🚀 go-pugleaf

John Wright, the original author, posted this article to the net over a
year ago but with the recient interest on the topic I thought a repost
would be useful. I have used these patches, particularly the environment 
size, over the years and not encountered any problems but your mileage
may vary. Enjoy!

Note: I lost my online copy due to multiple disk problems so had to
      reenter it by scanner and correct typos.
      Although I have checked the code for typos don't assume I caught them all.
      For most users deleting everything except the 1st patch is probably best
      but proceed with caution in any case and make sure you have backups and a
      bootable floppy before trying any of this. At the end I have included info
      on the offsets of the various patches for several versions of DOS we had.

If John Wright, the original author, is still around maybe he could make this
or a more current vesion avaliable for FTP from one of the common sites for
such things.

From ariel!nmvax!tut.cis.ohio-state.edulclca!ctrsol!ginosko!uunet!cs.dal.ca~lane ~c
Artlcle 36807 of comp.sys.ibm.pc:
Path: arlel!unmvax!t~t.cis~ohio-state~edu!cicalctrsol!linosko!uunet?cs.dal~ca~lane
>From: lan~cs-d~l-ca (John Wright/Dr. Pat Lane)
Newsgroups: comp.sys.ibm.pc
Subject: Patches to COMMAND.COM for DOS3.3x (LONG)
Summary: My old set of patches updated for several vers of DOS 3.3
Keywords: DOS COMMAND.COM patch environment size echo
Message-ID: 8~Au~l4~o44~r~ 647~ics~da
Date; 14 Aug 89 04:41:11 GMT
Organization: Math, Stats & CS, Dalhousie University, Halifax, NS, Canada
Lines: 325

Some time ago I posted a set of Patches for COMMAND.COM to do some nice
things like increace the default environment size and make ECHO OFF the
default for all batch files. All these patches had been Posted on Usenet
before but usually for a single version of DOS. I presented the patches
for several versions 3.10 to 3.30, from IBM, Microsoft and  Compaq.
In this posting I present the patches for versions: IBM PCDOS 3.30,
Microsoft MSDOS 3.30 and Compaq MSDOS 3.31. In a companion posting to
this one, I present patches for IBM PCDOS 4.00, Microsoft MSDOS 4.00
and Microsoft MSDOS 4.01.

Of course many of these Patches are no longer as useful since DOS now
provides other means of accomplishing the same things. But some, myself
included, may still find these patches to be preferable.

Since DOS 3.10, you can set the size of the environment space by using
the /E parameter of COMMAND.COM (least we forget, it's /E:xxxx where)
XXXX is in 16 byte paragraphs for DOS 3.10 or bytes for 3.20 and up).
This as usually done by using SHELL= in the CONFIG.SYS file to specify
the string DOS uses to call COMMAND.COM for the initial invocation . The
drawback to this is that it only applies to the first, bottom level
shell. If you run COMMAND again (without a /E) it will just use the
default size of 160 byte, (unless the environment is already bigger
than 160 bytes in which case it will use the environment size rounded
up to the next 16 bytes).  If you go into another program and "drop
down" into DOS or if you use a shell program to run DOS commands you,
you are actually re-running COMMAND with the default environment size
and it won't be long before you get that "Out of environment space"
message. It's amazing that DOS has gone so far without anybody doing
something about it (like making the default bigger or providing some
mechanism whereby the size specified in CONFIG.SYS would carry though
to all subsequent invocations of COMMAND).

Since DOS 3.30, you can prevent batch file lines from being displayed
by putting a '@' before them. An '@echo off' as the first line will
prevent all lines in the file (including the 'echo off') from being
echoed. This solution, while upwardly compatible, is Just plain ugly.
Also lt causes problems if you take your batch files to a system running
a lower version of DOS.

To produce a blank line, you can use 'echo' followed by an unprintable
character (such as DEL) which makes creatiny or editing the file more
difficult, but seems to work in every version of DOS. Since some version
of DOS (not sure which), 'echo.' works, though I've never seen it
documented. I include the 'echo-space' Patch mainly for upward
compatibility.


Users please note that these patches have been tested rather perfunctorily
and should be used with some caution.  Definitely keep an unpatched copy
of COMMAND.COM on a bootable disk.  If you mess up your regular copy of
COMMAND.COM, you won't be able boot with it; you'll have to boot from
the floppy and copy the original COMMAND.COM over the messed up one before
you can reboot normally.

