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• Diagonal Output of Assembly programe

  - by Yousuf Umar

  i have this assembly programe and i want to diagonal ouptut of this programe but i dont know how to put tabspace in assembly section .text global _start ;must be declared for using gcc _start: ;tell linker entry point mov edx, len ;message length mov ecx, msg ;message to write mov ebx, 1 ;file descriptor (stdout) mov eax, 4 ;system call number (sys_write) int 0x80 ;call kernel mov eax, 1 ;system call number (sys_exit) int 0x80 ;call kernel section .data msg db 'Y',10,'O',10,'U',10,'S',10,'U',10,'F' ;our dear string len equ $ - msg ;length of our dear string output of my programe is Y O U S U F output shoud like this Y O U S U F or is there any other way to write this programe and get this output

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• No Program Entry Point TASM Error

  - by Nathan Campos

  I'm trying to develop a simple kernel using TASM, using this code: ; beroset.asm ; ; This is a primitive operating system. ; ;********************************************************************** code segment para public use16 '_CODE' .386 assume cs:code, ds:code, es:code, ss:code org 0 Start: mov ax,cs mov ds,ax mov es,ax mov si,offset err_msg call DisplayMsg spin: jmp spin ;**************************************************************************** ; DisplayMsg ; ; displays the ASCIIZ message to the screen using int 10h calls ; ; Entry: ; ds:si ==> ASCII string ; ; Exit: ; ; Destroyed: ; none ; ; ;**************************************************************************** DisplayMsg proc push ax bx si cld nextchar: lodsb or al,al jz alldone mov bx,0007h mov ah,0eh int 10h jmp nextchar alldone: pop si bx ax ret DisplayMsg endp err_msg db "Operating system found and loaded.",0 code ends END Then I compile it like this: C:\DOCUME~1\Nathan\Desktop tasm /la /m2 beroset.asm Turbo Assembler Version 4.1 Copyright (c) 1988, 1996 Borland International Assembling file: beroset.asm Error messages: None Warning messages: None Passes: 2 Remaining memory: 406k C:\DOCUME~1\Nathan\Desktop tlink beroset, loader.bin Turbo Link Version 7.1.30.1. Copyright (c) 1987, 1996 Borland International Fatal: No program entry point C:\DOCUME~1\Nathan\Desktop What can I to correct this error?

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• nasm/yasm arguments, linkage to C++

  - by arionik

  Hello everybody, I've got a question concerning nasm and its linkage to C++. I declare a litte test function as extern "C" void __cdecl myTest( byte i1, byte i2, int stride, int *width ); and I call it like this: byte i1 = 1, i2 = 2; int stride = 3, width = 4; myTest( i1, i2, stride, &width ); the method only serves to debug assembly and have a look at how the stack pointer is used to get the arguments. beyond that, the pointer arguments value shall be set to 7, to figure out how that works. This is implemented like this: global _myTest _myTest: mov eax, [esp+4] ; 1 mov ebx, [esp+8] ; 2 mov ecx, dword [esp+16] ; width mov edx, dword [esp+12] ; stride mov eax, dword [esp+16] mov dword [eax], 7 ret and compiled via yasm -f win32 -g cv8 -m x86 -o "$(IntDir)\$(InputName).obj" "$(InputPath)" , then linked to the c++ app. In debug mode, everything works fine. the function is called a couple of times and works as expected, whereas in release mode the function works once, but subsequent programm operations fail. It seems to me that something's wrong with stack/frame pointers, near/far, but I'm quite new to this subject and need a little help. thanks in advance! a.

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• Am I Writing Assembly Or NASM?

  - by cam

  I'm fed up with this. I've been trying to just get a grip on assembly for awhile, but I feel like I'm coding towards my compiler rather than a language. I've been using this tutorial, and so far it's giving me hell. I'm using NASM, which may be the problem, but I figured it was the most popular one. I'm simply trying to learn the most general form of assembly, so I decided to learn x86. I keep running into stupid errors, like not being able to increment a variable. Here's the latest one: not being able to use div. mov bx, 0; mov cx, 0; jmp start; start: inc cx; mov ax, cx; div 3; <-- invalid combination of opcode and operand cmp ah,0; jz totalvalue; mov ax, cx; div 5; <-- invalid combination of opcode and operand cmp ah, 0; jz totalvalue; cmp cx, 1000; jz end; totalvalue: add bx,cx; jmp start; jmp end; end: mov ah,4ch; mov al,00; int 21h; Should I change compilers? It seems like division should be standard. Do I need to read two tutorials (one on NASM, and one on x86?). Any specific help on this problem?

