It is a trick/shortcut. say for example ldr r0,=main what would happen is the assembler would allocate a data word, near the instruction but outside the instruction path ldr r0,main_addr … b somewhere main_addr: .data main Now expand that trick to constants/immediates, esp those that cannot fit into a move immediate instruction: top: add r1,r2,r3 … Read more
You can switch to asm layout in gdb: (gdb) layout asm See here for more information.
Looking at the nasm source, it looks like: ‘oword”https://stackoverflow.com/”DO’ is 8 times as big as “word” (O for “octoword”), synonymous with dqword (“double-quad”); that would be 128 bits, corresponding to the size of an SSE vector register. ‘tword”https://stackoverflow.com/”DT’ is 80 bits (T for “ten bytes”), the full size of an Intel x87 floating point register. … Read more
The problem is that replacing call printf@PLT with call [printf@GOTPLT] requires that the compiler knows that the function printf exists in a shared library and not a static library (or even in just a plain object file). The linker can change call printf into call printf@PLT, jmp printf into jmp printf@PLT or even mov eax, … Read more
This site seems useful.
There are a lot of possibility how to mov 0 in to ax under IA32… lea eax, [0] mov eax, 0FFFF0000h //All constants form 0..0FFFFh << 16 shr ax, 16 //All constants form 16..31 shl eax, 16 //All constants form 16..31 And perhaps the most strange… 🙂 @movzx: movzx eax, byte ptr[@movzx + 6] //Because … Read more
The first is equate, similar to C’s: #define len 2 in that it doesn’t actually allocate any space in the final code, it simply sets the len symbol to be equal to 2. Then, when you use len later on in your source code, it’s the same as if you’re using the constant 2. The … Read more
I always liked the comprehensive explanation by Samael in the following thread: Explanation of the ALIGN MASM directive, How is this directive interpreted by the compiler? Quote: 1. USAGE ALIGN X The ALIGN directive is accompanied by a number (X). This number (X) must be a power of 2. That is 2, 4, 8, 16, … Read more
Check this very complete table of x86 opcodes on x86asm.net. Just CTRL+F and you’re done! Be sure to read the correct line tho, as C8 for example may appear in several locations.
R just stands for “register”. The AMD64 ISA extension added 8 additional general-purpose registers, named R8 through R15. The 64-bit extended versions of the original 8 registers had an R prefix added to them for symmetry. E stands for “extended” or “enhanced”. (Wikipedia says “extended”.) They are the “extended” versions of the 16-bit registers, in … Read more