UNDERCODE COMMUNITY

Forwarded from Backup Legal Mega

▁ ▂ ▄ ｕ𝕟𝔻Ⓔ𝐫Ć𝔬𝓓ⓔ ▄ ▂ ▁

🦑 PART 2 PRO CRACKING
> Crash Course in Assembly Language
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🦑𝕃𝔼𝕋'𝕊 𝕊𝕋𝔸ℝ𝕋 :

1) If you are already well familiar with the assembly
language, you may wish to skip this section. Cracking
demands the knowledge of assembly language. If you wish to

2) become a "serious" cracker, you might like to read up more
about this fascinating language. This section will only give
you enough info for intermediate level cracking.

3) At this point, you should familiarize yourself with
DEBUG and its commands as we will be using them shortly.

🦑 Registers
--------————-

1) One of the neato things that you will be fooling around
most often with are called the registers. Registers are like
variables (such as in BASIC) that are located within the CPU
itself. These registers may hold a positive integer from 0
to 255 or from 0 to 65535. They can also hold negative
integers from -128 to 127 or from -32768 to 32767. The
registers are given names as follows:

2) AX => accumulator - this register is most commonly used
for mathematical or I/O operations
BX => base - this register is used commonly as a base or
a pointer register (we'll talk more about this
later)
CX => count - used commonly for counting instructions
such as loops
DX => displacement - much like the base register

3) The registers stated above are considered general purpose
registers, since they can basically be used to store whatever
the user wants. Let's try putting some number in these
registers. Type in "R {enter}". You should see a bunch of
info, of which are four of the above mentioned registers.
Now, type in "RAX {enter}". Then type in a number like
8FABh.

4) Type in "R" again and noticed how the accumulator
(AX) has change its number.

5) These general purpose registers can also be "split" in
half into its higher and lower order components. Instead of
having one register AX, you can have two registers, AH and
AL. Note however that while you have a range of 0 to FFFFh
for AX, you will now have a range of 0 to FF for AH and AL.
You cannot change these directly in debug, but be aware that
programs will use it. If AX contains 0A4Ch, then AH will
contain 0Ah and AL will contain 4Ch.
6) The following are called the segment registers:

7) CS => code segment - the block of memory where the code
(instructions are located)
DS => data segment - the block of memory where data can
be accessed. In block move operations in which

8) huge blocks of memory are moved, this is commonly
the segment in which the CPU reads from.
ES => extra segment - also another data segment. In
block move operations in which huge blocks of
memory are moved, this is commonly the segment in
which the CPU writes to.
SS => stack segment - this is the block of memory in
which the CPU uses to store return addresses from
subroutines. (more on this later)

9) In introductory level of cracking, we don't mess around with
these registers. Later, we will see how we can use these to
trick a program into thinking other things, but that's later.
You can also change these registers in debug. Type in "RCS
{enter}". Then enter "0 {enter}" and notice how the CS
register changed.
There are other registers that we use to see what the
program is doing. These registers can also be change in
debug. Included are the following:

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