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• Intel Assembly Programming

  - by Kay

  class MyString{ char buf[100]; int len; boolean append(MyString str){ int k; if(this.len + str.len>100){ for(k=0; k<str.len; k++){ this.buf[this.len] = str.buf[k]; this.len ++; } return false; } return true; } } Does the above translate to: start: push ebp ; save calling ebp mov ebp, esp ; setup new ebp push esi ; push ebx ; mov esi, [ebp + 8] ; esi = 'this' mov ebx, [ebp + 14] ; ebx = str mov ecx, 0 ; k=0 mov edx, [esi + 200] ; edx = this.len append: cmp edx + [ebx + 200], 100 jle ret_true ; if (this.len + str.len)<= 100 then ret_true cmp ecx, edx jge ret_false ; if k >= str.len then ret_false mov [esi + edx], [ebx + 2*ecx] ; this.buf[this.len] = str.buf[k] inc edx ; this.len++ aux: inc ecx ; k++ jmp append ret_true: pop ebx ; restore ebx pop esi ; restore esi pop ebp ; restore ebp ret true ret_false: pop ebx ; restore ebx pop esi ; restore esi pop ebp ; restore ebp ret false My greatest difficulty here is figuring out what to push onto the stack and the math for pointers. NOTE: I'm not allowed to use global variables and i must assume 32-bit ints, 16-bit chars and 8-bit booleans.

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• How does loop address alignment affect the speed on Intel x86_64?

  - by Alexander Gololobov

  I'm seeing 15% performance degradation of the same C++ code compiled to exactly same machine instructions but located on differently aligned addresses. When my tiny main loop starts at 0x415220 it's faster then when it is at 0x415250. I'm running this on Intel Core2 Duo. I use gcc 4.4.5 on x86_64 Ubuntu. Can anybody explain the cause of slowdown and how I can force gcc to optimally align the loop? Here is the disassembly for both cases with profiler annotation: 415220 576 12.56% |XXXXXXXXXXXXXX 48 c1 eb 08 shr $0x8,%rbx 415224 110 2.40% |XX 0f b6 c3 movzbl %bl,%eax 415227 0.00% | 41 0f b6 04 00 movzbl (%r8,%rax,1),%eax 41522c 40 0.87% | 48 8b 04 c1 mov (%rcx,%rax,8),%rax 415230 806 17.58% |XXXXXXXXXXXXXXXXXXX 4c 63 f8 movslq %eax,%r15 415233 186 4.06% |XXXX 48 c1 e8 20 shr $0x20,%rax 415237 102 2.22% |XX 4c 01 f9 add %r15,%rcx 41523a 414 9.03% |XXXXXXXXXX a8 0f test $0xf,%al 41523c 680 14.83% |XXXXXXXXXXXXXXXX 74 45 je 415283 ::Run(char const*, char const*)+0x4b3 41523e 0.00% | 41 89 c7 mov %eax,%r15d 415241 0.00% | 41 83 e7 01 and $0x1,%r15d 415245 0.00% | 41 83 ff 01 cmp $0x1,%r15d 415249 0.00% | 41 89 c7 mov %eax,%r15d 415250 679 13.05% |XXXXXXXXXXXXXXXX 48 c1 eb 08 shr $0x8,%rbx 415254 124 2.38% |XX 0f b6 c3 movzbl %bl,%eax 415257 0.00% | 41 0f b6 04 00 movzbl (%r8,%rax,1),%eax 41525c 43 0.83% |X 48 8b 04 c1 mov (%rcx,%rax,8),%rax 415260 828 15.91% |XXXXXXXXXXXXXXXXXXX 4c 63 f8 movslq %eax,%r15 415263 388 7.46% |XXXXXXXXX 48 c1 e8 20 shr $0x20,%rax 415267 141 2.71% |XXX 4c 01 f9 add %r15,%rcx 41526a 634 12.18% |XXXXXXXXXXXXXXX a8 0f test $0xf,%al 41526c 749 14.39% |XXXXXXXXXXXXXXXXXX 74 45 je 4152b3 ::Run(char const*, char const*)+0x4c3 41526e 0.00% | 41 89 c7 mov %eax,%r15d 415271 0.00% | 41 83 e7 01 and $0x1,%r15d 415275 0.00% | 41 83 ff 01 cmp $0x1,%r15d 415279 0.00% | 41 89 c7 mov %eax,%r15d

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• help me improve my sse yuv to rgb ssse3 code

