I. Pieces, bits and bytes:
• BIT - The smallest possible piece of data. It can be either a 0 or a 1. If you put a bunch of bits together, you end up in the 'binary number system'
i.e. 00000001 = 1 00000010 = 2 00000011 = 3 etc.
• BYTE - A byte consists of 8 bits. It can have a maximal value of 255 (0-255). To make it easier to read binary numbers, we use the 'hexadecimal number system'. It's a 'base-16 system', while binary is a 'base-2 system'
• WORD - A word is just 2 bytes put together or 16 bits. A word can have a maximal value of 0FFFFh (or 65535d).
• DOUBLE WORD - A double word is 2 words together or 32 bits. Max value = 0FFFFFFFF (or 4294967295d).
• KILOBYTE - 1000 bytes? No, a kilobyte does NOT equal 1000 bytes! Actually, there are 1024 (32*32) bytes.
• MEGABYTE - Again, not just 1 million bytes, but 1024*1024 or 1,048,578 bytes.
II. Registers:
Registers are “special places” in your computer's memory where we can store data. You can see a register as a little box, wherein we can store something: a name, a number, a sentence. You can see a register as a placeholder.
On today’s average WinTel CPU you have 9 32bit registers (w/o flag registers). Their names are:
EAX: Extended Accumulator Register
EBX: Extended Base Register
ECX: Extended Counter Register
EDX: Extended Data Register
ESI: Extended Source Index
EDI: Extended Destination Index
EBP: Extended Base Pointer
ESP: Extended Stack Pointer
EIP: Extended Instruction Pointer
Generally the size of the registers is 32bit (=4 bytes). They can hold data from 0-FFFFFFFF (unsigned). In the beginning most registers had certain main functions which the names imply, like ECX = Counter, but in these days you can - nearly - use whichever register you like for a counter or stuff (only the self defined ones, there are counter-functions which need to be used with ECX). The functions of EAX, EBX, ECX, EDX, ESI and EDI will be explained when I explain certain functions that use those registers. So, there are EBP, ESP, EIP left:
EBP: EBP has mostly to do with stack and stack frames. Nothing you really need to worry about, when you start. ;)
ESP: ESP points to the stack of a current process. The stack is the place where data can be stored for later use (for more information, see the explanation of the push/pop instructions)
EIP: EIP always points to the next instruction that is to be executed.
There's one more thing you have to know about registers: although they are all 32bits large, some parts of them (16bit or even 8bit) can not be addressed directly.
The possibilities are:
A register looks generally this way:
So, EAX is the name of the 32bit register, AX is the name of the "Low Word" (16bit) of EAX and AL/AH (8bit) are the “names” of the "Low Part" and “High Part” of AX. BTW, 4 bytes is 1 DWORD, 2 bytes is 1 WORD.
REMARK: make sure you at least read the following about registers. It’s quite practical to know it although not that important.
All this makes it possible for us to make a distinction regarding size:
• i. byte-size registers: As the name says, these registers all exactly 1 byte in size. This does not mean that the whole (32bit) register is fully loaded with data! Eventually empty spaces in a register are just filled with zeroes. These are the byte-sized registers, all 1 byte or 8 bits in size:
o AL and AH
o BL and BH
o CL and CH
o DL and DH
• ii. word-size registers: Are 1 word (= 2 bytes = 16 bits) in size. A word-sized register is constructed of 2 byte-sized registers. Again, we can divide these regarding their purpose:
1.general purpose registers:
AX (word-sized) = AH + AL -> the '+' does *not* mean: 'add them up'. AH and AL exist independently, but together they form AX. This means that if you change AH or AL (or both), AX will change too!
-> 'accumulator': used to mathematical operations, store strings,..
BX -> 'base': used in conjunction with the stack (see later)
CX -> 'counter'
DX -> 'data': mostly, here the remainder of mathematical operations is stored
DI -> 'destination index': i.e. a string will be copied to DI
SI -> 'source index': i.e. a string will be copied from SI
2.index registers:
BP -> 'base pointer': points to a specified position on the stack (see later)
SP -> 'stack pointer': points to a specified position on the stack (see later)
3. segment registers:
CS -> 'code segment': instructions an application has to execute (see later)
DS -> 'data segment': the data your application needs (see later)
ES -> 'extra segment': duh! (see later)
SS -> 'stack segment': here we'll find the stack (see later)
4.special:
IP -> 'instruction pointer': points to the next instruction. Just leave it alone ;)
• iii. Doubleword-size registers:
2 words = 4 bytes = 32 bits. EAX, EBX, ECX, EDX, EDI…
If you find an 'E' in front of a 16-bits register, it means that you are dealing with a 32-bits register. So, AX = 16-bits; EAX = the 32-bits version of EAX.
III. The flags:
Flags are single bits which indicate the status of something. The flag register on modern 32bit CPUs is 32bit large. There are 32 different flags, but don't worry. You will mostly only need 3 of them in reversing. The Z-Flag, the O-Flag and the C-Flag. For reversing you need to know these flags to understand if a jump is executed or not. This register is in fact a collection of different 1-bit flags. A flag is a sign, just like a green light means: 'ok' and a red one 'not ok'. A flag can only be '0' or '1', meaning 'not set' or 'set'.
• The Z-Flag:
The Z-Flag (zero flag) is the most useful flag for cracking. It is used in about 90% of all cases. It can be set (status: 1) or cleared (status: 0) by several opcodes when the last instruction that was performed has 0 as result. You might wonder why "CMP" (more on this later) could set the zero flag, because it compares something - how can the result of the comparison be 0? The answer on this comes later ;)
• The O-Flag:
The O-Flag (overflow flag) is used in about 4% of all cracking attempts. It is set (status: 1) when the last operation changed the highest bit of the register that gets the result of an operation. For example: EAX holds the value 7FFFFFFF. If you use an operation now, which increases EAX by 1 the O-Flag would be set, because the operation changed the highest bit of EAX (which is not set in 7FFFFFFF, but set in 80000000 - use calc.exe to convert hexadecimal values to binary values). Another need for the O-Flag to be set, is that the value of the destination register is neither 0 before the instruction nor after it.
• The C-Flag:
The C-Flag (Carry flag) is used in about 1% of all cracking attempts. It is set, if you add a value to a register, so that it gets bigger than FFFFFFFF or if you subtract a value, so that the register value gets smaller than 0.
Best Regards,
Meghdad Shamsaei
