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x86 assembly

x86 assembly. JongBeom Lim. Reference. Tom Shanley , X86 Instruction Set Architecture: Comprehensive 32- and 64- Bit Coverage, MindShare Press, 2009 http://www.mindshare.com Jon Erickson, Hacking: the art of exploitation, No Starch Press, 2008. x86 Instruction Basics.

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x86 assembly

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  1. x86 assembly JongBeom Lim

  2. Reference • Tom Shanley, X86 Instruction Set Architecture: Comprehensive 32- and 64- Bit Coverage, MindShare Press, 2009 • http://www.mindshare.com • Jon Erickson, Hacking: the art of exploitation, No Starch Press, 2008

  3. x86 Instruction Basics • x86 is a CISC architecture with hundreds of instructions defined • General instruction categories • General Purpose instructions • Floating-Point instructions • Program Flow-related instructions • Hardware-related instructions • Simple x86 instructions: • MOV AX, BX • ADD AX, CX 15 0 • (BX => AX) • 2Ch (AX) • + 55h (CX) • --------------- • 81h (AX) • (CX+AX => AX) • 81 • ADD CX, AX (AX+CX => CX) Note: intel syntax (destination, source)

  4. Typical Instruction Variants • A number of the x86 instructions have multiple variants. Here are a few variants of the MOV and ADD instructions as examples: • MOV SI, 7000h ; move immediate to register • MOV BX, AX ; move ax register to bx register • MOV BX, [SI] ; move memory data to register • MOV [SI], BX ; move register data to memory • ADD AX, BX ; register to register add • ADD AX, [BX] ; memory to register add • ADD [BX], AX ; register to memory add • ADD AX, 20 ; immediate to register add • MOV 7000h, SI • MOV [7000h], SI √ MOVS [SI], [BX] ; moves data from one memory location to another • MOV [SI], [BX] Note: intel syntax (destination, source)

  5. Subset of Instructions • A small sampling of the general-purpose and floating-point x86 instructions are shown below: • ADD • SUB • INC • DEC • MUL • DIV • XOR • NOT • NEG • SHL • MOV • MOVS • XCHG • FADD • FMULP • FSIN • FXSAVE • PADDB • PAVGW • PCMPEQB • ADDPS • SUBSD • MULPD • DIVSS • CVTPI2PD • SQRTPS MMX instructions x87 instructions • [ ] • BX SSE instructions • AX • BX • ( ? * BX) • DX : AX SIMD operations (Single Instruction Multiple Data) Upper 16 bits Lower 16 bits http://www.intel.com/content/www/us/en/processors/architectures-software-developer-manuals.html

  6. Subset of Instructions (Continued) • Here is a small subset of the x86 instructions related to program flow: • JMP • JNZ • LOOP • CALL • RET • INT • IRET / 1 / 0 IP DEC CX ; CX = 2 LOOP Lable1 JNZ Label 1

  7. Instruction Composition Average instruction size: 32-bit code: 3.4 bytes 64-bit code: 3.8 bytes

  8. Traditional x86 Register Set

  9. x86-64 Register Set

  10. General-Purpose Registers (GPRs) (Accumulator) (Base) (Counter) (Data) • “L” means the Low byte • “H” means the High byte

  11. General-Purpose Registers (GPRs)

  12. General-Purpose Registers (GPRs) R8D (“D” means a Doubleword access) Dword = 4 bytes R8W (“W” means a Word access) Word = 2 bytes R8W (“B” means a Bord access) Quadword = Qword = 8 bytes

  13. Sized Operations with GPRs • 16-bit Operations • ADD AX, BX • ADD AX, R8W • ADD R9W, R8W • 64-bit Operations • ADD RAX, RBX • ADD RAX, R8 • ADD RAX, RAX • 8-bit Operations • ADD AL, BL • ADD AL, AH • ADD R8B, AL • ADD R8B, AH • 32-bit Operations • ADD EAX, EBX • ADD EAX, R8D • ADD R9W, EAX You cannot perform a byte operation using a high register (AH, BH, CH, DH) and one of the new registers (R8B – R15B). The operand sizes must match on most x86 instructions

  14. GPR Behavior 63 31 15 7 0 63 31 15 7 0 • ADD AL, 24h • ADD AH, 96h 63 31 15 7 0 63 31 15 7 0 • ADD AX, 96h 63 31 15 7 0 63 31 15 7 0 • ADD EAX, 242424h 63 31 15 7 0 63 31 15 7 0 The upper 32 bits are cleared when Performing a Dword operation.

  15. X86 is Little Endian • Little Endian – the least significant byte goes in the littlest address • movesi, 4000h • mov ax, 1234h • mov [esi], ax Memory • 4003h • 4002h • 4001h • 4000h 63 31 15 7 0 • 3FFFh • 3FFEh AH AL • 3FFDh AX • 3FFCh EAX RAX

  16. 0x250 Getting You Hands Dirty (Demo) #include <stdio.h> int main() { inti; for(i=0; i < 10; i++) { printf("Hello World!\n"); } }

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