  - by David McPaul

  Hello, I am looking to optimise some sse code I wrote for converting yuv to rgb (both planar and packed yuv functions). i am using SSSE3 at the moment but if there are useful functions from later sse versions thats ok. I am mainly interested in how I would work out processor stalls and the like. Anyone know of any tools that do static analysis of sse code? ; ; Copyright (C) 2009-2010 David McPaul ; ; All rights reserved. Distributed under the terms of the MIT License. ; ; A rather unoptimised set of ssse3 yuv to rgb converters ; does 8 pixels per loop ; inputer: ; reads 128 bits of yuv 8 bit data and puts ; the y values converted to 16 bit in xmm0 ; the u values converted to 16 bit and duplicated into xmm1 ; the v values converted to 16 bit and duplicated into xmm2 ; conversion: ; does the yuv to rgb conversion using 16 bit integer and the ; results are placed into the following registers as 8 bit clamped values ; r values in xmm3 ; g values in xmm4 ; b values in xmm5 ; outputer: ; writes out the rgba pixels as 8 bit values with 0 for alpha ; xmm6 used for scratch ; xmm7 used for scratch %macro cglobal 1 global _%1 %define %1 _%1 align 16 %1: %endmacro ; conversion code %macro yuv2rgbsse2 0 ; u = u - 128 ; v = v - 128 ; r = y + v + v >> 2 + v >> 3 + v >> 5 ; g = y - (u >> 2 + u >> 4 + u >> 5) - (v >> 1 + v >> 3 + v >> 4 + v >> 5) ; b = y + u + u >> 1 + u >> 2 + u >> 6 ; subtract 16 from y movdqa xmm7, [Const16] ; loads a constant using data cache (slower on first fetch but then cached) psubsw xmm0,xmm7 ; y = y - 16 ; subtract 128 from u and v movdqa xmm7, [Const128] ; loads a constant using data cache (slower on first fetch but then cached) psubsw xmm1,xmm7 ; u = u - 128 psubsw xmm2,xmm7 ; v = v - 128 ; load r,b with y movdqa xmm3,xmm0 ; r = y pshufd xmm5,xmm0, 0xE4 ; b = y ; r = y + v + v >> 2 + v >> 3 + v >> 5 paddsw xmm3, xmm2 ; add v to r movdqa xmm7, xmm1 ; move u to scratch pshufd xmm6, xmm2, 0xE4 ; move v to scratch psraw xmm6,2 ; divide v by 4 paddsw xmm3, xmm6 ; and add to r psraw xmm6,1 ; divide v by 2 paddsw xmm3, xmm6 ; and add to r psraw xmm6,2 ; divide v by 4 paddsw xmm3, xmm6 ; and add to r ; b = y + u + u >> 1 + u >> 2 + u >> 6 paddsw xmm5, xmm1 ; add u to b psraw xmm7,1 ; divide u by 2 paddsw xmm5, xmm7 ; and add to b psraw xmm7,1 ; divide u by 2 paddsw xmm5, xmm7 ; and add to b psraw xmm7,4 ; divide u by 32 paddsw xmm5, xmm7 ; and add to b ; g = y - u >> 2 - u >> 4 - u >> 5 - v >> 1 - v >> 3 - v >> 4 - v >> 5 movdqa xmm7,xmm2 ; move v to scratch pshufd xmm6,xmm1, 0xE4 ; move u to scratch movdqa xmm4,xmm0 ; g = y psraw xmm6,2 ; divide u by 4 psubsw xmm4,xmm6 ; subtract from g psraw xmm6,2 ; divide u by 4 psubsw xmm4,xmm6 ; subtract from g psraw xmm6,1 ; divide u by 2 psubsw xmm4,xmm6 ; subtract from g psraw xmm7,1 ; divide v by 2 psubsw xmm4,xmm7 ; subtract from g psraw xmm7,2 ; divide v by 4 psubsw xmm4,xmm7 ; subtract from g psraw xmm7,1 ; divide v by 2 psubsw xmm4,xmm7 ; subtract from g psraw xmm7,1 ; divide v by 2 psubsw xmm4,xmm7 ; subtract from g %endmacro ; outputer %macro rgba32sse2output 0 ; clamp values pxor xmm7,xmm7 packuswb xmm3,xmm7 ; clamp to 0,255 and pack R to 8 bit per pixel packuswb xmm4,xmm7 ; clamp to 0,255 and pack G to 8 bit per pixel packuswb xmm5,xmm7 ; clamp to 0,255 and pack B to 8 bit per pixel ; convert to bgra32 packed punpcklbw xmm5,xmm4 ; bgbgbgbgbgbgbgbg movdqa xmm0, xmm5 ; save bg values punpcklbw xmm3,xmm7 ; r0r0r0r0r0r0r0r0 punpcklwd xmm5,xmm3 ; lower half bgr0bgr0bgr0bgr0 punpckhwd xmm0,xmm3 ; upper half bgr0bgr0bgr0bgr0 ; write to output ptr movntdq [edi], xmm5 ; output first 4 pixels bypassing cache movntdq [edi+16], xmm0 ; output second 4 pixels bypassing cache %endmacro SECTION .data align=16 Const16 dw 16 dw 16 dw 16 dw 16 dw 16 dw 16 dw 16 dw 16 Const128 dw 128 dw 128 dw 128 dw 128 dw 128 dw 128 dw 128 dw 128 UMask db 0x01 db 0x80 db 0x01 db 0x80 db 0x05 db 0x80 db 0x05 db 0x80 db 0x09 db 0x80 db 0x09 db 0x80 db 0x0d db 0x80 db 0x0d db 0x80 VMask db 0x03 db 0x80 db 0x03 db 0x80 db 0x07 db 0x80 db 0x07 db 0x80 db 0x0b db 0x80 db 0x0b db 0x80 db 0x0f db 0x80 db 0x0f db 0x80 YMask db 0x00 db 0x80 db 0x02 db 0x80 db 0x04 db 0x80 db 0x06 db 0x80 db 0x08 db 0x80 db 0x0a db 0x80 db 0x0c db 0x80 db 0x0e db 0x80 ; void Convert_YUV422_RGBA32_SSSE3(void *fromPtr, void *toPtr, int width) width equ ebp+16 toPtr equ ebp+12 fromPtr equ ebp+8 ; void Convert_YUV420P_RGBA32_SSSE3(void *fromYPtr, void *fromUPtr, void *fromVPtr, void *toPtr, int width) width1 equ ebp+24 toPtr1 equ ebp+20 fromVPtr equ ebp+16 fromUPtr equ ebp+12 fromYPtr equ ebp+8 SECTION .text align=16 cglobal Convert_YUV422_RGBA32_SSSE3 ; reserve variables push ebp mov ebp, esp push edi push esi push ecx mov esi, [fromPtr] mov edi, [toPtr] mov ecx, [width] ; loop width / 8 times shr ecx,3 test ecx,ecx jng ENDLOOP REPEATLOOP: ; loop over width / 8 ; YUV422 packed inputer movdqa xmm0, [esi] ; should have yuyv yuyv yuyv yuyv pshufd xmm1, xmm0, 0xE4 ; copy to xmm1 movdqa xmm2, xmm0 ; copy to xmm2 ; extract both y giving y0y0 pshufb xmm0, [YMask] ; extract u and duplicate so each u in yuyv becomes u0u0 pshufb xmm1, [UMask] ; extract v and duplicate so each v in yuyv becomes v0v0 pshufb xmm2, [VMask] yuv2rgbsse2 rgba32sse2output ; endloop add edi,32 add esi,16 sub ecx, 1 ; apparently sub is better than dec jnz REPEATLOOP ENDLOOP: ; Cleanup pop ecx pop esi pop edi mov esp, ebp pop ebp ret cglobal Convert_YUV420P_RGBA32_SSSE3 ; reserve variables push ebp mov ebp, esp push edi push esi push ecx push eax push ebx mov esi, [fromYPtr] mov eax, [fromUPtr] mov ebx, [fromVPtr] mov edi, [toPtr1] mov ecx, [width1] ; loop width / 8 times shr ecx,3 test ecx,ecx jng ENDLOOP1 REPEATLOOP1: ; loop over width / 8 ; YUV420 Planar inputer movq xmm0, [esi] ; fetch 8 y values (8 bit) yyyyyyyy00000000 movd xmm1, [eax] ; fetch 4 u values (8 bit) uuuu000000000000 movd xmm2, [ebx] ; fetch 4 v values (8 bit) vvvv000000000000 ; extract y pxor xmm7,xmm7 ; 00000000000000000000000000000000 punpcklbw xmm0,xmm7 ; interleave xmm7 into xmm0 y0y0y0y0y0y0y0y0 ; extract u and duplicate so each becomes 0u0u punpcklbw xmm1,xmm7 ; interleave xmm7 into xmm1 u0u0u0u000000000 punpcklwd xmm1,xmm7 ; interleave again u000u000u000u000 pshuflw xmm1,xmm1, 0xA0 ; copy u values pshufhw xmm1,xmm1, 0xA0 ; to get u0u0 ; extract v punpcklbw xmm2,xmm7 ; interleave xmm7 into xmm1 v0v0v0v000000000 punpcklwd xmm2,xmm7 ; interleave again v000v000v000v000 pshuflw xmm2,xmm2, 0xA0 ; copy v values pshufhw xmm2,xmm2, 0xA0 ; to get v0v0 yuv2rgbsse2 rgba32sse2output ; endloop add edi,32 add esi,8 add eax,4 add ebx,4 sub ecx, 1 ; apparently sub is better than dec jnz REPEATLOOP1 ENDLOOP1: ; Cleanup pop ebx pop eax pop ecx pop esi pop edi mov esp, ebp pop ebp ret SECTION .note.GNU-stack noalloc noexec nowrite progbits