Following the presentation of the patches for the various DOS versions,
I list a couple of batch files I use for effecting the patches. Following
that I list the dis-assembled code around the patched areas so that those
with DOS versions other than those shown will be able to find the proper
offsets.

Remember to add 100h to offsets when patching with DEBUG.


OFFSETS for patch to COMMAND.COM for IBM PCDOS 3.30 or COMPAQ PCDOS 3.31

Default environment size of 512 bytes
     [0DB8] = 0A 00 -> 20 00
(any size environment from 160 to 32768 bytes (000A to 0800 hex) can be
 made the default by coding the ammount, divided by 16, in hexadecimal
 digits, at the address shown with the low order byte first)
ECho OFF for AUTOEXEC.BAT
     [1204] = 03 -> 02
Echo off for batch files except AUTOEXEC.BAT
     [1C68] = 01 -> 00
ECHO<space> produces blank line
     [3E29] =  E8 20 00 74 -> 83 F9 01 72


COMMAND.COMs in IBM 3.30 and COMPAQ 3.31 appear to be identical except
for version no., identification and copyright strings.


OFFSETS for patch to COMMAND.COM for Microsoft MSDOS 3.30

Default environment size of 512 bytes
     [0DB8] = 0A 00 -> 20 00
ECHO OFF for AUTOEXEC.BAT
     [1205] = 03 -> 02
Echo off for batch files except AUTOEXEC.BAT
     [1C68] = 01 -> 00
ECHO<space> produces blank line
     [3E29] = E8 20 00 74 -> 83 F9 01 72


Microsoft MSDOS 3.30 COMMAND.COM has some minor differences with IBM
PCDOS 3.30 (besides identification and copyright strings) and there is
one offset which is changed ([1205] instead of [1204]).

Following are two batch files PATCH.BAT and UNPATCH.BAT for installing
and un-installing the patches for IBM PC-DOS 3.30. It should be easy
enough to modify this for other DOS 3 verslons.

--------------------- PATCH.BAT for IBM PCDOS 3.30 ----------------------

echo off
echo INSTALL COMMAND.COM PATCHS FOR IBM PC-DOS 4.00
rem usage PATCH [filespec]
rem Patch 1. Default environment size is 512 bytes
rem Patch 2. ECHO OFF default for AUTOEXEC.BAT
rem Patch 3. ECHO OFF default for batch files except AUTOEXEC.BAT
rem Patch 4. ECHO-space produces blank line
if "%1"=="" goto :default
set $s=%1
echo About to modify %$s%
goto pause
:default
set $s=COMMAND.COM
echo About to Modify %$s% in the dcfault drive and directory
pause
:pause
if not exist %$s% goto error
echo creating PATCH.TMP...
echo e0EB8 >patch.tmp
echo 20 00 >>patch.tmp
rem end of patch 1
echo e1304 >>patch.tmp
echo 02 >>patch.tmp
rem end of patch 2
echo e1D68 >>patch.tmp
echo 00 >>patch.tmp
rem end of patch 3
echo e3F29 >>patch.tmp
echo 83 F9 01 72 >>patch.tmp
rem end of patch 4
echo w >>patch.tmp
echo q >>patch.tmp
debug %$s% <patch.tmp
echo Deleting PATCH.TMP
del patch.tmp
echo Modification complete. Reboot system . . .
pause
goto exit
:error
echo %$s% not found
:exit
-------------- END OF PATCH.BAT------------------
-------------- UNPATCH.BAT for IBM PCDOS 3.30 ----------------------
echo off
echo UNINSTALL COMMAND.COM PATCHES FOR IBM PC-DOS 3.30
rem usage PATCH [filespec]
rem Patch 1. Default environment size is 160 bytes
rem Patch 2. ECHO ON default for AUTOEXEC.BAT
rem Patch 3. ECHO ON default for batch files except AUTOEXEC.BAT
rem Patch 4. ECHO-space produces "ECHO is on|off"
if "%1"=="" goto :default
set $s=%1
echo About to modify %$s%
goto pause
:default
set $s=COMMAND.COM
echo About to Modify %$s% in the dcfault drive and directory
:pause
pause
if not exist %$s% goto error
echo creating PATCH.TMP...
echo e0EB8 >patch.tmp
echo 0A 00 >>patch.tmp
rem end of patch 1
echo e1304 >>patch.tmp
echo 03 >>patch.tmp
rem end of patch 2
echo e1D68 >>patch.tmp
echo 01 >>patch.tmp
rem end of patch 3
echo e3F29 >>patch.tmp
echo E8 20 00 74 >>patch.tmp
rem end of patch 4
echo w >>patch.tmp
echo q >>patch.tmp
debug %$s% <patch.tmp
echo Deleting PATCH.TMP
del patch.tmp
echo Modification complete. Reboot system . . .
pause
goto exit
:error
echo %$s% not found
:exit
-------------- END OF PATCH.BAT------------------


