Learn how to crack windows, programs manually part 2

Program terminated normally

That's the same program we did at the end of the previous chapter. Notice how you run the program you just entered with "g", and also notice how the set-up part is not there? That's because debug is just too limited to support that.
Another thing you can do with assemble is specify the address at which you want to start, by default this is 0100 since that's where all .COM files start.
Compare:

Compare takes 2 block of memory and displays them side by side, byte for byte. Lets do an example. Quite out of debug if you haven't already using "q". Now type "debug c:\command.com"

-c 0100 l 8 0200
10A3:0100 7A 06 10A3:0200

This command compared offset 0100 with 0200 for a length of 8 bytes. Debug responded with the location that was DIFFERENT. If 2 locations were the same, debug would just omit them, if all are the same debug would simply return to the prompt without any response.
Dump:

Dump will dump a specified memory segment. To test it, code that assembly program again:

C:\>debug
-a
107A:0100 MOV AH,02
107A:0102 MOV DL,41
107A:0104 INT 21
107A:0106 INT 20
-d 0100 l 8
107A:0100 B4 02 B2 41 CD 21 CD 20
...A.!.

The "B4 02 B2 41 CD 21 CD 20" is the program you just made in machine language.

B4 02 = MOV AH,02
B2 41 = MOV DL,41
CD 21 = INT 21
CD 20 = INT 20

The "...A.!." part is your program in ASCII. The "." represent non-printable characters. Notice the A in there.
Enter:

This is one of the hard commands. With it you can enter/change certain memory areas. Lets change our program so that it prints a B instead of an A.
-e 0103 <-- edit program at segment 0103
107A:0103 41.42 <-- change 41 to 42
-g
B

Program terminated normally
-
Wasn't that amazing?
Fill:

This command is fairly useless, but who knows....
It fills the specified amount of memory with the specified data. Lets for example clear out all memory from segment 0100 to 0108, which happens to be our program.
-f 0100 l 8 0 <-- file offset 0100 for a length of 8 bytes with 0
-d 0100 l 8 <-- verify that it worked
107A:0100 00 00 00 00 00 00 00 00 .......
Yep, it worked.
Go:

So far we used go (g) to start the program we just created. But Go can be used for much more. For example, lets say we want to execute a program at 107B:0100:
-r CS <-- set the CS register to point to 107B
CS 107A
:107B
-g =100

You can also set breakpoints.
-a <-- enter our original program so we have something
107A:0100 MOV AH,02 to work with
107A:0102 MOV DL,41
107A:0104 INT 21
107A:0106 INT 20
-g 102 <-- set up a break point at 107A:0102

At this point the program will stop, display all registers and the current instruction.
Hex:

This can be very useful. It subtracts and adds two hexadecimal values:
-h 2 1
0003 0001 <-- 2h + 1+ = 3h and 2h - 1h = 1h

This is very useful for calculating a programs length, as you will see later.
Input:

This is one of the more advanced commands, and I decided not to talk about it too much for now. It will read a byte of data from any of your computers I/O ports (keyboard, mouse, printer, etc).

-i 3FD
60

continue part3