https://github.com/jibsen/com2text

MS-DOS COM to Text Converters
https://github.com/jibsen/com2text

Last synced: 6 months ago
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MS-DOS COM to Text Converters

• Host: GitHub
• URL: https://github.com/jibsen/com2text
• Owner: jibsen
• License: apache-2.0
• Created: 2015-11-14T15:28:51.000Z (almost 10 years ago)
• Default Branch: master
• Last Pushed: 2015-11-24T14:25:30.000Z (almost 10 years ago)
• Last Synced: 2023-08-03T12:21:02.468Z (about 2 years ago)
• Language: Assembly
• Homepage:
• Size: 15.6 KB
• Stars: 2
• Watchers: 2
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md
  • License: LICENSE

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README

          

MS-DOS COM to Text Converters

=============================

Copyright 2000-2015 Joergen Ibsen

 

About

-----

The MS-DOS 16-bit [COM file](https://en.wikipedia.org/wiki/COM_file) format

is a very simple executable format containing no headers at all.

`c2t1` and `c2t2` convert such an executable file into a plain text file that

remains executable. This is done by encoding the bytes of the original file

in a process similar to Base64, and adding a decoder that is carefully craftet

to only contain x86 opcodes that happen to be text characters.

There were a number of such tools around, and I wrote this one back in 2000,

just to see how it was done.

Recently someone requested a decoder, so I dug out the source and made one,

and figured I might as well put it on GitHub.

Encoding

--------

`c2t1` transforms each byte into two characters by encoding 4 bit values in

`[0..15]` as ASCII characters between `k` (0x6B) and `z` (0x7A).

`c2t2` encodes 6 bit values into two distinct character ranges. Values in

`[0..5]` are encoded as ASCII characters between `(` (0x28) and `-` (0x2D),

and values in `[6..63]` as ASCII characters between `A` (0x41) and `z` (0x7A).

The bottom two bits of three consecutive bytes are stored as a separate

encoded 6 bit tag value.

`handler1.asm` and `handler2.asm` contain commented source of the handler code

which performs the decoding.

How to Build

------------

You can assemble the encoder sources using [NASM](http://www.nasm.us/):

    nasm c2t1.asm -fbin -o c2t1.com

    nasm c2t2.asm -fbin -o c2t2.com

Note that recent versions of Windows no longer support running COM files, so

you may need to use a virtual machine to actually use them.