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• Performance of C# method polymorphism with generics

  - by zildjohn01

  I noticed in C#, unlike C++, you can combine virtual and generic methods. For example: using System.Diagnostics; class Base { public virtual void Concrete() {Debug.WriteLine("base concrete");} public virtual void Generic<T>() {Debug.WriteLine("base generic");} } class Derived : Base { public override void Concrete() {Debug.WriteLine("derived concrete");} public override void Generic<T>() {Debug.WriteLine("derived generic");} } class App { static void Main() { Base x = new Derived(); x.Concrete(); x.Generic<PerformanceCounter>(); } } Given that any number of versions of Generic<T> could be instantiated, it doesn't look like the standard vtbl approach could be used to resolve method calls, and in fact it's not. Here's the generated code: x.Concrete(); mov ecx,dword ptr [ebp-8] mov eax,dword ptr [ecx] call dword ptr [eax+38h] x.Generic<PerformanceCounter>(); push 989A38h mov ecx,dword ptr [ebp-8] mov edx,989914h call 76A874F1 mov dword ptr [ebp-4],eax mov ecx,dword ptr [ebp-8] call dword ptr [ebp-4] The extra code appears to be looking up a dynamic vtbl according to the generic parameters, and then calling into it. Has anyone written about the specifics of this implementation? How well does it perform compared to the non-generic case?

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• What is Causing This Memory Leak in Delphi?

  - by lkessler

  I just can't figure out this memory leak that EurekaLog is reporting for my program. I'm using Delphi 2009. Here it is: Memory Leak: Type=Data; Total size=26; Count=1; The stack is: System.pas _UStrSetLength 17477 System.pas _UStrCat 17572 Process.pas InputGedcomFile 1145 That is all there is in the stack. EurekaLog is pointing me to the location where the memory that was not released was first allocated. According to it, the line in my program is line 1145 of InputGedcomFile. That line is: CurStruct0Key := 'HEAD' + Level0Key; where CurStruct0Key and Level0Key are simply defined in the procedure as local variables that should be dynamically handled by the Delphi memory manager when entering and leaving the procedure: var CurStruct0Key, Level0Key: string; So now I look at the _UStrCat procedure in the System Unit. Line 17572 is: CALL _UStrSetLength // Set length of Dest and I go to the _UStrSetLength procedure in the System Unit, and the relevant lines are: @@isUnicode: CMP [EAX-skew].StrRec.refCnt,1 // !!! MT safety JNE @@copyString // not unique, so copy SUB EAX,rOff // Offset EAX "S" to start of memory block ADD EDX,EDX // Double length to get size JO @@overflow ADD EDX,rOff+2 // Add string rec size JO @@overflow PUSH EAX // Put S on stack MOV EAX,ESP // to pass by reference CALL _ReallocMem POP EAX ADD EAX,rOff // Readjust MOV [EBX],EAX // Store MOV [EAX-skew].StrRec.length,ESI MOV WORD PTR [EAX+ESI*2],0 // Null terminate TEST EDI,EDI // Was a temp created? JZ @@exit PUSH EDI MOV EAX,ESP CALL _LStrClr POP EDI JMP @@exit where line 17477 is the "CALL _ReallocMem" line. So then what is the memory leak? Surely a simple concatenate of a string constant to a local string variable should not be causing a memory leak. Why is EurekaLog pointing me to the ReallocMem line in a _UStrSetLength routine that is part of Delphi? This is Delphi 2009 and I am using the new unicode strings. Any help or explanation here will be much appreciated.

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• Explanation of the disassembly of the simplest program (x86)

  - by noname

  The following code int _main() {return 0;} Compiled using the command: gcc -s -nostdlib -nostartfiles 01-simple.c -o01-simple.exe gcc version 4.4.1 (TDM-1 mingw32) OllyDbg produced this output: http://imgur.com/g81vK.png Can you explain what happens here? Analysis so far: // these two seems to be an idiom: PUSH EBP // places EBP on stack MOV EBP, ESP // overwrites EBP with ESP MOV EAX, 0 // EAX = 0 LEAVE // == mov esp, ebp // pop ebp // according to // http://en.wikipedia.org/wiki/X86_instruction_listings What is the meaning of all this?

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• [Assembly] jnz after xor?

  - by kotarou3

  After using IDA Pro to disassemble a x86 dll, I found this code (Comments added by me in pusedo-c code. I hope they're correct): test ebx, ebx ; if (ebx == false) jz short loc_6385A34B ; Jump to 0x6385a34b mov eax, [ebx+84h] ; eax = *(ebx+0x84) mov ecx, [esi+84h] ; ecx = *(esi+0x84) mov al, [eax+30h] ; al = *(*(ebx+0x84)+0x30) xor al, [ecx+30h] ; al = al XOR *(*(esi+0x84)+0x30) jnz loc_6385A453 Lets make it simpler for me to understand: mov eax, b3h xor eax, d6h jnz ... How does the conditional jump instruction work after a xor instruction?

