Lesson 5.1: Disk & CD-Rom access (basics)


LESSON 5 (1) - HOW TO CRACK, HANDS ON - Disk/CDROM access (plus
bypasses "on the fly")

Somewhere I have to put the bypasses (loader programs) in this
tutorial, allow me to put them here:

Preparing a loader to bypass a protection [MARIO ANDRETTI]
     At time the protectionists hook vectors in order to impose
a particular protection. In this (and similar) cases a good
crack-way is to prepare a "loader" program, that "de-hooks" the
vector used for the protection. This kind of crack can be used
also for internet cracking (on some firewall configurations, see
lesson A.2).
     As example let's take "Mario andretti racing challenge", a
stupid game that uses the SAME (!) protection scheme you'll still
find to day on some access routines of military servers around
the witlessly called "free" world.

In order to crack this cram you would prepare a loader on the
following lines:

loc   code           instruction        what's going on
:0100 EB44           JMP 0146
:0142 0000           <- storing for offset of INT_21
:0144 5887           <- storing for segment of INT_21
:0146 FA             CLI
:0147 0E             PUSH CS
:0148 1F             POP DS
:0149 BCB403         MOV SP,03B4
:014C FB             STI
:014D 8C1EA901       MOV [01A9],DS      <- save DS
:0151 8C1EAD01       MOV [01AD],DS         three
:0155 8C1EB101       MOV [01B1],DS         times
:0159 B82135         MOV AX,3521        <- get INT_21
:015C CD21           INT 21                in ES:BX
:015E 891E4201       MOV [0142],BX      <- store offset

:0162 8C064401       MOV [0144],ES      <- store segment
:0166 BA0201         MOV DX,0102
:0169 B82125         MOV AX,2521        <- set INT_21 to
:016C CD21           INT 21                DS:0102
:016E 0E             PUSH CS
:016F 07             POP ES             <- ES= current CS
:0170 BBB403         MOV BX,03B4
:0173 83C30F         ADD BX,+0F
:0176 B104           MOV CL,04
:0178 D3EB           SHR BX,CL          <- BX= 3C
:017A B8004A         MOV AX,4A00        <- Modify memory block
:017D CD21           INT 21                to 3C paragraphs
:017F BA9E01         MOV DX,019E        <- ds:dx=program name
:0182 BBA501         MOV BX,01A5        <- es:bx = param. block
:0185 B8004B         MOV AX,4B00        <- load
:0188 CD21           INT 21
:018A 2E8B164201     MOV DX,CS:[0142]   <- reset old int_21
:018F 2E8E1E4401     MOV DS,CS:[0144]
:0194 B82125         MOV AX,2521
:0197 CD21           INT 21
:0199 B8004C         MOV AX,4C00        <- terminate with return
:019C CD21           INT 21                code
:019E 6D612E636F6D00 ""
      0000           fence
:01A7 B2015887
:01AB B2015887
:O1AF B2015887
      0000           fence

let's now prepare a routine that hooks INT_21:

push all
CMP AX,2500    <- go on if INT_21 service 25
JNZ ret
CMP Word Ptr [0065], C00B <- go on if location 65 = C00B
JNZ ret
MOV  Byte Ptr [0060], EB  <- crack instructions
MOV  Byte Ptr [0061], 3C
MOV  Byte Ptr [0062], 40  <- INC AX
MOV  Byte Ptr [0063], 90  <- NOP
MOV  Byte Ptr [0064], 48  <- DEC AX
pop all
JMP  FAR CS:[0142]  <- JMP previous INT_21

     From now on this loader will work every time that a program
with location [0065] containing an 0R AX,AX instruction (0BC0:
it's the case of calls INT_21 service 25 (hook a vector),
the target program will be modified on the fly and will get, at
location [0060], the instruction JMP 3C locations ahead, despite
the fact that it has routines capable of self checking in order
to make sure it has not been modified.
     The most important thing is the routine that YOU write that
will precede the call to INT_21 (or any other INT) service 25 (or
any other service) in order to crack on the fly the offending
program. I'll show you another one, this one for [Reach for the
skies] (

