Gibberish on screen at boot

[tarun]

I have followed this official page for installation instructions https://archlinuxarm.org/platforms/armv6/raspberry-pi
Image link - http://os.archlinuxarm.org/os/ArchLinuxARM-rpi-3-latest.tar.gz
Device - Raspberry pi 3b+

But when I connect the pi to monitor and boot it up, it shows gibberish on screen. Nothing is legible. I could shut it down by logging in as root (by typing root 2 times blindly) and running "shutdown now". So I presume installation worked.

I do not have an ethernet cable, so I cannot ssh to the machine. I tried adding "gpu_mem=64" to /boot/config.txt but no luck.

Please suggest what to do. Thanks in advance.