bugnano/fmhex

Accessing a DOS machine without floppy

Accessing a DOS machine without floppy

While researching on how to send files to a DOS machine via serial port, I stumbled upon this website:

https://jcoppens.com/soft/howto/bootstrap/index.en.php

This was interesting but I had 2 problems with that:

1. QBasic is only available from MS-DOS version 5 onwards. My Olivetti M24 has MS-DOS 3.3, so I couldn't use the QBasic program provided.
2. The QBasic program does not work if the hex file is larger than 64k.

So I rewrote the program in GW-BASIC and removed the 64k limitation.

While I was at it, I also rewrote the binary to hex converter in Python, so that it's easier to run on modern Linux systems.

While continuing the research, I stumbled upon the BOO file format from The Kermit Project:

https://www.columbia.edu/kermit/archivefiles/boo.html

which is an encoding similar to Base64, that encodes 3 bytes into 4 printable ASCII characters, and implements a rudimentary RLE compression on consecutive NUL bytes.

In particular the msbpcg.com file caught my attention. It is a DOS COM executable, but consisting only of printable ASCII characters, that decodes BOO files.

While I was at it, given that the program to encode BOO files is written in pre-standard C, I rewrote it in Python, so that it's easier to run on modern Linux systems.

Video tutorials

https://www.youtube.com/watch?v=7kecyLA1IWM

https://www.youtube.com/watch?v=sMRAU8F5wEs (Italian)

The problem

MS-DOS does not have a serial communication program included, but if the only choice we have to communicate with the DOS machine is the serial port, how do we put the serial communication program on the DOS machine?

Luckily on DOS it is still possible to transfer small text files via the serial port (by small I mean under 64k), with a baud rate of at most 9600bps.

Now we have a problem: The serial communication program is a binary file, but we can only send text files.

Solution 1: HEX files

Here's where tohex.py comes into play: it converts a binary file to its hex representation, generating a text file.

Note that if the generated text file is bigger than 64k, transferring it to the DOS machine could fail, so it's better to split it on chunks of less than 64k, ideally splitting it at the end of the line. I've included the splithex.py program that splits the hex file every 1350 lines, resulting in files of about 46k.

After that, we can send the hex file(s) to the DOS machine.

Now we have another problem: We need to reconstruct the binary file on the DOS machine, as it cannot execute the hex files directly.

Here's where fmhex.bas comes into play: it converts a hex file to binary. Being written in GW-BASIC, it is compatible with any MS-DOS version, as MS-DOS versions prior to 5 include GW-BASIC, and versions 5 and above include QBasic, which is compatible with GW-BASIC source files.

How to use

Let's say that we want to transfer the ZM.EXE file to the DOS machine.

First of all, we need to convert it to hex, so, from our Linux machine we run:

python3 tohex.py ZM.EXE zm.hex

Now the problem is that zm.hex is about 92k, so it's too big to be transferred as-is. We need to split it, so we need to run:

python3 splithex.py zm.hex 1350

This creates the files zm1.hex and zm2.hex.

Now we're ready to send the files to the DOS machine.

After connecting the DOS machine to the Linux machine with a null-modem cable, from the DOS machine we need to setup the serial port for 9600 baud, no partiy, 8 bit data, 1 stop bit:

mode com1:9600,n,8,1,p

From the Linux machine we need to use a serial communication program to transfer the file to the DOS machine. In this example we'll use minicom:

minicom -b 9600 -D /dev/ttyUSB0

After that we need to prepare the DOS machine to receive the first file:

copy com1: zm1.hex

To transfer the file, from the Linux machine press CTRL-A followed by S, then select ascii as the file transfer method, then select the file zm1.hex.

After the transfer has been completed, we need to tell the DOS machine that the file transfer has been completed, so from our Linux machine, press CTRL-Z.

Repeat the procedure for the zm2.hex and fmhex.bas files, starting from the copy command on the DOS machine.

After all files have been transferred, we need to reconstruct the zm.hex file from the 2 splitted files, so from the DOS machine run:

copy zm1.hex+zm2.hex zm.hex

And now we can reconstruct the binary file:

gwbasic fmhex.bas

Of course, if you're running MS-DOS version 5 or above, substitute gwbasic with qbasic.

Type zm.hex for the input archive and zm.exe for the output archive.

Note that it might seem that the DOS machine has frozen, but be patient. On an Olivetti M24 reconstructing the zm.exe file takes about 10 minutes.

After that, we can use the zm.exe file to transfer files at higher baud rates and reliably, using the Zmodem protocol.

Solution 2: BOO files (recommended)

Here's where toboo.py comes into play: it converts a binary file to the BOO format, which is a text file.

Note that if the generated text file is bigger than 64k, transferring it to the DOS machine could fail, so it's better to split it on chunks of less than 64k, ideally splitting it at the end of the line. I've included the splithex.py program that splits the BOO file every 800 lines, resulting in files of about 57k. Refer to Solution 1 on how to use splithex.py, and substitute 1350 with 800.

After that, we can send the BOO file(s) to the DOS machine.

Now we have another problem: We need to reconstruct the binary file on the DOS machine, as it cannot execute the BOO files directly.

Here's where msbpcg.com comes into play: it converts a BOO file to binary. Being a DOS COM executable consisting only of printable ASCII characters, it can easily be transferred like the other text files, and it can be directly executed on the DOS machine.

How to use

Let's say that we want to transfer the ZM.EXE file to the DOS machine.

First of all, we need to convert it to BOO, so, from our Linux machine we run:

python3 toboo.py ZM.EXE zm.boo

The resulting zm.boo file is about 57k, so it's OK to be transferred in a single shot.

After connecting the DOS machine to the Linux machine with a null-modem cable, from the DOS machine we need to setup the serial port for 9600 baud, no partiy, 8 bit data, 1 stop bit:

mode com1:9600,n,8,1,p

From the Linux machine we need to use a serial communication program to transfer the file to the DOS machine. In this example we'll use minicom:

minicom -b 9600 -D /dev/ttyUSB0

After that we need to prepare the DOS machine to receive the first file:

copy com1: zm.boo

To transfer the file, from the Linux machine press CTRL-A followed by S, then select ascii as the file transfer method, then select the file zm.boo.

After the transfer has been completed, we need to tell the DOS machine that the file transfer has been completed, so from our Linux machine, press CTRL-Z.

Repeat the procedure for the msbpcg.com file, starting from the copy command on the DOS machine.

After all files have been transferred, we can reconstruct the binary file:

msbpcg zm.boo

After that, we can use the zm.exe file to transfer files at higher baud rates and reliably, using the Zmodem protocol.

License

Unlicense