push all
CMP  AH,3D      <- is it service 3D? (open file)
JNZ  ret        <- no, so ret
CMP  DX,13CE    <- you wanna open file at 13CE?
JNZ  ret        <- no, so ret
MOV  AX,[BP+04] <- in this case
CMP  Byte Ptr [B6DA],74 <- old instructions
JNZ  015B
CMP  Byte Ptr [B6DB],0F <- ditto
JNZ  015B
CMP  Byte Ptr [B6DC],80 <- ditto, now we now where we are
JNZ  015B
MOV  Byte Ptr [B6DA],EB <- crack
MOV  Byte Ptr [B697],40 <- camouflaged  no-opping
MOV  Byte Ptr [B698],48 <- cam          nop
MOV  Byte Ptr [B699],90 <- cam          nop
MOV  Byte Ptr [B69A],40 <- cam          nop
MOV  Byte Ptr [B69B],48 <- cam          nop
MOV  DX,CS:[0165]
MOV  DS,CS:[0167]
MOV  AX,2521  <- set hook
INT  21
POP  all
JMP  FAR CS:[0165]
Here you did change the instruction 740F in the instruction EB0F,
and you did "noop" the instructions at B697-B69B. (Well, more
elegantly than "noop" them with "90" bytes, you choose a INC AX,
DEC AX, NOP, INC AX, DEC AX sequence instead! There are sound
reasons to use a sequence of "working" instructions instead of
NOPs: recent protection schemes "smell" patched nops inside the
program and trash everything if they find more than -say- three
consecutive NOPs! You should always try to choose THE LESS
INTRUSIVE and MORE "CAMOUFLAGED" solution when you crack!)
     You can apply this kind of crack, on the same lines, to many
programs that perform self checking of the code and hook the

     Now we may come to the subject of this lesson:
     As usual, let's begin from the beginning: history is always
the key that allows an understanding of present and future, in
cracking matters too. As the older 5 1/4 inch big black floppy
disks were still used (the 320K/8 tracks or 360K/9 tracks ones,
that were really "floppy" and have nowadays almost disappeared)
one of the more common methods to protect a program, was to
format the "master" (key) disk in a weird way. Old floppy disk
for the PC did usually store 360K at 9 sectors per track.
     Some basics for those of you that do not know anything: in
order to defeat this kind of cracks you need to know two things:

the floppy disk parameter block (FDPB) and the interrupt routines
dealing with format/read disk (basically INT_13).
     Most often, the protection scheme is to either format one
or more sectors or tracks with sector sizes other than the
standard 512 bytes, or to either give one of the sectors a wild
sector number like 211 or just not format a whole track of
eight/nine/15 sectors. If you, for instance, have got the same
(very old) copy of VisiCalc master I do, you'll find that sector
8 on track 39 is missing entirely. The interrogation with
assembly or with an "ad hoc" utility (I use the tools I wrote
myself, but you 'll be able to find many such utilities in public
domain, the oldest one, from 1984 (!) being the seasoned [U-ZAP]
an "Ultra utility" from the "Freesoft company") will tell you
which sector numbers were altered, their size in bytes, and if
they were formatted with a CRC error (another not so fancy
     The floppy disk parameters are stored in the BIOS: interrupt
vector 1E contains the address of the floppy disk parameter
block. The FDPB's contents are the following:
Offset    Function                 crackworthy?        Example
0    Step rate & head unload            no                  DF
1    head load time                     no                  02
2    Motor on delay                     no                  25
3    Number of bytes per sector         yes                 02
4    Last sector number                 yes                 12
5    Gap length                         yes                 1B
6    Data track length                  yes                 FF
7    Format gap length                  yes                 54
8    Format byte                        no                  F6
9    Head settle time                   no                  0F
A    Motor start time                   no                  02

0)   Offset #0: the left "nybble" (single digit) of this value
     is the step rate time for the disk drive head. The right
     nybble is the disk head unload time. These values are best
     left alone.
1)   Offset #1: again, don't fool around with these values. The
     left nybble is the disk head load time, and the right
     nybble is the direct memory access mode select.
2)   Wait time until motor is turned off. Not normally of use.
3)   Bytes-per-sector value: AH-HAH! If you place a "0" in this
     value, the PC expects all sectors to be 128 bytes long. A
     "1" means a  sector size of 256 bytes, a "2" means 512
     bytes (this is the standard DOS value), and a "3" means
     1024 bytes per sector.
4)   Highest sector number on a track: this is used for
     formatting and tells DOS how many sectors there are on each
5)   Gap length for diskette reads: this is what you fool around
     with if you keep getting CRC errors when you try to read a
     non-standard size sector. Normally, you can just leave this
     alone except when formatting with a U-Format tool.
6)   Data length: This contains the number of bytes in a sector
     when the value in table byte #4 doesn't contain a 0, 1, 2,
     or 3.
7)   Number of bytes in the gap between sectors: this is also
     only used when formatting special tracks.
8)   Format fill byte: When formatting, this is the
     initialization byte that will be placed in all new sectors.
9)   Head settle time: leave this alone.
A)   Motor start time: don't fool with this either.
In order to modify globally the number of tracks on a given disk
and the number of sectors per track you can always format with
the DOS command switches "/t:" and "/n:"
                  FORMAT /t:tracks /n:sectors

