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1990-06-19
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MSBAAA.HLP MS-DOS KERMIT 3.0 "BOOTSTRAPPING" June 1990
MS-DOS Kermit 3.0 is distributed on diskette in binary executable form on a
5.25-inch IBM DOS-format diskette in the book "Using MS-DOS Kermit", by
Christine M. Gianone, published by Digital Press, Bedford, MA, 1990, order
number EY-C204E-DP (phone 1-800-343-8321). It is also available from Kermit
Distribution at Columbia University and wherever computer books are sold.
The MSB*.* files are for people who cannot get MS-DOS Kermit on diskette.
MS-DOS Kermit (and many other Kermit programs) are often distributed using a
special encoding called "boo" (short for "bootstrap") format, developed
especially for distribution of MS-DOS Kermit over networks, e-mail, and
communication lines. MS-DOS Kermit has grown to have so many features that
the binary program image (the .EXE file) has become quite large. But binary
files are generally not compatible with the common labeled tape formats (e.g.
ANSI D), electronic mail, or raw downloading -- methods commonly used for
Kermit software distribution.
A common practice is to encode .EXE and other binary files into printable
characters, such as hexadecimal digits, for transportability. A simple "hex"
encoding results in two characters per 8-bit binary byte, plus CRLFs added
every 80 (or less) hex characters to allow the file to pass through
card-oriented networks like BITNET. A hex file is therefore more than twice
as large as the original binary file.
A .BOO file is a more compact, but somewhat more complicated, encoding. Every
three binary bytes (24 bits) are split up into four 6-bit bytes with 48 (ASCII
character "0") added to each, resulting in four ASCII characters ranging from
"0" (ASCII 48) to "o" (ASCII 111), with CRLFs added at or near "column 76".
The resulting file size would therefore be about 4/3 the .EXE file size. This
is still quite large, so .BOO files also compress consecutive null (zero)
bytes. Up to 78 consecutive nulls are compressed into two characters. Tilde
("~") is the null-compression lead-in, and the following character indicates
how many nulls are represented (subtract 48 from this character's ASCII
value). For instance "~A" means 17 consecutive nulls; "~~" means 78 of them.
Repeated nulls are very common in .EXE files.
4-for-3 encoding combined with null compression reduces the size of the
encoded file to approximately the same size as the original .EXE file, and
sometimes even smaller. The first line of a .BOO file is the name (in plain
text) of the original file. Here's what the first few lines of a typical .BOO
file look like:
MSVIBM.EXE
CEYP0Id05@0P~3oomo2Y01FWeP8@007P000040HB4001`W~28bL005\W~2JBP00722V0ZHPYP:
\8:H2]R2V0[`PYP:68>H2S23V0YHPiP:Xg800;Qd~2UWD006Yg~2Ogl009]o~2L8000;20~~~~
~~~~~~~:R2H008TV?P761T410<H6@P40j4l6RRH0083l17@PP?`1M@?YSP20o0Ee0nUD0h3l
1WD3jO@3]0VjW03=8L?X4`N0o01h1\H6~20l>0i7n0o1]e7[@2\PO=8LH60@00Raj>04^97Xh0
PROGRAMS FOR HANDLING .BOO FILES
Kermit Distribution includes several useful .BOO-file programs:
MSBPCT.BAS
This Microsoft BASIC program can be used on any PC or PS/2 that has BASIC
to decode a .BOO file into an .EXE file. It's about 50 lines, so it can
be typed in.
MSBPCT.BOO
BASIC programs run rather slowly, so .BOO-file decoders have also been
written in high-level languages like C. The MSBPCT.EXE file that was
produced by compiling MSBPCT.C is encoded into MSBPCT.BOO, which can be
decoded back into MSBPCT.EXE using MSBPCT.BAS. Once you've done that, you
don't need to run the slow BASIC version any more, which is a blessing,
because the MS-DOS Kermit .BOO file takes up to half an hour to decode
using the BASIC version (depending on the system), but only seconds using
MSBPCT.EXE.
MSBPCT.*
There are .BOO-file decoders written in other languages too, like
assembler, Turbo Pascal, Fortran, etc. Take your pick. They all do the
same thing.
MSBMKB.*
This is the program for encoding an .EXE file into a .BOO file. It is
written in C, compiled, and translated (by itself) into .BOO format,
suitable for decoding back into .EXE form by any of the MSBPCT programs.
Also in other languages, including Fortran and Turbo Pascal.
MSBPCG.*
MSBPCG.ASM is an assembly language program that compiles into a very
short executable program that consists entirely of printable ASCII
characters (MSBPCG.COM). This program can be used, like the MSBPCT
programs, to decode BOO files.
MSBRCV.BAS
This is a receive-only Kermit program in Microsoft Basic for the IBM PC
and PS/2, short enough to type in, usable for downloading a real Kermit
from a host that has Kermit to a PC that doesn't. See "Kermit, A File
Transfer Protocol" by Frank da Cruz for a detailed explanation.
MSBHEX.* are C programs for producing and decoding straight hex files.