1 / 37

C Programming and Assembly Language

C Programming and Assembly Language. Janakiraman V – jramaanv@yahoo.com NITK Surathkal 2 nd August 2014. Motivation. Do you know how all this is implemented in assembly?. Agenda. Brief introduction to the 8086 processor architecture Describe commonly used assembly instructions

sissy
Download Presentation

C Programming and Assembly Language

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. C Programming and Assembly Language Janakiraman V – jramaanv@yahoo.com NITK Surathkal 2nd August 2014

  2. Motivation Do you know how all this is implemented in assembly?

  3. Agenda • Brief introduction to the 8086 processor architecture • Describe commonly used assembly instructions • Use of stack and related instructions • Translate high level function calls into low level assembly language • Familiarize the calling conventions • Explain how variables are passed and accessed

  4. 8086 Architecture • ALU – Arithmetic and Logical unit – The heart of the processor • Control Unit – Decodes instructions, Controls the execution flow • Registers – Implicit memory locations within the processor • Registers – Serve as arguments to most operations • Flags – All ALU operations will set particular bits after execution

  5. Registers • EAX – Stores integer return values • ECX – Stores the counters for loops and also stores “THIS” pointer • EIP –Instruction pointer. Stores the address of the next instruction to be executed • ESP – The Stack pointer. Implicitly changed during Call/ Ret instructions. • EBP – Base pointer. Used to access local variables and function parameters.

  6. Registers Contd… • EBX – A general purpose register • ESI– The source index register for string instructions • EDI - The destination index registers for string instructions • EFL – Flag register. Stores the flag bits of various flags like Carry, Zero, etc. • Segment registers point to a segment of memory. EDS, ESS, EES, ECS • EDX – Stores high 32 bits of 64 bit values

  7. Instruction Set • Data transfer • Arithmetic and logical • Stack Operations • Branching and Looping • Function calls • String Instructions • Prefix to instructions

  8. Data transfer instructions • MOV Destination, Source - Format • Data transfer is always from RIGHT to LEFT. • Source Register is unaffected. • LEA – Load effective address. • Loads the offset Address of the specified variable into the destination. • Equivalent of int y = &x;

  9. Arithmetic and Logical instructions • Operation destination, source – Format • ADD AX, BX • SUB AX, [BX] • OR AX, [BX+4] • XOR AX, AX – Fastest way to clear registers

  10. Exercise 1 int x=4, y=6, a=3, b=2; __asm { MOV EAX, x MUL y ADD EAX, a SUB EAX, b MOV EBX, x XOR EBX, y MOV ECX, a AND ECX, b OR EBX, ECX } • Write an assembly program to evaluate the following expression. (All variables are 32 bit integers) • EAX = x*y + a – b • EBX =( x^y) | ( a&b)

  11. Branching and Looping • JMP Addr – Loads EIP with Addr • Conditional Jumps • Transfers control based on a condition • Based on state of one or more flags • ALU operation sets flags

  12. Exercise 2 Multiplication by repeated addition. int x =9, y=10, z=0; __asm { XOR EAX, EAX MOV EBX, y MULT: ADD EAX, x DEC EBX JNZ MULT MOV z, EAX } String length of a constant string char* pChar = “Test data"; MOV EDI, pChar XOR ECX, ECX COMPARE: CMP [EDI], 0 JNZ INCREASE JMP DONE INCREASE: INC ECX INC EDI JMP COMPARE DONE: MOV len, ECX • Write an assembly program to evaluate the expression “ z = x * y ”using • Repeated addition • MUL instruction • Write an assembly program to calculate the string length of a constant string

  13. Stack Operations • PUSH: PUSH EAX • ESP decreases by 4/ 2/ 1 • Data is moved on to top of stack • Used extensively to pass parameters to functions. • POP: POP EAX • ESP increases 4/ 2/ 1 • Data is copied to the destination • Compliment of PUSH

