Kernel zeroes memory?

I am using Debian squeeze and have noticed that memory is always zeroed. Is this new in linux distributions ? Some time ago, I believe I could use puts() and garbage would be output.

I run this test program many times but the commented results are always the same. (I have randomize_va_space=2 in sysctl.conf so I know that memory in different locations is being used at each run.)

char *a = malloc(50000000);
a[49999999] = '\0';
puts(a); // it outputs nothing since all are zeroes
printf("%p\n", a);
if(a[5000] == '\0') // this condition is always true
{
    puts("It is a nul char.");
}

Is it possible to make the system not zero memory ? What options could this Debian squeeze installation have activated that always zero memory ?