Notably, Dave Nutting Associates (Bushnell's former employer, who also distributed Computer Space) had played around with an Intel 4004 in 1974 and then demonstrated (to Bally) a CPU based system with a frame buffer in September, which evolved into the Intel 8080-based board that ran Midway's Gun Fight. Atari would have probably been aware of this (Nutting Associates had filed a patent.) So, something along the lines of Intel 4004 or 8080 machine code, maybe M6800.