     If you want to find out what the existing parameters are,
run [Debug.exe] or [Symdeb.exe] and enter the following commands:
-    d 0:78    l 4                 <- get FDPB address
 0000:0070     22 05 00       <- debugger's likely response
-    d 0:522   l a                 <- get 10 FDPB values
 0000:520 DF 02 25 02 12 1B FF...  <- see preceding table

     Remember that all standard disk formats under DOS support
a sector size of 512 bytes, therefore, for one-sided 5.25 inch
               40t*8s*512b=163.840 bytes (160Kb)
               40t*9s*512b=184.320 bytes (180Kb)
and for two-sided 5.25 inch floppies:
           40t*8s*512b*2sides=327.680 bytes (320Kb)
           40t*9s*512b*2sides=368.640 bytes (360Kb)
     Beginning with DOS version 3.0 (Yeah, more and more
history!) a new floppy disk format has been supported: The IBM
AT (80286 CPU) introduced the so called "high capacity" 5.25 u-
inch floppy, capable of storing 1.2M at 15 sectors per track:
          80t*15s*512b*2sides=1.228.800 bytes (1.2Mb)
     Later on were introduced the to-day universally used 3.5
inch floppies, the ones inside a rigid small plastic cartridge,
and we have, similarly:

             3.5-inch double sided/double density      720K
            3.5-inch double sided/quad density (HD)    1440K
              3.5-inch double sided/high density       2880K

[INT_13, AH=18, Set media type for format]
     In order to create weird layouts, the protectionists use
interrupt 13h, service 18h, that specifies to the formatting
routines the number of tracks and sectors per track to be placed
on the media:
*    Registers on entry: AH=18h; CH=Nø of tracks; CL= Sectors
     per track; DL= Drive number (A=0; B=1;C=2... bit 7 is set
     if the drive is an hard disk)
*    Registers on Return: DI: Offset address of 11-byte
     parameter table; ES: Segment address of 11-byte parameter

[INT_13, AH=2, Read disk sectors]
In order to read them, they have to use INT_13, service 2, read
disk sectors, with following layout:
*    Registers on entry: AH=2h; AL= Nø of sectors; BX= Offset
     address of data buffer; CH=track; CL= Sector; DH= Head
     (side) number; DL= Drive number; ES: Segment address of
     data buffer.
*    Registers on Return: AH= return code. If the carry flag is
     not set, AH=0, therefore the weird sector has been read, if
     on the contrary the carry flag is set, AH reports the
     status byte as follows:
76543210  HEX  DEC       Meaning
1         80h  128       Time out - drive crazy
 1        40h  064       Seek failure, could not move to track
  1       20h  032       Controller kaputt
   1      10h  016       Bad CRC on disk read
    1     09h  009       DMA error - 64K boundary crossed
    1     08h  008       DMA overrun
     1    04h  004       Bad sector - sector not found
      11  03h  003       Write protect!
      1   02h  002       Bad sector ID (address mark
       1  01h  001       Bad command

[Return code AH=9: DMA boundary error]
     One of the possible errors should be explained, coz it is
used in some protection schemes: AH=9 DMA boundary error, means
that an illegal boundary was crossed when the in formation was
placed into RAM. DMA (Direct memory access) is used by the disk
service routines to place information into RAM. If a memory
offset address ending in three zeros (ES:1000, ES: 2000...) falls
in the middle of the area being overlaid by a sector, this error
will occur.

[INT_13, AH=4 Verify disk sectors]
     Another possible protection interrupt is interrupt 13H,
service 4, Verify disk sectors. Disk verification takes place on
the disk and DOES NOT involve verification of the data on the
disk against data in memory! This function has no buffer
specification, does not read or write a disk: it causes the
system to read the data in the designated sector or sectors and
to check its computed cyclic redundancy check (CRC) against data
stored on the disk. See INT_13, AH=2 registers and error report.

