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Unit 4: Classification of Computers

In the 1960s, most mainframes had no explicitly interactive interface. They accepted sets of punched
cards, paper tape, and/or magnetic tape and operated solely in batch mode to support back office Notes
functions, such as customer billing. Teletype devices were also common, for system operators, in
implementing programming techniques. By the early 1970s, many mainframes acquired interactive
user interfaces and operated as timesharing computers, supporting hundreds of users simultaneously
along with batch processing. Users gained access through specialized terminals or, later, from
personal computers equipped with terminal emulation software. By the 1980s, many mainframes
supported graphical terminals, and terminal emulation, but not graphical user interfaces. This format
of end-user computing reached mainstream obsolescence in the 1990s due to the advent of personal
computers provided with GUIs. After 2000, most modern mainframes have partially or entirely
phased out classic terminal access for end-users in favour of Web user interfaces.
Historically, mainframes acquired their name in part because of their substantial size, and because
of requirements for specialized heating, ventilation, and air conditioning (HVAC), and electrical
power, essentially posing a “main framework” of dedicated infrastructure. The requirements of
high-infrastructure design were drastically reduced during the mid-1990s with CMOS mainframe
designs replacing the older bipolar technology. IBM claimed that its newer mainframes can reduce
data center energy costs for power and cooling, and that they could reduce physical space
requirements compared to server farms.
Characteristics
The ability to run (or host) multiple operating systems, and thereby operate as a host of a collective
of virtual machines. In this role, a single (via the z/VM operating system). Many mainframe
customers run two machines: one in their primary data center, and one in their backup data center—
fully active, partially active, or on standby—in case there is a catastrophe affecting the first building.
Test, development, training, and production workload for applications and databases can run on a
single machine, except for extremely large Mainframes are designed to handle very high volume
input and output (I/O) and emphasize throughput computing.
Since the mid-1960s, mainframe designs have included several subsidiary computers (called channels
or [[CDC 6600#Peripheral Processors.28PPs.29shops to deal with massive databases and files.
Gigabyte to terabyte-size record files are not unusual. Compared to a typical PC, mainframes
commonly have hundreds to thousands of times as much data storage online, and can access it
much faster. Other server families also offload I/O processing and emphasize throughput computing.

Mainframe return on investment (ROI), like any other computing platform, is dependent
on its ability to scale, support mixed workloads, reduce labor costs, deliver
uninterrupted service for critical business applications, and several other risk-adjusted
cost factors.

Mainframes also have execution integrity characteristics for fault tolerant computing. For example,
z900, z990, System z9, and System z10 servers effectively execute result-oriented instructions twice,
compare results, arbitrate between any differences (through instruction retry and failure isolation),
then shift workloads “in flight” to functioning processors, including spares, without any impact to
operating systems, applications, or users. This hardware-level feature, also found in HP’s NonStop
systems, is known as lock-stepping, because both processors take their “steps” (i.e., instructions)
together. Not all applications absolutely need the assured integrity that these systems provide, but
many do, such as financial transaction processing.
Personal Computer

A personal computer (PC) is any general-purpose computer whose size, capabilities, and original
sales price make it useful for individuals, and which is intended to be operated directly by an end-

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