Game boy color (8-bit cpu, 2MHz, 40k ram). Supposedly the guy (Palmer) who created the commercial gba version had done a tech demo for gbc, but Carmack decided

Game boy color (8-bit cpu, 2MHz, 40k ram). Supposedly the guy (Palmer) who created the commercial gba version had done a tech demo for gbc, but Carmack decided it was too stripped down and proposed a commander keen port for gbc at the time instead. Gba came out a couple of years later and was powerful enough.