I read that a 64-bit machine actually uses only 48 bits of address (specifically, I'm using Intel core i7). I would expect that the extra 16 bits (bits 48-63) are irrelevant for the address, and w...

I'm working through Computer Systems, A Programmer's Perspective (3rd edition), and Practice Problem 3.3 contains the following line: movb $0xF, (%ebx) I'm supposed to find out what's wrong with...

I already learned that on the x86-64 platform using any 64-bit register would need a REX prefix, and any address less than 64 bits would require an address-size prefix. On x86-64 bit: E3 rel8 is jr...

I'm learning assembly and facing the following problem: In the function asm_func4: push rbp ; preserve rbp call _strlen ; get rdi strlen mov rdx, rax ; copy strlen to reverse counter sub...

Consider inline assembly like this: uint64_t flags; asm ("pushf\n\tpop %0" : "=rm"(flags) : : /* ??? */); Nonwithstanding the fact that there is probably some kind of intrinsic to get the conten...

typedef struct foo { void (*const t)(struct foo *f); } foo; void t(struct foo *f) { } void (*const myt)(struct foo *f) = t; foo f = {.t = t}; int main(void) { f.t(&f); myt(&f); r...

I have this simple python/numba code: from numba import njit import numba as nb @nb.njit(nb.uint64(nb.uint64)) def popcount(x): b=0 while(x > 0): x &= x - nb.uint64(1) b+=1 return b @...

In a book I read the following: 32-bit processors have 2^32 possible addresses, while current 64-bit processors have a 48-bit address space My expectation was that if it's a 64-bit processor, ...

While watching this talk by Matt Godbolt, I was astonished to see that Clang, if instructed to compile for the Haswell¹ architecture, works out that the following code int foo(int a) { int count =...

I have this simple function that is called from many inner-most loops. uint16_t getBit(uint16_t val, uint16_t n) { return (val & (1 << n)) >> n; } This version of the programs tak...

I've been reverse engineering the EnterCriticalSection function on Windows 10 and found this interesting spin-loop: It goes: lbl_loop: mov ecx, [rsp+60h] mov ecx, [rsp+60h] mov ecx, [rsp+60h] pau...

I am trying to call a function - that should have an absolute address when compiled and linked - from machine code. I am creating a function pointer to the desired function and trying to pass that ...

I am experimenting with the way parameters are passed to a function when compiling C++ code. I tried to compile the following C++ code using the x64 msvc 19.35/latest compiler to see the resulting ...

I debug a program with gdb for fun, with stack guard so it writes the canary to the stack from fs:28h. Out of curiosity, I trying to find the memory address that fs:28h points to. I encounter two p...

My primary development machine is x86_64 while some of my deploy environments are arm7vl (Raspberry Pi). For most Python development, this isn't a problem, but some Python libraries are are only av...

Is there a way to remove the array bounds check in C#? here is what I want to achieve: public static int F(int[] M, int i) { return M[i]; // I can guarantee that [i] will never be outside of [0, ...

I have the following Java code (all arrays are initialized before we call "arrays" and all are of size "arraySize") int arraySize = 64; float[] a; float[] b; float[] result; p...

I have stumbled upon this kernel page: https://www.kernel.org/doc/html/v5.9/x86/x86_64/fsgs.html and wanted to play around with LD_PRELOAD to see whether I can allocate a value at a load-time and r...

I want to replace the lowest byte in an integer. On x86 this is exactly mov al, [mem] but I can't seem to get compilers to output this. Am I missing an obvious code pattern that is recognized, am I...

I have the following code, which doesn't compile with x86_64 GCC 13: #include <iostream> #include <stdfloat> int main() { std::cout << std::float128_t{1} << '\n'; } This ...

given this code lp: vpaddq ymm0, ymm1 vpaddq ymm3, ymm4 add rbx, rax add rcx, rax vpaddq ymm1, ymm2 vpaddq ymm4, ymm5 sub rax, 0xA jge lp according to the https://uica.uops.info this is ...

Consider the following almost leaf function: int almost_leaf(int* x) { if (__builtin_expect(*x >= 0, true)) { return *x; } return x_was_negative() + 1; } It is almost leaf in the sense that...

At the C level, the recommendation is typically to pass anything bigger than word size (8 bytes on x86-64) by pointer, and anything smaller by value (implying by register). The argument is supposed...

According to Wikipedia: A page fault is a trap to the software raised by the hardware when a program accesses a page that is mapped in the virtual address space, but not loaded in physical memor...

Profiling suggests that this function here is a real bottle neck for my application: static inline int countEqualChars(const char* string1, const char* string2, int size) { int r = 0; for (int j...