ 80 $ 70 do 'c RTS +base i -base ! 4 +loop get.ints ;

Thanks, Phil. I'll deal with more NEON stuff in the next column where I'll also have some time to write about the new release of NEON, version 2.0. Here only a few headlines:

• NEON 2.0 is fully HFS and Switcher compatible; a new class pathlist has been added to specify HFS search paths. The assembler supports the new ROM calls.

• Applications install as a one piece file, no need to keep a copy of the installed NEON kernel on the disk.

• A decompiler has been added that will handle simple definitions, classes, objects and methods.

• A number of bugs, including the famous IC! have been fixed.

More details about NEON 2.0 and other NEON things later. Till then, good holidays.

Listing 1: XMODEM batch file downloader

( Implementation of Modem7 file transfer protocol, 
 © 9/86 J. Langowski  for MacTutor )

only forth also mac also i/o also assembler

60 user fileID

10 constant maxerr   0 constant nul24 constant can
4 constant eot   6 constant ack  21 constant nak
1 constant soh   26 constant ctrl-z
117 constant bdnmch

" Select file on volume to upload:" constant seldirtext

variable time-ct variable nak-ct variable cksum    
variable filename 128 vallot
variable xbuffer 128 vallot
variable replyrecord 74 vallot

header parblock 12 allot  ( 12 bytes junk )
header iocompl   4 allot  ( at PB + 12 )
header iores     8 allot  ( at PB + 16 )
header ioref#    4 allot  ( at PB + 24 )
header iomisc    4 allot  ( at PB + 28 )
100 allot ( some elbow room )

: long-timeout  1000 time-ct ! ; (  17 sec timeout )
: short-timeout 100  time-ct ! ; ( 1.7 sec timeout )

: baud-rate ( - )
 1CC0A COMM1 mode  ( 9600 BAUD Xon/Xoff )
 ABORT" Serial Driver Error" ;  (8 bit, No Par, 2Stop)
: modout comm1 output emit console output ;
: fin can modout ;  : fini fin close-file ;

: timed-read { | tstart -- char }
 call tickcount -> tstart
 comm1 input
 ?terminal if key console input exit then
 call tickcount tstart -  time-ct @ > 
 console input  -1

: ?contrlx ?terminal 
 if key 24 = 
 if fini 1 abort" Control-X abort" then

: wait-ack { | errs -- }
 0 -> errs 
 begin  errs maxerr >  nak-ct @ maxerr > or
 if fini 1 abort" Max error count exceeded." then
 timed-read case
 -1  of errs 1+ -> errs ." Timeout" cr endof
 can of 1 abort" Remote Cancel" endof
 ack of 0 nak-ct !  exit endof
 nak of 1 nak-ct +! exit endof
 ( disp-chr 0 )

: send-setup
 standard-getfile  0= 
 if fin 1 abort" Cancelled." then
 0 nak-ct !

: read-sect { sector# | sect-start -- }
 sector# 128 * -> sect-start
 128 0
 do i sect-start + virtual c@ i xbuffer + c! loop

: send-filename
 ack modout 0 cksum c!
 12 1 do filename i + 
 c@ 127 and dup 95 > if 32 - then
 dup modout dup emit
 cksum c@ + cksum c!
 loop cr
 ctrl-z modout 
 cksum c@ ctrl-z + cksum c!
: send-hdr { sector# | -- }
 soh modout
 sector# 1+ 255 and dup modout 255 xor modout
 ." Sending " sector# . cr

: send-sect ( send sector in xbuffer via XMODEM )
 0 ( sum )
 128 0 do xbuffer i + c@ dup modout + 255 and loop
 cksum c!
: send-cksum cksum c@ modout ;

: end-send
 begin cr ." Sending EOT -" eot modout wait-ack
 nak-ct @ 0= until
 ." ack'd"

: get-file-length ( -- sectors )
 fileID w@ ['] ioref# w!  0 ['] iocompl !
 ['] parblock call geteof . cr
 ['] iomisc @ 128 /mod swap 
 if 1+ then
 dup ." File has " . ."  sectors" cr 
: send ( current file via XMODEM, closes file on exit )
 get-file-length 0
 i read-sect ( into xbuffer )
 i send-hdr send-sect send-cksum
 wait-ack nak-ct @ 0=

