Back to basics. Everything needed to learn ASM is available for free online.
Art of ASM book:
It starts you off with High Level Assembly which is more like a traditional programming language. Towards the end, you’ll switch to actual assembly little endian full on ASM programming.
All course files are available here:
I. Pieces, bits and bytes:
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’
MEGABYTE – Again, not just 1 million bytes, but 1024*1024 or 1,048,578 bytes.
Registers are place holders where we can store data.
On today’s average WinTel CPU you have 9 32bit registers (w/o flag registers). Their names are:
–EAX: Accumulator : used for performing calculations, and used to store return values from function calls. Basic operations such as add, subtract, compare use this general-purpose register
–ECX : Counter : used for iterations. ECX counts downward.
–EDX : Data : this is an extension of the EAX register. It allows for more complex calculations (multiply, divide) by allowing extra data to be stored to facilitate those calculations.
–EBX : Base (does not have anything to do with base pointer). It has no general purpose and can be used to store data.
–ESP : stack pointer
–EBP : base pointer
–ESI : source index : holds location of input data
–EDI : destination index : points to location of where result of data operation is stored
–EIP : 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.
ESP: ESP points to the stack of a current process. The stack is the place where data can be stored for later use.
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:
32bit Register 16bit Register 8bit Register
EAX AX AH/AL
EBX BX BH/BL
ECX CX CH/CL
EDX DX DH/DL
ESI SI —–
EDI DI —–
EBP BP —–
ESP SP —–
EIP IP —–
A register looks generally this way:
|————————— EAX: 32bit (=1 DWORD =4BYTES) ————————-|
|——- AX: 16bit (=1 WORD =2 BYTES) —-|
|- AH:8bit (=1 BYTE)-|- AL:8bit (=1 BYTE)-|
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.
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:
AL and AH
BL and BH
CL and CH
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:
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
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)
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)
IP -> ‘instruction pointer’: points to the next instruction. Just leave it alone 😉
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 32bits large. There are 32 different flags but we 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 (zero flag). 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 (overflow flag). 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 kcalc 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 (Carry flag). 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.
IV. Segments en offsets
A segment is a piece in memory where instructions (CS), data (DS), the stack (SS) or just an extra segment (ES) are stored. Every segment is divided in ‘offsets’. In 32-bits applications (Windows 95/98/ME/2000), these offsets are numbered from 00000000 to FFFFFFFF. 65536 pieces of memory thus 65536 memory addresses per segment. The standard notation for segments and offsets is:
SEGMENT : OFFSET = Together, they point to a specific place (address) in memory.
See it like this:
A segment is a page in a book : An offset is a specific line at that page.
V. The stack:
The Stack is a part in memory where you can store different things for later use. See it as a pile of books in a chest where the last put in is the first to grab out. Or imagine the stack as a paper basket where you put in sheets. The basket is the stack and a sheet is a memory address (indicated by the stack pointer) in that stack segment. Remember following rule: the last sheet of paper you put in the stack, is the first one you’ll take out! The command ‘push’ saves the contents of a register onto the stack. The command ‘pop’ grabs the last saved contents of a register from the stack and puts it in a specific register.
VI. INSTRUCTIONS (alphabetical)
Please note, that all values in ASM mnemonics (instructions) are *always* hexadecimal.
Most instructions have two operators (like “add EAX, EBX”), but some have one (“not EAX”) or even three (“IMUL EAX, EDX, 64”). When you have an instruction that says something with “DWORD PTR [XXX]” then the DWORD (4 byte) value at memory offset [XXX] is meant. Note that the bytes are saved in reverse order in the memory (WinTel CPUs use the so called “Little Endian” format. The same is for “WORD PTR [XXX]” (2 byte) and “BYTE PTR [XXX]” (1 byte).
Most instructions with 2 operators can be used in the following ways (example: add):
add eax,ebx ;; Register, Register
add eax,123 ;; Register, Value
add eax,dword ptr  ;; Register, Dword Pointer [value]
add eax,dword ptr [eax] ;; Register, Dword Pointer [register]
add eax,dword ptr [eax+00404000] ;; Register, Dword Pointer [register+value]
add dword ptr ,eax ;; Dword Pointer [value], Register
add dword ptr ,123 ;; Dword Pointer [value], Value
add dword ptr [eax],eax ;; Dword Pointer [register], Register
add dword ptr [eax],123 ;; Dword Pointer [register], Value
add dword ptr [eax+404000],eax ;; Dword Pointer [register+value], Register
add dword ptr [eax+404000],123 ;; Dword Pointer [register+value], value
Syntax: ADD destination, source
The ADD instruction adds a value to a register or a memory address. It can be used in
These instruction can set the Z-Flag, the O-Flag and the C-Flag and some others.