  14. Exercise 3 • Write an assembly program to swap two integers x and y. • Write a C program to swap two numbers using a function Swap(int* pX, int* pY). Implement the Swap function directly in assembly language Swap two integers. int x=4, y=5; __asm { PUSH x PUSH y POP x POP y } Function to swap variables void swap(int* pX, int* pY) { __asm { MOV EAX, pX MOV EBX, pY PUSH DWORD PTR [EAX] PUSH DWORD PTR [EBX] POP DWROD PTR [EAX] POP DWORD PTR [EBX] } }

  15. Function calls • CALL – CALL ADDR • Used for function calls. • Implicitly pushes the EIP on to the stack. • Reads the address specified (ADDR) and loads EIP with ADDR. • RET – RET n • Used to return to the calling function. • Implicitly pops the DWORD on the TOS into EIP. • ‘n’ Specifies the number to be added to ESP after returning. Used for stack clean up.

  16. int g_iVar = 5; void main() { int z=0; z = Fn(2,4); g_iVar = z; } int Fn(int x, int y) { int z=0; z = x+ y return z; } Compile the C program!!

  17. C and assembly language - FAQ • How are function calls in ‘C’ translated into assembly? • How are parameters passed to the function? • What does it mean to say local variables are stored on stack? Scope of local variables! • How are global variables accessed?

  18. C and Assembly language Contd…. • Cannot pass many parameters in registers • Scope – Desirable feature • Stack – Ideal to store local variables • ESP cannot be used to access the local variables • EBP is used to access them!!!

  19. Parameters, Local and Global variables • Before a function is called parameters are pushed onto stack • Parameters are accessed by [EBP +n] • Local variables are accessed by [EBP –n] • Integers are returned in EAX • Global variables are accessed by direct address values

  20. void main() { int z=0; MOV z, 0 z = Fn(2,4); PUSH 0x00000004 PUSH 0x00000002 CALL Fn MOV z, EAX g_iVal = z; MOV [g_iVar], EAX } int Fn(int x, int y) { int z=0; MOV z, 0 z = x+ y; MOV EAX, x ADD EAX, y MOV z, EAX return z; RET } Compile the C program Contd…

  21. CODE SEGMENT – Function – main() . int z = 0; C100 MOV [EBP-4], 0 z = Fn(2,4); C101 PUSH 0x00000004 C102 PUSH 0x00000002 C103 Call C200 C104 MOV [EBP-4], EAX g_iVar = z; C105 MOV [g_iVar], EAX . . STACK SEGMENT Compile the C Program Contd…. ESP ESP ESP ESP ESP EBP

  22. CODE SEGMENT – Function – Fn() C200 MOV EBP, ESP C201 SUB ESP, 0x40 int z=0; C202 MOV [EBP-4], 0 z = x+ y C203 MOV EAX, [EBP+4] C204 ADD EAX, [EBP+8] C205 MOV [EBP-4], EAX return z; C206 ADD ESP, 0x40 C206 RET STACK SEGMENT Compile the C Program Contd…. ESP Local variable space Z ESP ESP EBP EBP

  23. CODE SEGMENT – Function – main() . int z = 0; C100 MOV [EBP-4], 0 z = Fn(2,4); C101 PUSH 0x00000004 C102 PUSH 0x00000002 C103 Call C200 C104 MOV [EBP-4], EAX g_iVar = z; C105 MOV [g_iVar], EAX C106 RET STACK SEGMENT Stack corruption!!!!! You have accessed the stack of the function “Fn()” You computer will now REBOOT!!!!! EBP ESP

  24. CODE SEGMENT – Function – main() . int z = 0; C100 MOV [EBP-4], 0 z = Fn(2,4); C101 PUSH 0x00000004 C102 PUSH 0x00000002 C103 Call C200 C104 MOV [EBP-4], EAX g_iVar = z; C105 MOV [g_iVar], EAX . . STACK SEGMENT Compile the C Program Contd…. ESP ESP ESP ESP ESP EBP

  25. CODE SEGMENT – Function – Fn() C200 PUSH EBP C202 MOV EBP, ESP C203 SUB ESP, 0x40 int z=0; C204 MOV [EBP-4], 0 z = x+ y C205 MOV EAX, [EBP+8] C206 ADD EAX, [EBP+12] C207 MOV [EBP-4], EAX return z; C208 ADD ESP, 0x40 C209 POP EBP C20A RET 8 STACK SEGMENT Compile the C Program Contd…. ESP Local variable space Z ESP ESP ESP ESP EBP EBP EBP