: prelude
 short-timeout  begin timed-read -1 = until
 long-timeout  ." Awaiting initial NAK: "
 wait-ack cr   short-timeout

: send-one send-setup prelude send ;

: end-batch ack modout eot modout ;

( *** file and directory  handling routines *** )

: getVolID { index | pb -- VRefNum / errcode 0 }
 ( ID for volume no. index )
   ['] parblock -> pb
 0 ['] iocompl !
 filename pb 18 + !
 index pb 28 + w!
 pb call getvolinfo
 ?dup if 0 else pb 22 + w@ then

CODE SFGetFile  ( x y prompt numTypes typeList ReplyRecord )
 MOVE.L (A6)+,A1 ( reply record )
 MOVE.L (A6)+,A0 ( type list )
 MOVE.L (A6)+,D3 ( numTypes )
 MOVE.L (A6)+,D2 ( prompt )
 MOVE.L (A6)+,D1 ( y-coord )
 MOVE.L (A6)+,D0 ( x-coord )

 MOVE.W D0,-(A7)
 MOVE.W D1,-(A7) ( where )
 MOVE.L D2,-(A7) ( prompt )
 MOVE.L #0,-(A7) ( no filter proc )
 MOVE.W D3,-(A7) ( num Types )
 MOVE.L A0,-(A7) ( type list )
 MOVE.L #0,-(A7) ( no dialog hook )
 MOVE.L A1,-(A7) ( reply record )

 MOVE.W #2,-(A7)

: promptvolID  ( -- VRefNum flag )
 100 50 seldirtext -1 0 replyrecord 
 replyrecord 6 + w@ l_ext
 replyrecord w@ l_ext

 MOVE.L (A6)+,A1
 MOVE.L (A6)+,A0
 MOVE.L (A6)+,D3
 MOVE.L (A6)+,D2
 MOVE.L (A6)+,D1
 MOVE.L (A6)+,D0

 MOVE.W D0,-(A7)
 MOVE.W D1,-(A7)
 MOVE.L D2,-(A7)
 MOVE.L D3,-(A7)
 MOVE.L A0,-(A7)
 MOVE.L A1,-(A7)

 MOVE.W #1,-(A7)

: $openWD { name | pb -- VRefNum / errcode 0 }
   ['] parblock -> pb
 0 ['] iocompl !
 name pb 18 + !
 0 pb 28 + w!
 pb 1 call HFSDispatch
 ?dup if 0 else pb 22 + w@ then

: closeWD { volume | pb -- rescode }
   ['] parblock -> pb
 0 ['] iocompl !
 volume pb 22 + w!
 pb 2 call HFSDispatch

: setVol { volume | pb -- res code }
   ['] parblock -> pb
 0 ['] iocompl !
 0 pb 18 + !
 volume pb 22 + w!
 pb call setVol
: getidxfile { volume index | pb -- res code }
   ['] parblock -> pb
 0 ['] iocompl !
 filename pb 18 + !( ioNamePtr )
 volume pb 22 + w! ( ioVRefNum ) 
 0 pb 26 + w!  ( ioFVersNum )
 index pb 28 + w!( ioFDirIndex )
 pb call getfileinfo 
 dup 0= if ." Found: " filename count type cr then

: openidxfile { volume index | -- res code }
 volume index getidxfile
 ?dup 0= if filename $open then

: find-files { | volume index -- }
 if -> volume 1 -> index
 volume index getidxfile
 index 1+ -> index
 else ." cancelled" . cr 
( *** main definitions for batch file transfer *** )
: display-filename
 12 1 do filename i + c@ 127 and dup 95 > if 32 - then emit
 loop cr
: send-batch{ volume | index }
 1 -> index  0 nak-ct ! 
 begin cr
 filename 1+ 11 32 fill
 volume index getidxfile 0= 
 volume setvol drop 
 filename $open dup 0> 
 if   fileID w!
 prelude send-filename
 timed-read cksum c@ <> 
 ." Checksum Error in filename, restarting" cr
 bdnmch modout
 ack modout
 send ( closes file )
 else drop 
 index 1+ -> index
 wait-ack end-batch
: upload 
 if send-batch 
 else cr ." Cancelled" cr

Community Search:
MacTech Search:

