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| layout | permalink | title |
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| default | /RE101/section1.3/ | Fundamentals |
Go Back to Reverse Engineering Malware 101
Section 1.3: Fundamentals
x86 Assembly Language
The C programming is a high level language interpreted by the compiler that converts code into machine instructions called assembly language. By using a disassembler tool we can get the assembly language of a compiled C program.
The Intel 8086 and 8088 were the first CPUs to have an instruction set that is now commonly referred to as x86. Intel Architecture 32-bit (IA-32) sometimes also called i386 is the 32-bit version of the x86 instruction set architecture.
The x86 architecture is little-endian, meaning that multi-byte values are written least significant byte first.
How we see it:
| A0 | A1 | A2 | A3 |
Stored as Little Endian
| A3 | A2 | A1 | A0 |
Opcodes and Instructions
Each Instruction represents opcodes (hex code) that tell the machine what to do next.
Three categories of instructions:
- Data Movement/Access
- Arithmetic / Logic
- Control-Flow
Common Instructions
- mov, lea (data movement, data access)
- add, sub (arithmetic)
- or, and, xor (Logic)
- shr, shl (Logic)
- ror, rol (Logic)
- jmp, jne, jnz, jnb (Control Flow)
- push, pop, call, leave, enter, ret (Control Flow)
Example below is moving value at 0xaaaaaaaa into ecx.
| Instruction | Opcode |
|---|---|
mov ecx,[0xaaaaaaaa]; |
8B 0D AA AA AA AA |
Use the search page below or open the Search Instructions page to search for functions discussed above
Registers
The image below is what registers will look like in a debugger.
General-Purpose Registers [1]1
| Register | Description |
|---|---|
| EAX | Accumulator Register |
| EBX | Base Register |
| ECX | Counter Register |
| EDX | Data Register |
| ESI | Source Index |
| EDI | Destination Index |
| EBP | Base Pointer |
| ESP | Stack Pointer |
Segment Registers
| Register | Description |
|---|---|
| SS | Stack Segment, Pointer to the stack |
| CS | Code Segment, Pointer to the code |
| DS | Data Segment, Pointer to the data |
| ES | Extra Segment, Pointer to extra data |
| FS | F Segment, Pointer to more extra data |
| GS | G Segment, Pointer to still more extra data |
EFLAGS Register
| ID | Name | Description |
|---|---|---|
| CF | Carry Flag | Set if the last arithmetic operation carried (addition) or borrowed (subtraction) a bit beyond the size of the register. This is then checked when the operation is followed with an add-with-carry or subtract-with-borrow to deal with values too large for just one register to contain |
| PF | Parity Flag | Set if the number of set bits in the least significant byte is a multiple of 2 |
| AF | Adjust Flag | Carry of Binary Code Decimal (BCD) numbers arithmetic operations |
| ZF | Zero Flag | Set if the result of an operation is Zero (0) |
| SF | Sign Flag | Set if the result of an operation is negative |
| TF | Trap Flag | Set if step by step debugging |
| IF | Interruption Flag | Set if interrupts are enabled |
| DF | Direction Flag | Stream direction. If set, string operations will decrement their pointer rather than incrementing it, reading memory backwards |
| OF | Overflow Flag | Set if signed arithmetic operations result in a value too large for the register to contain |
| IOPL | I/O Privilege Level field (2 bits) | I/O Privilege Level of the current process |
| NT | Nested Task flag | Controls chaining of interrupts. Set if the current process is linked to the next process |
| RF | Resume Flag | Response to debug exceptions |
| VM | Virtual-8086 Mode | Set if in 8086 compatibility mode |
| AC | Alignment Check | Set if alignment checking of memory references is done |
| VIF | Virtual Interrupt Flag | Virtual image of IF |
| VIP | Virtual Interrupt Pending flag | Set if an interrupt is pending |
| ID | Identification Flag | Support for CPUID instruction if can be set |
Instruction Pointer
The EIP register contains the address of the next instruction to be executed.
Hello World
Click Image to Enlarge
Calling a Function
Arguments on the Stack
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Local Variables on the Stack
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