The Game Boy has a custom SM83-based CPU, often called the SM83, which can be considered either as a stripped-down Z80, or a slightly upgraded 8080. It runs at a whopping 4MHz, though it can be simplified to a 1MHz, since, except in a few extremely edge-y cases, everything take a multiple of 4 cycles. Game Boy Color software can ask the console to double its CPU speed.
SM83ベースと説明されていて「 a stripped-down Z80, or a slightly upgraded 8080(余分なものを除いたZ80、もしくは少しアップグレードした8080)」と説明されています。こちらの記事やその参考に挙げられてるドキュメントにも記載の無かったので、私にとっては初耳です。
"SM83": Someone on the GBDev Discord server claims to have seen datasheets for other Sharp microcontrollers or systems on chip that have the same instruction set as the Game Boy CPU, and they call the core "SM83". I'd like to get the identity of this datasheet into a form that I can cite, as things said only in chat tend to disappear down a memory hole more quickly than things said in a newsgroup post, mailing list post, or other forum post with a URL.