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• Endianness inside CPU registers

  - by Abhishek Tamhane

  I need help understanding endianness inside CPU registers of x86 processors. I wrote this small assembly program: section .data section .bss section .text global _start _start: nop mov eax, 0x78FF5ABC mov ebx,'WXYZ' nop ; GDB breakpoint here. mov eax, 1 mov ebx, 0 int 0x80 I ran this program in GDB with a breakpoint on line number 10 (commented in the source above). At this breakpoint, info registers shows the value of eax=0x78ff5abc and ebx=0x5a595857. Since the ASCII codes for W, X, Y, Z are 57, 58, 59, 5A respectively; and intel is little endian, 0x5a595857 seems like the correct byte order (least significant byte first). Why isn't then the output for eax register 0xbc5aff78 (least significant byte of the number 0x78ff5abc first) instead of 0x78ff5abc?

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• Beginner assembly should you put value into register before push?

  - by Daniel

  Which one is better/faster/preferred 1: mov eax, 5 push eax mov eax, [someAddress] push eax 2: push 5 push [someAddress]

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• link nasm program for mac os x

  - by Fry Constantine

  i have some problems with linking nasm program for macos: GLOBAL _start SEGMENT .text _start: mov ax, 5 mov bx, ax mov [a], ebx SEGMENT .data a DW 0 t2 DW 0 fry$ nasm -f elf test.asm fry$ ld -o test test.o -arch i386 ld: warning: in test.o, file was built for unsupported file format which is not the architecture being linked (i386) ld: could not find entry point "start" (perhaps missing crt1. fry$ nasm -f macho test.asm fry$ ld -o test test.o -arch i386 ld: could not find entry point "start" (perhaps missing crt1.o) can anyone help me?

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• First ASM program

  - by Tal

  Hello, I'm trying to run my first ASM 8086 program on MASM on Windows Vista 64bit OS. I put this program on my MASM editor: .model small .stack .data message db "Hello world, I'm learning Assembly !!!", "$" .code main proc mov ax,seg message mov ds,ax mov ah,09 lea dx,message int 21h mov ax,4c00h int 21h main endp end main and the MASM editor gives me this output that I got no idea what's wrong with the program: Assembling: D:\masm32\First.asm D:\masm32\First.asm(9) : error A2004: symbol type conflict D:\masm32\First.asm(19) : warning A4023: with /coff switch, leading underscore required for start address : main _ Assembly Error Where is the problem with this code? This is my first ASM program please remember. Thank you :)

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• Using an array in embedded x86 assembly??

  - by Mark V.

  Hey all I have a method (C++) that returns a character and takes an array of characters as its parameters. I'm messing with assembly for the first time and just trying to return the first character of the array in the dl register. Here's what I have so far: char returnFirstChar(char arrayOfLetters[]) { char max; __asm { push eax push ebx push ecx push edx mov dl, 0 mov eax, arrayOfLetters[0] xor edx, edx mov dl, al mov max, dl pop edx pop ecx pop ebx pop eax } return max; } For some reason this method returns a ? Any idea whats going on? Thanks

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• Printf in assembler doesn't print

