Google Groups no longer supports new Usenet posts or subscriptions. Historical content remains viewable.
Dismiss

executable ascii

184 views
Skip to first unread message

Uno

unread,
Nov 26, 2003, 4:20:52 PM11/26/03
to
I'd like to manually write ececutable ascii text
(without encoding - decoding routines)
but I don't know how to translate instructions like int 21h for example.
I've read this page: http://lf.8k.com/BAT/FVAR.HTM
but I can't understand how it works.
Thanks a lot


bitRAKE

unread,
Nov 26, 2003, 11:41:03 PM11/26/03
to
Uno, in regard to the article: AX is loaded with the end address of
the destination code buffer which is loaded into SP; then code bytes
are constructed in reverse in AX and pushed on the stack (filling the
buffer); finally the code buffer is jumped to. Operations and
immediate values are chosen to insure the code bytes are printable
characters. This same method can be used for any program.

lallous

unread,
Nov 26, 2003, 11:44:16 PM11/26/03
to
Hello,

AFAIK, You will need some sort of encoder/decoder that will convert a given
instruction opcode into a printable character. The hard part is to write the
decoder (or loader) in printable characters then append the rest of the
encoded program into it, something like:
XY?/:01AALAS <- say this is the decoder that will decode the next chunks:
L7KAXKXAO01!?~0_0
.
.
.
<- this was your original program encoded.
After decoding you jump to original entry point.

Time ago I saw a program name 'com2txt' or 'comtotxt' that does that.

Regards,
Elias
"Uno" <nos...@no.spam> wrote in message
news:UO8xb.105905$hV.38...@news2.tin.it...

Matt Taylor

unread,
Nov 27, 2003, 1:04:15 AM11/27/03
to
"Uno" <nos...@no.spam> wrote in message
news:UO8xb.105905$hV.38...@news2.tin.it...

Refer to the Intel Architecture Manuals Volume 2 available on Intel's
website. It details how instructions are encoded, and Appendix A lists all
of the opcodes. Volume 3 of the x86-64 manuals also talks about encodings,
and the x86-64 encodings are quite similar to existing ones. The website
www.sandpile.org is also a good generally reference for x86 opcodes.

-Matt


Terje Mathisen

unread,
Nov 27, 2003, 4:27:48 AM11/27/03
to
Matt Taylor wrote:

Funny you should ask! :-)

I happen to have written the ultimate executable ascii program:

The bootstrap loader gets away with the least amount of selfmodification
possible: A single two-byte instruction (a backwards branch).

It uses much less than the regular ascii set, in fact it stays within
the 70+ characters defined in the MIME standard as being capable of
surviving all mail gateways. (This BTW is really hard, because it makes
stuff like POP BX, SI, DI or BP impossible.)

It is more or less self-relocating, in that it will survive most forms
of reformatting:

You can replace the CRLF line terminators with a single CR (Mac) or LF
(Unix), or remove them completely (Word: Each paragraph becomes a single
line).

After the first two 64-character lines, which contains the level 1
bootstrap, the rest can be reformatted any way you like, including
inserting/removing any amount of whitespace.

The first-level bootstrap code (after self-modification) contains a
two-to-one character decoder, capable of generating any binary value
from a pair of characters in the MIME Base64 character set. It does this
by taking the first character and then subtracting the second char
_twice_, which effectively affords the needed shift.

The second-level bootstrap code generated by this process contains what
is probably the world's shortest MIME Base64 decoder (i.e. a 4-to-3
binary decoder).

Finally, the resulting binary is either written to disk, or relocated
and executed in-place.

I'll include just the first level bootstrap, if I do anything more than
that I'll probably trigger some 'no binary posts' filter somewhere:

ZRYPQIQDYLRQRQRRAQX,2,NPPa,R0Gc,.0Gd,PPu.F2,QX=0+r+E=0=tG0-Ju E=
EE(-(-GNEEEEEEEEEEEEEEEF 5BBEEYQEEEE=DU.COM=======(c)TMathisen95


Terje

PS. I wrote this program over a ski weekend in the montains, with no PC
access: I had brought along a list of possible opcodes with MIME ascii
encoding.
--
- <Terje.M...@hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"


Uno

unread,
Nov 27, 2003, 5:08:28 PM11/27/03
to

"bitRAKE" <bit...@home.com> wrote in message news:3e9d5e9c.03112...@posting.google.com...

Could you explain me better? I'm interested in this topic too,
Thanks!


bitRAKE

unread,
Nov 28, 2003, 2:36:15 PM11/28/03
to
Terje, very talented solution!

Rickey Bowers Jr.

Terje Mathisen

unread,
Nov 28, 2003, 4:32:58 PM11/28/03
to
bitRAKE wrote:

> Terje, very talented solution!
>

Glad you liked it. :-)

I do feel that this is probably the best single hack I've ever written,
it was extremely satisfying to work out how to do it after first
deciding that it was impossible.

A few days later I suddenly discovered that even though all the POP reg
opcodes for memory-adressing registers were out of bounds, POPA is OK!

This does mean that the mime version of the startup code won't run on
the original 1981 PC, since the POPA opcode was introduced a year later,
with the 80186 cpu.

BTW, did you notice the way I'm using INC BP (ascii 'E') as a NOP?

One important element is the need to align all jump targets properly, so
as to fit within the MIME set. I also use a pair of E's as a buffer in
case the first CRLF pair is modified.

That CRLF pair is itself hidden within the immediate part of an

CMP AX,0A0Dh

opcode, if the CRLF pair is removed or shortened, the following pair of
E's will be included instead.

Terje

0 new messages