Error reading /dev/mem

Hi, I have a system call that maps virtual to physical addresses. I have disabled CONFIG_STRICT_DEVMEM and am reading from /dev/mem to verify that the contents of the syscall-returned physical address match the contents of the virtual address. It works fine up to a point. Reading beyond byte 935321597 in /dev/mem throws a "bad address" error. Not sure it's relevant but I'm running on virtual box and specifying 1GB of RAM. Tried with 2GB of RAM and hit the same problem. Any idea why I can't read beyond the above point? Thanks.