Syntax: AND destination, source
The AND instruction uses a logical AND on two values.
This instruction *will* clear the O-Flag and the C-Flag and can set the Z-Flag.
To understand AND better, consider those two binary values:
If you AND them, the result is 0001000100
When two 1 stand below each other, the result is of this bit is 1, if not: The result
Syntax: CALL something
The instruction CALL pushes the RVA (Relative Virtual Address) of the instruction that
follows the CALL to the stack and calls a sub program/procedure.
CALL can be used in the following ways:
CALL 404000 ;; MOST COMMON: CALL ADDRESS
CALL EAX ;; CALL REGISTER – IF EAX WOULD BE 404000 IT WOULD BE SAME AS THE ONE ABOVE
CALL DWORD PTR [EAX] ;; CALLS THE ADDRESS THAT IS STORED AT [EAX]
CALL DWORD PTR [EAX+5] ;; CALLS THE ADDRESS THAT IS STORED AT [EAX+5]
CDQ(Convert DWord (4Byte) to QWord (8 Byte))
CDQ is an instruction that always confuses newbies when it appears first time. It is
mostly used in front of divisions and does nothing else then setting all bytes of EDX
to the value of the highest bit of EAX. (That is: if EAX <80000000, then EDX will be
00000000; if EAX >= 80000000, EDX will be FFFFFFFF).
Syntax: CMP dest, source
The CMP instruction compares two things and can set the C/O/Z flags if the result fits.
CMP EAX, EBX ;; compares eax and ebx and sets z-flag if they are equal
CMP EAX, ;; compares eax with the dword at 404000
CMP ,EAX ;; compares eax with the dword at 404000
Syntax: DEC something
dec is used to decrease a value (that is: value=value-1)
dec can be used in the following ways:
dec eax ;; decrease eax
dec [eax] ;; decrease the dword that is stored at [eax]
dec  ;; decrease the dword that is stored at 
dec [eax+401000] ;; decrease the dword that is stored at [eax+401000]
The dec instruction can set the Z/O flags if the result fits.
Syntax: DIV divisor
DIV is used to divide EAX through divisor (unsigned division). The dividend is always
EAX, the result is stored in EAX, the modulo-value in EDX.
mov eax,64 ;; EAX = 64h = 100
mov ecx,9 ;; ECX = 9
div ecx ;; DIVIDE EAX THROUGH ECX
After the division EAX = 100/9 = 0B and ECX = 100 MOD 9 = 1
The div instruction can set the C/O/Z flags if the result fits.
Syntax: IDIV divisor
The IDIV works in the same way as DIV, but IDIV is a signed division.
The idiv instruction can set the C/O/Z flags if the result fits.
Syntax: IMUL value
IMUL multiplies either EAX with value (IMUL value) or it multiplies two values and puts
them into a destination register (IMUL dest, value, value) or it multiplies a register
with a value (IMUL dest, value).
If the multiplication result is too big to fit into the destination register, the
O/C flags are set. The Z flag can be set, too.
Syntax: INC register
INC is the opposite of the DEC instruction; it increases values by 1.
INC can set the Z/O flags.
Syntax: int dest
Generates a call to an interrupt handler. The dest value must be an integer (e.g., Int 21h).
INT3 and INTO are interrupt calls that take no parameters but call the handlers for
interrupts 3 and 4, respectively.
