There are a couple of methods to get around this, but the simplest method involves glitching the crystal by grounding one of its leads. The ROM is readable while booting, but afterwards the address space of the ROM is remapped for interrupt vectors and other uses.
Reading the contents of the Game Boy ROM is a bit challenging. wanted to learn more, so he used a clock-glitching technique to dump the device’s ROM and made some interesting discoveries about its copyright protection and boot process along the way.
While there’s a ton of information about the boot ROM and operation of the original Game Boy, not much is known about these clones. Got his hands on a Game Fighter, a clone of the original Game Boy.