Accessing raw CPUID information is actually very easy, here is a C++ class for that which works in Windows, Linux and OSX: #ifndef CPUID_H #define CPUID_H #ifdef _WIN32 #include <limits.h> #include <intrin.h> typedef unsigned __int32 uint32_t; #else #include <stdint.h> #endif class CPUID { uint32_t regs[4]; public: explicit CPUID(unsigned i) { #ifdef _WIN32 __cpuid((int *)regs, (int)i); … Read more
In x86-64, 32-bit instructions implicitly zero-extend: bits 32-63 are cleared (to avoid false dependencies). So sometimes that’s why you’ll see odd-looking instructions. (Is mov %esi, %esi a no-op or not on x86-64?) However, in this case the previous mov-load is also 32-bit so the high half of %rax is already cleared. The mov %eax, %eax … Read more
Recent GAS in binutils has a .nops N pseudo-instruction that expands to the requested number of bytes for the target: .nops size[, control]
TL:DR: intptr_t might be best in some cases because the signed-overflow boundary is in the middle of the “non-canonical hole”. Treating a value as negative instead of huge may be better if wrapping from zero to 0xFF…FF or vice versa is possible, but pointer+size for any valid size can’t wrap a value from INT64_MAX to … Read more
You can put the argv array onto the stack and load the address of it into rsi. The first member of argv is a pointer to the program name, so we can use the same address that we load into rdi. xor edx, edx ; Load NULL to be used both as the third ; … Read more
It’s undefined behaviour, of course, to pass arguments not corresponding to the format, so the language cannot tell us why the output changes. We must look at the implementation, what code it produces, and possibly the operating system too. My setup is different from yours, Linux 3.1.10-1.16-desktop x86_64 GNU/Linux (openSuSE 12.1) with gcc-4.6.2. But it’s … Read more
The number of instructions executed in program 1) is high because of linking the program with system library’s at runtime? Yep, dynamic linking plus CRT (C runtime) startup files. used -static and which reduces the count by a factor of 1/10. So that just left the CRT start files, which do stuff before calling main, … Read more
X86-64 system calls use syscall instruction. This instruction saves return address to rcx, and after that it loads rip from IA32_LSTAR MSR. I.e. rcx is immediately destroyed by syscall. This is the reason why rcx had to be replaced for system call ABI. This same syscall instruction also saves rflags into r11, and then masks … Read more
What’s probably happening is that the 32-bit build is using the 80-bit FPU registers to do the calculation and the 64-bit build is using the SIMD operations using 64-bit values, causing a slight discrepancy. Note that both answers agree to 14 decimal places, which is about the best you can hope for with 64-bit floating … Read more
An x86 instruction stream is not self-synchronizing, and can only be unambiguously decoded forward. You need to know a valid start-point to decode. The last byte of an immediate can be a 0x90 which decodes as a nop, or in general a 4-byte immediate or displacement can have byte-sequences that are valid instructions, or whatever … Read more