I fired the image to sdcard. then boot the PI, and use commend line type "top",i find out that archlinux just use 189Mb ram only. then i modify "/boot/config.txt" file, append "gpu_mem=16 \n gpu_mem_512=16", reboot it . use "top", but it was 189Mb ram.
how can i do ? i want archlinux to use 512ram.