     The CRC is a checksum, that detects general errors. When a
sector is written to disk, an original CRC is calculated AND

WRITTEN ALONG with the sector data. The verification service
reads the sector, recalculates the CRC, and compares the
recalculated CRC with the original CRC.

     We saw that some protection schemes attempt to disguise
interrupt calls. This is particularly frequent in the disk access
protection schemes that utilize INT_13 (the "disk" interrupt).
     If you are attempting to crack such programs, the usual
course of action is to search for occurrences of "CD13", which
is machine language for interrupt 13. One way or another, the
protection scheme has to use this interrupt to check for the
special sectors of the disk. If you examine a cross section of
the program, however, you'll find programs which do not have
"CD13" in their machine code, but which clearly are checking the
key disk for weird sectors. How comez?
     There are several techniques which can be used to camouflage
the protection scheme from our nice prying eyes. I'll describe
here the three such techniques that are more frequent:
1)   The following section of code is equivalent to issuing an
INT 13 command to read one sector from drive A, side 0, track
29h, sector ffh, and then checking for a status code of 10h:
     cs:1000   MOV  AH,02     ;read operation
     cs:1002   MOV  AL,01     ;1 sector to read
     cs:1004   MOV  CH,29     ;track 29h
     cs:1006   MOV  CL,FF     ;sector ffh
     cs:1008   MOV  DX,0000   ;side 0, drive A
     cs:100B   XOR  BX,BX     ;move 0...
     cs:100D   MOV  DS,BX     ; DS register
     cs:100F   PUSHF          ;pusha flags
     cs:1010   PUSH CS        ;pusha CX
     cs:1011   CALL 1100      ;push address for next
                              instruction onto stack and branch
     cs:1014   COMP AH,10     ;check CRC error
     cs:1017   ... rest of verification code
     cs:1100   PUSHF          ;pusha flags

     cs:1101   MOV  BX,004C   ;address of INT_13 vector
     cs:1104   PUSH [BX+02]   ;push CS of INT_13 routine
     cs:1107   PUSH [BX]      ;push IP of INT_13 routine
     cs:1109   IRET           ;pop IP,CS and flags
Notice that there is no INT 13 command in the source code, so if
you had simply used a debugger to search for "CD13" in the
machine code, you would never have found the protection routine.

2)   Another technique is to put in a substitute interrupt
instruction, such as INT 10, which looks harmless enough, and
have the program change the "10" to "13 (and then back to "10")
on the fly. A search for "CD13" would turn up nothing.

3)   The best camouflage method for interrupts I have ever
cracked (albeit not on a INT 13) was a jump to a section of the
PROGRAM code that reproduces in extenso the interrupt code. This
elegant (if a little overbloated) disguise mocks every call to
the replicated interrupt.

Old good [] has been called the "swiss army knife" of
the cracker. It allows a lot of nice things, inter alia the
loading, reading, modifying and writing of absolute sectors of
the disks. The sector count starts with the first sector of track
0, next sector is track 0, second side (if double sided), then,
back to the first side, track 1, and so on, until the end of the
disk. Up to 80h (128) sectors can be loaded at one time. To use
you must specify starting address, drive (0=A, 1=B, etc...),
starting sector and number of sectors to load.
                               -   l 100 0 10 20
This instruction tells DEBUG to load, starting at DS:0100, from
drive A, sector 10h for 20h sectors. This allows at times the
retrieval of hidden and/or weird formatted data. If you get an
error, check the memory location for that data. Often times, part
of the data has been transferred before the error occurs, and the
remainder can be manually entered or gathered through repetitive

Bear all this in mind learning the following cracks.
Let's now crack an "oldie" primitive:
MS Flight simulator (old version 2.12, from 1985!)
This old program used -in 1985!- following beautiful protection
scheme: on the disk you had only a "stub", called FS.COM with few
bytes, which had following instructions:

loc   code           instruction        what's going on
:0100 FA             CLI                ;why not?
:0101 33C0           XOR AX,AX          ;ax=0