Below, I present sections of code from IBM PCDOS 3.30, dis-assembled with
DEBUG so that those with versions of DOS other than the ones listed above
can find the offsets for their COMMAND.COM.

As you probably know, various OEM's (IBM, Compaq, AT&T, Tandy, Olivetti,
Toshlba, etc.) all license MS-DOS from Microsoft. (I've heard something
to the effect that IBM did the development work on DOS 4.0 and sold it
back to Microsoft, but I'm not sure about that). Anyway, the OEMs are
free to make their own modifications, supposedly to tailor the system to
their hardware so, there are a multitude of MS/PC-DOS versions, all slightly
different at the code level. Since they get the source, modify it, and re-
compile, the smallest change can result in a completely different binary.

ln every DOS 3 version I've seen so far, the sections of code to be patched
are unchanged, they are simply shifted in Position so it's just a question
of locating the correct offset, by searching for strings of bytes from the
sections of code listed below. The main trick here is to avoid addresses
in the code which are likely to be different (most absoluto addresses will
be different while relative addresses may or may not be different). Assuming
you're harldy with DEBUG or a similar debugger and know some 8088 assembly,
you should be able to pull it off. If DEBUG's search command is too limited,
you might try Unassembling the whole COMMAND.COM to a text file and using
an editor or lister utility to search.

Here are the sections of code from IBM PCDOS 3.30:

First, a minor formality,

-d 1490 l9f
2320:1490  28 43 29 43 6F 70 79 72-69 67 68 74 20 49 6E 74   (C)Copyright Int   
2320:14A0  65 72 6E 61 74 69 6F 6E-61 6C 20 42 75 73 69 6E   ernational Busin   
2320:14B0  65 73 73 20 4D 61 63 68-69 6E 65 73 20 43 6F 72   ess Machines Cor   
2320:14C0  70 20 31 39 38 31 2C 20-31 39 38 37 0D 0A 20 20   p 1981, 1987..     
2320:14D0  20 20 20 20 20 20 20 20-20 20 20 28 43 29 43 6F              (C)Co   
2320:14E0  70 79 72 69 67 68 74 20-4D 69 63 72 6F 73 6F 66   pyright Microsof   
2320:14F0  74 20 43 6F 72 70 20 31-39 38 31 2C 20 31 39 38   t Corp 1981, 198   
2320:1500  36 0D 0A 00 4C 69 63 65-6E 73 65 64 20 4D 61 74   6...Licensed Mat   
2320:1510  65 72 69 61 6C 20 2D 20-50 72 6F 67 72 61 6D 20   erial - Program    
2320:1520  50 72 6F 70 65 72 74 79-20 6F 66 20 49 42 4D      Property of IBM
                                                                               

The environment size

-u e9c l29                                                                      
2320:0E9C A10200        MOV     AX,[0002]                                       
2320:0E9F 8C1E1B0D      MOV     [0D1B],DS                                       
2320:0EA3 8C1E1F0D      MOV     [0D1F],DS                                       
2320:0EA7 8C1E130D      MOV     [0D13],DS                                       
2320:0EAB A3E50B        MOV     [0BE5],AX                                       
2320:0EAE C70652164500  MOV     WORD PTR [1652],0045                            
2320:0EB4 C70650160A00  MOV     WORD PTR [1650],000A      <-- ch. to whatever
2320:0EBA BA6B4D        MOV     DX,4D6B                                         
2320:0EBD B104          MOV     CL,04                                           
2320:0EBF D3EA          SHR     DX,CL                                           
2320:0EC1 89165A16      MOV     [165A],DX                                       

Just searching for '0A 00' should find this one.