  26. CODE SEGMENT – Function – main() . int z = 0; C100 MOV [EBP-4], 0 z = Fn(2,4); C101 PUSH 0x00000004 C102 PUSH 0x00000002 C103 Call C200 C104 MOV [EBP-4], EAX g_iVar = z; C105 MOV [g_iVar], EAX C106 Epilogue STACK SEGMENT ESP ESP ESP EBP

  27. Function calls in C - Summary • Function call gets translated to CALL addr • Prologue • Store the current EBP on stack • Set up the stack - Initialize the EBP • Allocate space for local variables. • Execute the function accordingly • Epilogue • Set the ESP to its original value • Set the EBP back to its original value

  28. Stack clean up • When? • Happens after returning from a function • Why? • Undo the effect of pushing parameters • How? • RET N or ADD ESP, N

  29. void main() { int z = 0; z = Function(2, 4); } /*Contd……*/ Prologue MOV [EBP-4], 0 PUSH 0x00000004 PUSH 0x00000002 CALL Function MOV [EBP-4], EAX Epilogue Contd…… C Program Assembly Contd…

  30. int Function(int a, int b) { int c=0; c = a + b; return c; } PUSH EBP MOV EBP, ESP --------- Prologue SUB ESP, N MOV [EBP-4], 0 MOV EAX, [EBP + 8] --- Body ADD EAX, [EBP+12] MOV [EBP-4], EAX ADD ESP, N POP EBP ----------------- Epilogue RET 8 C Program Assembly Contd…

  31. Calling conventions • __cdecl • Default calling convention of C functions • Needed for variable argument list • Caller cleans the stack - ADD ESP, N instruction • __stdcall • Faster than the __cdecl call. • Callee cleans the stack - RET N instruction • Contd……

  32. Back to Exercise 3 Write a C program to swap two numbers using a function Swap(int* pX, int* pY). Implement the Swap function directly in assembly language Function to swap variables void swap(int* pX, int* pY) { __asm { PUSH DWORD PTR [[EBP+4]] PUSH DWORD PTR [[EBP+8]] POP DWROD PTR [[EBP+4]] POP DWORD PTR [[EBP+8]] } } Function to swap variables void swap(int* pX, int* pY) { __asm { PUSH DWORD PTR [pX] PUSH DWORD PTR [pY] POP DWROD PTR [pX] POP DWORD PTR [pY] } } Function to swap variables void swap(int* pX, int* pY) { __asm { MOV DWORD PTREAX, [EBP+4] MOV DWORD PTREBX, [EBP+8] PUSH DWORD PTR [EAX] PUSH DWORD PTR [EBX] POP DWROD PTR [EAX] POP DWORD PTR [EBX] } } Double indirection is not a valid instruction

  33. struct stTest { int x; int y; }; void FnTest(stTest* pSt) { pSt->x = 0; pSt->y = 1; } void main() { stTest obj; FnTest(&obj); } class clsTest { int x; int y; public: void FnTest() { x = 0; y=1; } }; void main() { clsTest obj; obj.FnTest(); } What about C++?

  34. Calling convention Contd… • this call – The C++ calling convention • Behaves like the __cdecl call in most ways • This pointer is passed in the ECX register • Stores the this pointer in [EBP-4] location on stack

  35. String Instructions • Uses ESI, EDI as its operands. • After the operation ESI and EDI are automatically Incremented/ Decremented depending on the direction flag. • Usually used with the Prefix instructions. • Very efficient for standard looping instructions.

  36. Prefix to instructions • REP – REP MOVSB • Used to repeat instructions unconditionally • Implicitly decrements ECX by 1 after each execution • Stops once ECX = 0 • REPNE/ REPE – REPE SCASB • Used to repeat instructions conditionally • Implicitly decrements ECX by 1 after each execution • Stops once ECX = 0 or ZERO flag is set/ reset

  37. Optimized C functions • Memcpy • Strlen • Memset

More Related