:0103 8ED0           MOV SS,AX          ;ss=0
:0105 BCB0C0         MOV SP,C0B0        ;SP=C0B0
:0108 8EC0           MOV ES,AX          ;ES=0
:010A 26C70678003001 MOV Wptr ES:[0078],0130 ;Wp 0:78=130
:0111 268C0E7A00     MOV ES:[007A],CS   ;0:7A=Segment
:0116 BB0010         MOV BX,1000        ;BX=1000
:0119 8EC3           MOV ES,BX          ;ES=1000
:011B 33DB           XOR BX,BX          ;BX=0
:011D B80102         MOV AX,0201        ;AH=2 AL=1 sector
:0120 BA0000         MOV DX,0000        ;head=0 drive=0
:0123 B96501         MOV CX,0165        ;track=1 sector=65 (!)
:0126 CD13           INT 13             ;INT 13/AH=2
:0128 B83412         MOV AX,1234        ;AX=1234
:012B EA00000010     JMP 1000:0000      ;JMP to data we just read

:0130 CF             IRET               ;Pavlovian, useless ret

     You see what's happening in this old protection scheme,
don't you? Herein you can watch the same snap that happens in
more recent (much more recent) protection schemes (as you'll see
in the next lesson): the protection searches for a weird
formatted sector and/or for particular data.
     That should be no problem for you any more: you should just
reverse engineer everything (and that goes on pretty quickly:
just watch and break on the INT_13 calls), fetch the "weird"
data, tamper the whole crap and have your soup as you like it.
     One more word about "old" protection schemes. Be careful not
to spurn them! Some of them are
     -- CLEVER
     -- STILL USED
     -- DIFFICULT TO CRACK... I mean, this older DOS programs had

nice protections... it's pretty annoying to crack windows
programs that require a registration number: as you saw in Lesson
3, you just type your name and a serial number of your choice in,
say "666666666", break into the program with WINICE, search the
"666666666" and search too, for good measure, your own name, set
a memory read breakpoint where the number dwells and look at the
code that manipulates your input. As [Chris] rightly pointed out,
you can even rip the code straight out of the program and create
a key generator which will produce a valid code. This code will
work for any name you typed in only in the "pure maths
manipulation" protection schemes, and will on the contrary be
specific, following the name you typed in, the "alpha-maths
manipulation" protection schemes (like MOD4WIN, see the Windows
lessons), watch in this case the "pseudo-random xoring" of the
letters that compose your name.
     -- STUNNING, coz new ideas have always been infrequent, and
they are getting more and more rare in this objectionable world
of lazy, incapable programmers patronizing us with ill-cooked
outrages like Windows'95... yeah, as usual there is no
"development" at all, quite the contrary, I would say. Take a
step backward, sip a good Martini-Wodka (please remember that
only Ice cubes, Dry Martini, Wodka Moskovskaja, Schweppes'
"Indian tonic" a green olive from Tuskany and a maltese lemon
zest will really be perfect) and watch from your balcony, with
unsullied eyes, your town and the people around you: slaves
everywhere, leaving home at 7.30 in the morning, stinking in a
progression of identical cars, forced to interminably watch
advertisement panels and endlessly listen to boorish publicity,
happy to go to work (if they happen to have the "luck" to work,
in this inequitable society) the whole day long in order to
produce other cars in order to buy, one day, a new car with a
different colour...
     Why people don't look at the stars, love each other, feel
the winds, ban the stinking cars from the places where they live
and eat, study colours... name yourself a not-consumistic
activity? Why don't they read any poems any more? No poetry any
more, in the grey society of the publicity-spots slaves...poetry
will soon be forbidden, coz you cannot CONSUME as you read poems,
and in this farce of a society you are BOUND to consume, that's
the only thing they want you to do... you are CULTIVATED to
consume... no books worth to read any more... stupid american
conventional cram everywhere... boy, at times I'm missing some
well placed neutron bombs, the ones that would kill all these
useless zombies and leave noble books and good Wodka untouched.
It's difficult to believe in democracy any more... if I ever
did... all the useless zombie do -unfortunately- vote, and they
do vote for "smiling semblances", for "conventionally minded
idiots" that so act as if they would "really" be like what they
"look" like and could not care less about anything else than
making bucks and defend intolerant and petty patterns. The slaves
choose the people they have "seen" on TV... as if the egyptians
would VOTE for their pharaohs, exhilarated under the whips of
publicity... sorry, at times I forget that you are here for the
cracks, and could not care less about what I think...

     You 'll obtain the OTHER missing lessons IF AND ONLY IF you
mail me back (via with some tricks of the trade
I may not know that YOU discovered. Mostly I'll actually know
them already, but if they are really new you'll be given full
credit, and even if they are not, should I judge that you
"rediscovered" them with your work, or that you actually did good
work on them, I'll send you the remaining lessons nevertheless.
Your suggestions and critics on the whole crap I wrote are also

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