caching - Predict cache line length using time measure -

hi it's first post here hi everyone.

i have problem predict cache line size in gnu as. wrote program in c calls function written in assembly.

here function

.section .text .section .data  .global time  time: pushl %ebp xor %edx, %edx xor %eax, %eax cpuid rdtsc popl %ebp    ret 

it measure cpu cycles

c code is:

#include <stdio.h>  const int size = 256;  void main(){  unsigned long long cykl, cykl1, cykl2;  unsigned char matrix[size];  char bla;  int i,j,k;      for(i=0 ; i<size; i++)         {         cykl1 = time();         bla = matrix[i];         cykl2 = time();         cykl = cykl2 - cykl1;          printf("i=%d: %lld \n",i, cykl);         }        } 

i ran program, can't see time difference. know cache line lenght 64 bytes.

time should rise every time load next 64bytes of array, right?

i gratefull advice why can't work properly.

i think there 3 problems.

first, program call might not assembly routine, time(2) system call returns current time in seonds. assembly routine name should prefixed underscore, i.e., _time, in *.s file.

you can declare routine __asm__ keyword. see: http://gcc.gnu.org/onlinedocs/gcc-4.8.0/gcc/asm-labels.html

second, memory access might eliminated (or reordered) gcc optimizer. should check assembly code generated c code.

third, rdtsc not serialized. in other words, cpu might reorder access memory , rdtsc insturction, or executed in parallel. should insert instructions prevent reordering.

see description of rdtsc in "intel® 64 , ia-32 architecture software developer's manual volume 2b: instruction set reference, m-z": http://www.intel.com/content/www/us/en/processors/architectures-software-developer-manuals.html