Software Updates via MacUpdate

Microsoft Office 2016 16.11 - Popular pr...
Microsoft Office 2016 - Unmistakably Office, designed for Mac. The new versions of Word, Excel, PowerPoint, Outlook, and OneNote provide the best of both worlds for Mac users - the familiar Office... Read more
Adobe Photoshop CC 2018 19.1.2 - Profess...
Photoshop CC 2018 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Photoshop customer). Adobe Photoshop CC 2018, the industry standard... Read more
Adobe Dreamweaver CC 2018 -...
Dreamweaver CC 2018 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Dreamweaver customer). Adobe Dreamweaver CC 2018 allows you to... Read more
Adobe Flash Player - Plug-in...
Adobe Flash Player is a cross-platform, browser-based application runtime that provides uncompromised viewing of expressive applications, content, and videos across browsers and operating systems.... Read more
Drive Genius 5.2.0 - $79.00
Drive Genius features a comprehensive Malware Scan. Automate your malware protection. Protect your investment from any threat. The Malware Scan is part of the automated DrivePulse utility. DrivePulse... Read more
MegaSeg 6.0.6 - Professional DJ and radi...
MegaSeg is a complete solution for pro audio/video DJ mixing, radio automation, and music scheduling with rock-solid performance and an easy-to-use design. Mix with visual waveforms and Magic... Read more
ffWorks 1.0.7 - Convert multimedia files...
ffWorks (was iFFmpeg), focused on simplicity, brings a fresh approach to the use of FFmpeg, allowing you to create ultra-high-quality movies without the need to write a single line of code on the... Read more
Dash 4.1.5 - Instant search and offline...
Dash is an API documentation browser and code snippet manager. Dash helps you store snippets of code, as well as instantly search and browse documentation for almost any API you might use (for a full... Read more
Evernote 7.0.3 - Create searchable notes...
Evernote allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at anytime, from... Read more
jAlbum Pro 15.3 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. You can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly... Read more

Latest Forum Discussions

See All

All the best games on sale for iPhone an...
It might not have been the greatest week for new releases on the App Store, but don't let that get you down, because there are some truly incredible games on sale for iPhone and iPad right now. Seriously, you could buy anything on this list and I... | Read more »
Everything You Need to Know About The Fo...
In just over a week, Epic Games has made a flurry of announcements. First, they revealed that Fortnite—their ultra-popular PUBG competitor—is coming to mobile. This was followed by brief sign-up period for interested beta testers before sending out... | Read more »
The best games that came out for iPhone...
It's not been the best week for games on the App Store. There are a few decent ones here and there, but nothing that's really going to make you throw down what you're doing and run to the nearest WiFi hotspot in order to download it. That's not to... | Read more »
Death Coming (Games)
Death Coming Device: iOS Universal Category: Games Price: $1.99, Version: (iTunes) Description: --- Background Story ---You Died. Pure and simple, but death was not the end. You have become an agent of Death: a... | Read more »
Hints, tips, and tricks for Empires and...
Empires and Puzzles is a slick match-stuff RPG that mixes in a bunch of city-building aspects to keep things fresh. And it's currently the Game of the Day over on the App Store. So, if you're picking it up for the first time today, we thought it'd... | Read more »
What You Need to Know About Sam Barlow’s...
Sam Barlow’s follow up to Her Story is #WarGames, an interactive video series that reimagines the 1983 film WarGames in a more present day context. It’s not exactly a game, but it’s definitely still interesting. Here are the top things you should... | Read more »
Pixel Plex Guide - How to Build Better T...
Pixel Plex is the latest city builder that has come to the App Store, and it takes a pretty different tact than the ones that came before it. Instead of being in charge of your own city by yourself, you have to work together with other players to... | Read more »
Fortnite Will Be Better Than PUBG on Mob...
Before last week, if you asked me which game I prefer between Fortnite Battle Royale and PlayerUnknown’s Battlegrounds (PUBG), I’d choose the latter just about 100% of the time. Now that we know that both games are primed to hit our mobile screens... | Read more »
Siege of Dragonspear (Games)
Siege of Dragonspear 2.5.12 Device: iOS Universal Category: Games Price: $9.99, Version: 2.5.12 (iTunes) Description: Experience the Siege of Dragonspear, an epic Baldur’s Gate tale, filled with with intrigue, magic, and monsters.... | Read more »
7 Wonders Guide - Should You Buy The Lea...
The fantastic mobile version of 7 Wonders just got updated with an expansion that adds Leaders to the game. This new content adds a whole layer of depth to the game, but before you spend $1.99 to buy it blindly, check out this breakdown of exactly... | Read more »

