No whole phisical memory, no 64bit

[szzsqing1]

My boot is quite different. On my HDD there is not installed any OS. I use only live systems (mainly Knoppix in live mode) run from ISO files copied to HDD. The boot is provided by an installed GRUB2 (made with SystemRescCD) and manually edited grub.cfg file.
But reading your reply I have found the solution.
This was the wrong menuentry in the grub.cfg file:

menuentry "KNOPPIX_V7.6.1DVD-2016-01-16-EN 64b hu London" {
    set ISO=boot-isos/KNOPPIX_V7.6.1DVD-2016-01-16-EN.iso
    loopback loop /$ISO
    linux (loop)/boot/isolinux/linux knoppix64 bootfrom=/dev/sda1/$ISO nostartsound noeject noprompt lang=hu tz=Europe/London --
    initrd (loop)/boot/isolinux/minirt.gz
}

The problem was that I should have used "linux64", not "linux". First I tried "linux64" and "knoppix64" but the system could not started. So here is the good version:

menuentry "KNOPPIX_V7.6.1DVD-2016-01-16-EN 64b hu London" {
    set ISO=boot-isos/KNOPPIX_V7.6.1DVD-2016-01-16-EN.iso
    loopback loop /$ISO
    linux (loop)/boot/isolinux/linux64 knoppix bootfrom=/dev/sda1/$ISO nostartsound noeject noprompt lang=hu tz=Europe/London --
    initrd (loop)/boot/isolinux/minirt.gz
}

Now Knoppix - like SystemRescCD - can see 3,7G.

knoppix@Microknoppix:~$ free -h
              total        used        free      shared  buff/cache   available
Mem:           3,7G        241M        2,5G        143M        999M        3,2G
Swap:          2,7G          0B        2,7G
knoppix@Microknoppix:~$

Thank you for your help.