  - by Gaim

  Hi there, I have got a homework to hack program using buffer overflow ( with disassambling, program was written in C++, I haven't got the source code ). I have already managed it but I have a problem. I have to print some message on the screen, so I found out address of printf function, pushed address of "HACKED" and address of "%s" on the stack ( in this order ) and called that function. Called code passed well but nothing had been printed. I have tried to simulate the environment like in other place in the program but there has to be something wrong. Do you have any idea what I am doing wrong that I have no output, please? Thanks a lot EDIT: This program is running on Windows XP SP3 32b, written in C++, Intel asm there is the "hack" code CPU Disasm Address Hex dump Command Comments 0012F9A3 90 NOP ;hack begins 0012F9A4 90 NOP 0012F9A5 90 NOP 0012F9A6 89E5 MOV EBP,ESP 0012F9A8 83EC 7F SUB ESP,7F ;creating a place for working data 0012F9AB 83EC 7F SUB ESP,7F 0012F9AE 31C0 XOR EAX,EAX 0012F9B0 50 PUSH EAX 0012F9B1 50 PUSH EAX 0012F9B2 50 PUSH EAX 0012F9B3 89E8 MOV EAX,EBP 0012F9B5 83E8 09 SUB EAX,9 0012F9B8 BA 1406EDFF MOV EDX,FFED0614 ;address to jump, it is negative because there mustn't be 00 bytes 0012F9BD F7DA NOT EDX 0012F9BF FFE2 JMP EDX ;I have to jump because there are some values overwritten by the program 0012F9C1 90 NOP 0012F9C2 0090 00000000 ADD BYTE PTR DS:[EAX],DL 0012F9C8 90 NOP 0012F9C9 90 NOP 0012F9CA 90 NOP 0012F9CB 90 NOP 0012F9CC 6C INS BYTE PTR ES:[EDI],DX ; I/O command 0012F9CD 65:6E OUTS DX,BYTE PTR GS:[ESI] ; I/O command 0012F9CF 67:74 68 JE SHORT 0012FA3A ; Superfluous address size prefix 0012F9D2 2069 73 AND BYTE PTR DS:[ECX+73],CH 0012F9D5 203439 AND BYTE PTR DS:[EDI+ECX],DH 0012F9D8 34 2C XOR AL,2C 0012F9DA 2066 69 AND BYTE PTR DS:[ESI+69],AH 0012F9DD 72 73 JB SHORT 0012FA52 0012F9DF 74 20 JE SHORT 0012FA01 0012F9E1 3120 XOR DWORD PTR DS:[EAX],ESP 0012F9E3 6C INS BYTE PTR ES:[EDI],DX ; I/O command 0012F9E4 696E 65 7300909 IMUL EBP,DWORD PTR DS:[ESI+65],-6F6FFF8D 0012F9EB 90 NOP 0012F9EC 90 NOP 0012F9ED 90 NOP 0012F9EE 31DB XOR EBX,EBX ; hack continues 0012F9F0 8818 MOV BYTE PTR DS:[EAX],BL ; writing 00 behind word "HACKED" 0012F9F2 83E8 06 SUB EAX,6 0012F9F5 50 PUSH EAX ; address of "HACKED" 0012F9F6 B8 3B8CBEFF MOV EAX,FFBE8C3B 0012F9FB F7D0 NOT EAX 0012F9FD 50 PUSH EAX ; address of "%s" 0012F9FE B8 FFE4BFFF MOV EAX,FFBFE4FF 0012FA03 F7D0 NOT EAX 0012FA05 FFD0 CALL EAX ;address of printf This code is really ugly because I am new in assembler and there mustn't be null bytes because of buffer-overflow bug

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• Disassembler that tracks what value is where

  - by Martin C. Martin

  So lately I've been looking at the disassembly of my C++ code, and having to manually track what's in each register, like this: 95: 48 8b 16 mov (%rsi),%rdx ; %rdx = raggedCross.sink 98: 48 8b 42 38 mov 0x38(%rdx),%rax ; %rax = sink.table 9c: 8b 4a 10 mov 0x10(%rdx),%ecx ; %ecx = sink.baseCol 9f: 48 8b 70 50 mov 0x50(%rax),%rsi ; %rsi = table.starts a3: 89 c8 mov %ecx,%eax ; %eax = baseCol a5: 83 c1 1c add $0x1c,%ecx ; %ecx = baseCol + 1 And so on. The comments are mine, added by hand, from looking up the offset of various fields (e.g. sink, table, baseCol, starts) in the C++ classes. It's straight forward to do, but tedius and time consuming: the perfect thing for a program to be doing. gdb seems to know the offset of various fields within a struct: I can do &((Table *)0x1200)-starts and it tells the the right address. So, this information is around. Is there some disassembler that can use this info to annotate the code for me? Failing that, I could write my own. Where does gdb get the offsets?

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• Inline assembler get address of pointer Visual Studio

  - by Joe

  I have a function in VS where I pass a pointer to the function. I then want to store the pointer in a register to further manipulate. How do you do that? I have tried void f(*p) { __asm mov eax, p // try one FAIL __asm mov eax, [p] // try two FAIL __asm mov eax, &p // try three FAIL } Both 1 and 2 are converted to the same code and load the value pointed to. I just want the address. Oddly, option 1 works just fine with integers. void f() { int i = 5; __asm mov eax, i // SUCCESS? }

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• why this assembly program is loaded from the address 0B3D:0000?

  - by viperchaos

  I have seen a assembly program written from a book about assemble: assume cs:code code segment dw 0123h,0456h,0789h,0abch,0defh,0fedh,0cbah,0987h mov bx,0 mov ax,0 mov cx,8 s: add ax,cs:[bx] add bx,2 loop s mov ax,4c00h int 21h code ends end This program's function is to add eight numbers. The author compiled this program in the DOS and use the DEBUG to see how this program be loaded. The author use the R command and got that DS = 0B2DH ES = 0B2D SS = 0B3D CS = 0B3D IP = 0000 And then the author said that this program is loaded from the address 0B3D:0000. I'm a confused that why this program is loaded from the address 0B3D:0000? Is this because the existence of the Program Segment Prefix(PSP)? If the answer is the existence of the PSP, what is in the PSP?