Echo off for AUTOEXEC.BAT

-u 12fd l27                                                                     
2320:12FD A1970B        MOV     AX,[0B97]                                       
2320:1300 C606ED0B03    MOV     BYTE PTR [0BED],03       <-- ch. to 02
2320:1305 C706FC0B0100  MOV     WORD PTR [0BFC],0001                            
2320:130B 8EC0          MOV     ES,AX                                           
2320:130D 33FF          XOR     DI,DI                                           
2320:130F B000          MOV     AL,00                                           
2320:1311 AA            STOSB                                                   
2320:1312 B001          MOV     AL,01                                           
2320:1314 AA            STOSB                                                   
2320:1315 33C0          XOR     AX,AX                                           
2320:1317 AB            STOSW                                                   
2320:1318 AB            STOSW                                                   
2320:1319 AA            STOSB                                                   
2320:131A AB            STOSW                                                   
2320:131B AB            STOSW                                                   
2320:131C B8FFFF        MOV     AX,FFFF                                         
2320:131F B90A00        MOV     CX,000A                                         
2320:1322 F3            REPZ                                                    
2320:1323 AB            STOSW                                                   

Searching for '03 C7 06' works in the 3.30 versions I've seen. Searching
for '8E C0 33 FF B0 00' worked in 3.20 - 3.30. All those STOSW/B's should
stand out pretty well.


Echo off for other batch files

-u 1d55                                                                         
2320:1D55 26            ES:                                                     
2320:1D56 803EFF0B01    CMP     BYTE PTR [0BFF],01                              
2320:1D5B 7403          JZ      1D60                                            
2320:1D5D E8A407        CALL    2504                                            
2320:1D60 E8AA1C        CALL    3A0D                                            
2320:1D63 26            ES:                                                     
2320:1D64 A0ED0B        MOV     AL,[0BED]                                       
2320:1D67 2401          AND     AL,01                     <-- ch. to 00
2320:1D69 50            PUSH    AX                                              
2320:1D6A 33C0          XOR     AX,AX                                           
2320:1D6C 26            ES:                                                     
2320:1D6D F706970BFFFF  TEST    WORD PTR [0B97],FFFF                            
2320:1D73 7414          JZ      1D89                                            

Look for '24 01 50 33 C0'.


Echo-blank produces blank line

-u 3f12 l25                                                                     
2320:3F12 E89400        CALL    3FA9                                            
2320:3F15 7212          JB      3F29                                            
2320:3F17 8E1E9142      MOV     DS,[4291]                                       
2320:3F1B 7506          JNZ     3F23                                            
2320:3F1D 800EED0B01    OR      BYTE PTR [0BED],01                              
2320:3F22 C3            RET                                                     
2320:3F23 8026ED0BFE    AND     BYTE PTR [0BED],FE                              
2320:3F28 C3            RET                                                     
2320:3F29 E82000        CALL    3F4C                    ->  83F901  CMP CX,+01
2320:3F2C 7409          JZ      3F37                    ->  7209    JB  3F37
2320:3F2E BA8200        MOV     DX,0082                                         
2320:3F31 E810F1        CALL    3044                                            
2320:3F34 E9AFF0        JMP     2FE6                                            

Look for 'BA 82 00 E8'.

Good luck and let me know lf you have problems with these patches.

--
John Wright ////////////           Phone: 902-424-3805 or 902-424-6527
Post: c/o Dr Pat Lane, Biology Dept, Dalhousie U, Hlalifax N.S., CANADA B3H-4H8
Cdn/Eannet:lane@cs.dal.cdn Uucp:lane@dalcs.uucp or {uunet watmath}!dalcsllane
Arpa;lane~dalc,.uucp~uunet.uu.net  Internet:lane~c,.dal.c~
--
John Wright ////////////           Phone: 902-424-3805 or 902-424-6527

========================================================================
end of original article
========================================================================
Offset information for some other DOS versions
========================================================================
========================================================================

Version         Environment     Echo-1      Echo-2      Echo-3
                                                           
MSDOS 3.10      0DE1            112B        19C7        38E8
MSDOS 3.20      0E2F            123E        1BAC        3B48
MSDOS 3.21      0E2F            123E        1BBC        3B58
PCDOS 3.30      0EB8            1304        1D68        3F29
MSDOS 3.30+     0EB8            1304        1DF7        3FF9
Telex 3.30      0EB8            1304        1DF7        3F49
Telex 3.21      0D4F            115E        1ABC        3A18

Enjoy.
Mike Prine
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