The CDC 6400, a member of the CDC 6000 series, was a mainframe computer made by Control Data Corporation in the 1960s. The central processing unit was architecturally compatible with the CDC 6600. But in contrast to the 6600, which had 10 parallel functional units which could work on multiple instructions at the same time, the 6400 had a unified arithmetic unit, which could only work on a single instruction at a time. This resulted in a slower, lower-performance CPU, but one that cost significantly less. Memory, peripheral processor-based input/output (I/O), and peripherals were otherwise identical to the 6600.