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• Inline assembler getaddress of pointer Visual Studio

  - by Joe

  I have a function in VS where I pass a pointer to the function. I then want to store the pointer in a register to further manipulate. How do you do that? I have tried void f(*p) { __asm mov eax, p // try one FAIL __asm mov eax, [p] // try two FAIL __asm mov eax, &p // try three FAIL } Both 1 and 2 are converted to the same code and load the value pointed to. I just want the address. Oddly, option 1 works just fine with integers. void f() { int i = 5; __asm mov eax, i // SUCCESS? }

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• What am I doing wrong? (Simple Assembly Loop)

  - by sunnyohno

  It won't let me post the picture. Btw, Someone from Reddit.programming sent me over here. So thanks! TITLE MASM Template ; Description ; ; Revision date: INCLUDE Irvine32.inc .data myArray BYTE 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 .code main PROC call Clrscr mov esi, OFFSET myArray mov ecx, LENGTHOF myArray mov eax, 0 L1: add eax, [esi] inc esi loop L1 call WriteInt exit main ENDP END main Results in: -334881242

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• procedure that swaps the bytes (low/high) of a Word variable

  - by Altar

  Hi. I have this procedure that swaps the bytes (low/high) of a Word variable (It does the same stuff as System.Swap function). The procedure works when the compiler optimization is OFF but not when it is ON. Can anybody help me with this? { UNSAFE! IT IS NOW WORKING WHEN COMPILER OPTIMIZATION IS ON ! } procedure SwapWord_NotWorking(VAR TwoBytes: word); asm Mov EBX, TwoBytes Mov AX, [EBX] XCHG AL,AH Mov [EBX], AX end;

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• Why is DivMod Limited to Words (<=65535)?

  - by Andreas Rejbrand

  In Delphi, the declaration of the DivMod function is procedure DivMod(Dividend: Cardinal; Divisor: Word; var Result, Remainder: Word); Thus, the divisor, result, and remainder cannot be grater than 65535, a rather severe limitation. Why is this? Why couldn't the delcaration be procedure DivMod(Dividend: Cardinal; Divisor: Cardinal; var Result, Remainder: Cardinal); The procedure is implemented using assembly, and is therefore probably extremely fast. Would it not be possible for the code PUSH EBX MOV EBX,EDX MOV EDX,EAX SHR EDX,16 DIV BX MOV EBX,Remainder MOV [ECX],AX MOV [EBX],DX POP EBX to be adapted to cardinals? How much slower is the naïve attempt procedure DivModInt(const Dividend: integer; const Divisor: integer; out result: integer; out remainder: integer); begin result := Dividend div Divisor; remainder := Dividend mod Divisor; end; that is not (?) limited to 16-bit integers?

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• Messing with the stack in assembly and c++

  - by user246100

  I want to do the following: I have a function that is not mine (it really doesn't matter here but just to say that I don't have control over it) and that I want to patch so that it calls a function of mine, preserving the arguments list (jumping is not an option). What I'm trying to do is, to put the stack pointer as it was before that function is called and then call mine (like going back and do again the same thing but with a different function). This doesn't work straight because the stack becomes messed up. I believe that when I do the call it replaces the return address. So, I did a step to preserve the return address saving it in a globally variable and it works but this is not ok because I want it to resist to recursitivy and you know what I mean. Anyway, i'm a newbie in assembly so that's why I'm here. Please, don't tell me about already made software to do this because I want to make things my way. Of course, this code has to be compiler and optimization independent. My code (If it is bigger than what is acceptable please tell me how to post it): // A function that is not mine but to which I have access and want to patch so that it calls a function of mine with its original arguments void real(int a,int b,int c,int d) { } // A function that I want to be called, receiving the original arguments void receiver(int a,int b,int c,int d) { printf("Arguments %d %d %d %d\n",a,b,c,d); } long helper; // A patch to apply in the "real" function and on which I will call "receiver" with the same arguments that "real" received. __declspec( naked ) void patch() { _asm { // This first two instructions save the return address in a global variable // If I don't save and restore, the program won't work correctly. // I want to do this without having to use a global variable mov eax, [ebp+4] mov helper,eax push ebp mov ebp, esp // Make that the stack becomes as it were before the real function was called add esp, 8 // Calls our receiver call receiver mov esp, ebp pop ebp // Restores the return address previously saved mov eax, helper mov [ebp+4],eax ret } } int _tmain(int argc, _TCHAR* argv[]) { FlushInstructionCache(GetCurrentProcess(),&real,5); DWORD oldProtection; VirtualProtect(&real,5,PAGE_EXECUTE_READWRITE,&oldProtection); // Patching the real function to go to my patch ((unsigned char*)real)[0] = 0xE9; *((long*)((long)(real) + sizeof(unsigned char))) = (char*)patch - (char*)real - 5; // calling real function (I'm just calling it with inline assembly because otherwise it seems to works as if it were un patched // that is strange but irrelevant for this _asm { push 666 push 1337 push 69 push 100 call real add esp, 16 } return 0; }

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