History of Virtualization and Architectural Evolution - Oracle Solaris 10 System Virtualization Essentials - page 328

In today's data centers, server virtualization is primarily used to consolidate
multiple low-utilization physical servers, reducing costs for power, cooling, rack
space, software licensing, and administrative effort. This usage contrasts with the
original economic driver for virtualization: to avoid the high acquisition costs as-
sociated with the then-expensive computer systems. Despite the different reasons
driving virtualization trends in the past and in the present, the effects are similar:
multiple virtual computing environments on the same physical machine.
While server virtualization is now available on almost every computer product
line and in multiple architectural forms, the original and most widely used form
of server virtualization is the virtual machine , pioneered by early hypervisors that
ran on IBM mainframes in the 1960s and 1970s. This development inspired and
influenced many technologies that emerged in the following decades—obviously
the current mainframe hypervisor, z/VM, but also hypervisors for other system
families such as Xen, VMware, and Oracle VM VirtualBox. The early hypervi-
sors also influenced hardware/firmware solutions like Amdahl's Multiple Domain
Facility, IBM's PR/SM and LPARs, and Sun Microsystems' Dynamic Domains,
from the Enterprise 10000 in the late 1990s to the current SPARC Enterprise
M-Series Dynamic Domains, and finally to SPARC-based Logical Domains (now
called Oracle VM Server for SPARC), as well as operating system virtualization
solutions, such as BSD Jails and Oracle Solaris Containers.
Challenges and issues faced in the early days still confront modern virtual-
ization technologies, making an understanding of early systems valuable and
relevant.
CTSS and the Early Hypervisors
Virtualization has its roots in the Compatible Time-Sharing System (CTSS),
written at Massachusetts Institute of Technology (MIT) in the mid-1960s. CTSS
was among the first operating systems to share a single computer among multiple
interactive users. Previously, computers ran in batch mode, and programmers
often had to wait for dedicated time periods to use the one-and-only computer.
The CTSS project, under the direction of Professor Fernando Corbató, was the
first to time-slice a single computer between interactive users at terminals. CTSS
introduced architectural features that would be central to subsequent virtual-
ization environments—namely, address relocation to permit separate address
spaces for each user, preemptive multitasking of user processes, and isolation of
physical traps, devices, and interrupts from user processes. In addition, it pro-
vided a virtualized form of the FORTRAN batch environment typically run on
that processor. CTSS also introduced early forms of shell scripts, text formatting,
and inter-user messaging.