These are the most important jumps and the condition that needs to be met, so that
they’ll be executed (Important jumps are marked with * and very important with **):
JA* – Jump if (unsigned) above – CF=0 and ZF=0
JAE – Jump if (unsigned) above or equal – CF=0
JB* – Jump if (unsigned) below – CF=1
JBE – Jump if (unsigned) below or equal – CF=1 or ZF=1
JC – Jump if carry flag set – CF=1
JCXZ – Jump if CX is 0 – CX=0
JE** – Jump if equal – ZF=1
JECXZ – Jump if ECX is 0 – ECX=0
JG* – Jump if (signed) greater – ZF=0 and SF=OF (SF = Sign Flag)
JGE* – Jump if (signed) greater or equal – SF=OF
JL* – Jump if (signed) less – SF != OF (!= is not)
JLE* – Jump if (signed) less or equal – ZF=1 and OF != OF
JMP** – Jump – Jumps always
JNA – Jump if (unsigned) not above – CF=1 or ZF=1
JNAE – Jump if (unsigned) not above or equal – CF=1
JNB – Jump if (unsigned) not below – CF=0
JNBE – Jump if (unsigned) not below or equal – CF=0 and ZF=0
JNC – Jump if carry flag not set – CF=0
JNE** – Jump if not equal – ZF=0
JNG – Jump if (signed) not greater – ZF=1 or SF!=OF
JNGE – Jump if (signed) not greater or equal – SF!=OF
JNL – Jump if (signed) not less – SF=OF
JNLE – Jump if (signed) not less or equal – ZF=0 and SF=OF
JNO – Jump if overflow flag not set – OF=0
JNP – Jump if parity flag not set – PF=0
JNS – Jump if sign flag not set – SF=0
JNZ – Jump if not zero – ZF=0
JO – Jump if overflow flag is set – OF=1
JP – Jump if parity flag set – PF=1
JPE – Jump if parity is equal – PF=1
JPO – Jump if parity is odd – PF=0
JS – Jump if sign flag is set – SF=1
JZ – Jump if zero – ZF=1
LEA(Load Effective Address)
Syntax: LEA dest,src
LEA can be treated the same way as the MOV instruction. It isn’t used too much for its
original function, but more for quick multiplications like this:
lea eax, dword ptr [4*ecx+ebx]
which gives eax the value of 4*ecx+ebx
Syntax: MOV dest,src
This is an easy to understand instruction. MOV copies the value from src to dest and src
stays what it was before.
There are some variants of MOV:
MOVS/MOVSB/MOVSW/MOVSD EDI, ESI: Those variants copy the byte/word/dword ESI points to,
to the space EDI points to.
MOVSX: MOVSX expands Byte or Word operands to Word or Dword size and keeps the sign of the
MOVZX: MOVZX expands Byte or Word operands to Word or Dword size and fills the rest of the
space with 0.
Syntax: MUL value
This instruction is the same as IMUL, except that it multiplies unsigned. It can set the
This instruction does absolutely nothing, but slide to the next command.
OR(Logical Inclusive Or)
Syntax: OR dest,src
The OR instruction connects two values using the logical inclusive or.
This instruction clears the O-Flag and the C-Flag and can set the Z-Flag.
To understand OR better, consider those two binary values:
If you OR them, the result is 1101011111
Only when there are two 0 on top of each other, the resulting bit is 0. Else the resulting
bit is 1.
Syntax: POP dest
POP loads the value of byte/word/dword ptr [esp] and puts it into dest. Additionally it
increases the stack by the size of the value that was popped of the stack, so that the next
POP would get the next value.
Syntax: PUSH operand
PUSH is the opposite of POP. It stores a value on the stack and decreases it by the size
of the operand that was pushed, so that ESP points to the value that was PUSHed.
Repeat Following String Instruction: Repeats ins until CX=0 or until indicated condition
(ZF=1, ZF=1, ZF=0, ZF=0) is met. The ins value must be a string operation such as CMPS, INS,
LODS, MOVS, OUTS, SCAS, or STOS.
RET does nothing but return from a part of code that was reached using a CALL instruction.
RET digit cleans the stack before it returns.
Syntax: SUB dest,src
SUB is the opposite of the ADD command. It subtracts the value of src from the value of
dest and stores the result in dest.
SUB can set the Z/O/C flags.
Syntax: TEST operand1, operand2
This instruction is in 99% of all cases used for “TEST EAX, EAX”. It performs a Logical
AND(AND instruction) but does not save the values. It only sets the Z-Flag, when EAX is 0
or clears it, when EAX is not 0. The O/C flags are always cleared.
Syntax: XOR dest,src
The XOR instruction connects two values using logical exclusive OR (remember OR uses
This instruction clears the O-Flag and the C-Flag and can set the Z-Flag.
To understand XOR better, consider those two binary values:
If you OR them, the result is 1100011011
When two bits on top of each other are equal, the resulting bit is 0. Else the resulting
bit is 1.
The most often seen use of XOR is “XOR, EAX, EAX”. This will set EAX to 0, because when
you XOR a value with itself, the result is always 0.