Mainframes are the workhorses of the computing industry. They process transactions for about every industry, and handle the brunt of the economy. Their MTBF (Mean Time Between Failures) is measured in decades (typically 20-50 years). A comparison to a home computer is hard to make, they are in an entirely different league, playing an entirely different game.
Data Intense vs. CPU Intense
Mainframe processors such as these work in what is referred to as ‘Data Intensive’ computing environments. This is different from multi-cored processing that focuses on ‘CPU Intensive’ computing. CPU intense has a relatively small data set, but most perform a lot of work on that set of data, or do the same instruction on a set of data (such as graphics). CPU Intense processing can often be sped up with the addition of more processing cores. Data Intense processing does not see as much benefit from adding cores. Its biggest bottleneck is accessing the data, thus the System z tends to have VERY large caches, and very high bandwidth memory. They typically operate on transactional type data, where the processing has to operate in a certain order (A has to be done before B which has to finish before C etc).
IBM was one of the first, and continues to be one of the largest suppliers of such systems. Starting with the System/360 introduced in 1964 to the zSeries today. The zSeries was first launched in 2000 with the z900, a significant upgrade from the System/390. Data addressing was moved to 64-bits (from 31 bits) yet backwards compatibility (all the way back to the 360) is maintained. The z900 ran at 775MHz and was built with a 35 die MCM containing 20 Processing Units (PUs) and 32MB of L2 Cache.