cache. Any loop that fit into the cache (which is usually called in-stack) runs very fast, without referencing main memory for instruction fetch. The lower-end models do not contain an instruction stack. However, since up to four instructions are packed into each 60-bit word, some degree of prefetching is inherent in the design.

As with predecessor systems, the Cyber 170 series has eight 18-bit address registers (A0 through A7), eight 18-bit index registers (B0 through B7), and eight 60-bit