Price Scanner via

B&H drops prices on 15″ MacBook Pros up t...
B&H Photo has dropped prices on new 2017 15″ MacBook Pros, now up to $300 off MSRP and matching Adorama’s price drop yesterday. Shipping is free, and B&H charges sales tax for NY & NJ... Read more
Apple restocks Certified Refurbished 2017 13″...
Apple has restocked Certified Refurbished 2017 13″ 2.3GHz MacBook Pros for $200-$230 off MSRP. A standard Apple one-year warranty is included with each MacBook, models receive new outer cases, and... Read more
13″ Space Gray Touch Bar MacBook Pros on sale...
Adorama has new 2017 13″ Space Gray Touch Bar MacBook Pros on sale for $150 off MSRP. Shipping is free, and Adorama charges sales tax in NY & NJ only: – 13″ 3.1GHz/256GB Space Gray MacBook Pro (... Read more
Best deal of the year on 15″ Apple MacBook Pr...
Adorama has New 2017 15″ MacBook Pros on sale for up to $300 off MSRP. Shipping is free, and Adorama charges sales tax in NJ and NY only: – 15″ 2.8GHz Touch Bar MacBook Pro Space Gray (MPTR2LL/A): $... Read more
Save $100-$150+ on 13″ Touch Bar MacBook Pros...
B&H Photo has 13″ Touch Bar MacBook Pros on sale for $100-$150 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 13″ 3.1GHz/256GB Space Gray MacBook Pro... Read more
Current deals on 27″ Apple iMacs, models up t...
B&H Photo has 27″ iMacs on sale for up to $150 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 27″ 3.8GHz iMac (MNED2LL/A): $2149 $150 off MSRP – 27″ 3... Read more
Thursday Deal: 13″ 2.3GHz MacBook Pro for $11...
B&H Photo has the 13″ 2.3GHz/128GB Space Gray MacBook Pro on sale for $100 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 13-inch 2.3GHz/128GB Space... Read more
How to save $100-$190 on 10″ & 12″ iPad P...
Apple is now offering Certified Refurbished 2017 10″ and 12″ iPad Pros for $100-$190 off MSRP, depending on the model. An Apple one-year warranty is included with each model, and shipping is free: –... Read more
Silver 12″ 1.3GHz MacBook on sale at B&H...
B&H Photo has the 2017 12″ 1.3GHz Silver MacBook on sale for $1399.99 including free shipping plus sales tax for NY & NJ residents only. Their price is $200 off MSRP, and it’s the lowest... Read more
Amazon offers 21″ Apple iMacs for up to $150...
Amazon 21″ iMacs on sale today for $50-$150 off MSRP, depending on the model. Shipping is free: – 21″ 3.4GHz 4K iMac (MNE02LL/A): $1349.99 $150 off MSRP – 21″ 3.0GHz iMac (MNDY2LL/A): $1199 $100 off... Read more

Jobs Board

Data Scientist, *Apple* Ecosystem (AMP Anal...
# Data Scientist, Apple Ecosystem (AMP Analytics) Job Number: 113428728 Santa Clara Valley, California, United States Posted: 24-Jan-2018 Weekly Hours: 40.00 **Job Read more
*Apple* Solutions Consultant - Apple (United...
# Apple Solutions Consultant Job Number: 113523441 Orange, CA, California, United States Posted: 21-Feb-2018 Weekly Hours: 40.00 **Job Summary** Are you passionate Read more
*Apple* Professional Learning Specialist - A...
# Apple Professional Learning Specialist Job Number: 113456892 Englewood, NJ, New Jersey, United States Posted: 02-Feb-2018 Weekly Hours: 40.00 **Job Summary** The Read more
*Apple* Retail Online, Senior Financial Anal...
# Apple Retail Online, Senior Financial Analyst Job Number: 57228835 Santa Clara Valley, California, United States Posted: 12-Feb-2018 Weekly Hours: 40.00 **Job Read more
Site Reliability Engineer, *Apple* Pay - Ap...
# Site Reliability Engineer, Apple Pay Job Number: 113356036 Santa Clara Valley, California, United States Posted: 12-Jan-2018 Weekly Hours: 40.00 **Job Summary** Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.