How to convert old CDC mainframe PRUs to bytes?

Question

On the old CDC 6600 running the Kronos operating system developed by Seymour Cray was a I believe a 60-bit mainframe. It referred to units of storage as PRUs. What was a PRU and how can it be converted to bytes? I read a disk storage device held, for example 200,000 PRUs in the late 1970s. I'm curious to find out what size this is in modern times.

Answer 1

Quoted directly from Wikipedia:

The central processor had 60-bit words, whilst the peripheral processors had 12-bit words. CDC used the term "byte" to refer to 12-bit entities used by peripheral processors

Assuming a PRU is one "byte" that would yield (200,000 * 12 / 8) = 300,000 8 bit bytes of storage. This seems a bit "small", even for the day.

According to the description of a disk mass storage unit pictured in the CDC 6400 6500 66000 Reference Manual, it held 500 million bits of data (or about 60MB of 8 bit storage / 40MB 12 bit storage). This was a very large device for the time. I remember working of a VAX 11/70 (super mini) in the early 80's that had three whopping 67MB drives - thought I had died and gone to heaven.

This does not answer what a PRU is but does shed some light on the size of mass storage devices used on "super computers" in the 70's