Introduction to Virtual
Machines
From
“Virtual Machines”
Smith and Nair
Chapter 1
Introduction
1-1
Two fundamental notions in
computer system design
 Levels of Abstraction …
 ….separated by well-defined Interfaces
 Keys to managing complexity in computer
systems.
Introduction
1-2
Abstraction
 Abstraction allows lower levels of design to be
ignored/simplified while designing higher levels.
 E.g. Details of hard disk abstracted by operating
system into multiple variable sized partitions and
their file systems.
 Disadvantage: Sometimes low-level details are
necessary to optimize for performance. E.g. File
systems might use better layout if they knew the
disk geometry.
Introduction
1-3
Interfaces
 Allow computer design tasks to be decoupled so
that different development teams can work
independently at different levels of abstraction.
 E.g. Instruction set: Intel and AMD implement the
same IA-32 (x86) instruction set interface.

Software designers don’t need to worry about their
different implementations.
 Disadvantage: Components designed for one
interface cannot work on another


E.g. x86 vs IBM PowerPC
Diversity of interfaces can be restrictive for
applications.
Introduction
1-4
Virtualization
 Provides a way to increase flexibility.
 Real system (and its interfaces) appear to
be a set of virtual systems (and virtual
interfaces).
 Virtualization vs. abstraction
 Virtualization
does not necessarily hide the
level of details of the real system
Introduction
1-5
Example: Disk Virtualization
Virtual
Disk 1
Virtual
Disk 2
Virtualization
File 1
File 2
Interface
Real
Disk
Introduction
1-6
Virtual Machines
 Same concept as disk virtualization in last slide
 Implemented by adding layers of software to the
real machine to support the desired VM
architecture.

E.g. Virtual PC on Apple MAC/PowerPC emulates
Windows/x86.
 Uses:
 Multiple OSes on one machine
 Isolation,
 Enhanced security
 Platform emulation
 On-the-fly optimization
 Realizing ISAs not found in physical machines
Introduction
1-7
Virtualization – Isomorphism
 Maps a virtual guest system to a real host system.
Si
e(Si)
Si’
Guest
V(Si)
Si’
V(Sj)
e’(Si’)
Sj’
Host
Introduction
1-8
Computer Architecture
User ISA : 7
System ISA : 8
Syscalls : 3
ABI : 3, 7
API : 2,7
Introduction
1-9
Machine Interfaces
Application Binary Interface
(Process View)
ISA Interface
(OS View)
Introduction
1-10
Two Types of VMs
Process VMs
System VMs
Introduction
1-11
Process Virtual Machine
•Virtualizing software translates instructions from one platform to another.
•Helps execute programs developed for a different OS or different ISA.
•VM terminates when guest process terminates.
Introduction
1-12
System Virtual Machine
 Provides a complete system environment
 OS+user processes+networking+I/O+display+GUI
 Lasts as long as host is alive
Introduction
1-13
Virtual Machine Applications
Emulation &
Optimization
Replication
Composition
 Emulation: Mix-and-match cross-platform portability
 Optimization: Usually done with emulation for platform-
specific performance improvement
 Replication: Multiple VMs on single platform
 Composition: form more complex flexible systems
Introduction
1-14
Types of Process Virtual Machines
 Multiprogramming
 Standard OS syscall interface + instruction set
 Can support multiple processes with its own address space and
virtual machine view.
 Emulators
 Support one instruction set on hardware designed for another
IA-32 Windows APP
Windows NT

Interpreter:
Runtime
Digital FX!32
System
Alpha ISA
• Fetches, decodes and emulates the execution of individual source
instructions. Can be slow.

Dynamic Binary Translator:
• Blocks of source instructions converted to target instructions.
• Translated blocks cached to exploit locality.
Introduction
1-15
Types of Process Virtual Machines (contd)
 Same ISA Binary Optimizers
 Optimize code on the fly
 Same as emulators except source and target ISAs are the same.
 High-Level Language VMs
 Virtual ISA (bytecode) designed for platform independence
 Platform-dependent VM executes virtual ISA
 E.g. Sun’s JVM and Microsoft’s CLI (part of .NET)
 Both are stack-based VMs that run on register-based m/c.
Introduction
1-16
Types of System VMs
 Originally developed for large mainframes
 Today:
 Secure way of partitioning major software systems on a
common platform
 Ability to run multiple OSes on one platform
 Platform replication provided by VMM
 VMM controls access to hardware resources
 When guest OS performs a privileged operation, VMM
intercepts it, checks for correctness and performs the
operation.
 Transparent to guest OS.
Introduction
1-17
Classic System VMs
 Try to execute natively on the host ISA
 VMM directly controls hardware
 Provides all device drivers
 Traditional mainframe model
Introduction
1-18
Hosted VMs
 Similar to classic system VM
Operates in process space
 Relies on host OS to provide drivers
 E.g. VMWare

Introduction
1-19
Whole System VMs: Emulation
 Host and Guest ISA are different
 Hosted VM + emulation
 So emulation is required
 E.g. Virtual PC (Windows on MAC)
Introduction
1-20
Co-designed VMs
 Performance improvement of existing ISA
 Customized microarchitecture and ISA at
hardware level
 Native ISA not exposed to applications
 VMM


co-designed with native ISA
Part of native hardware implementation
 Emulation/translation
 E.g. Transmeta Crusoe
 Native ISA based on VLIW
 Guest ISA = x86
 Goal power savings
Introduction
1-21
Taxonomy
Introduction
1-22
Versatility
Java App
JVM
Linux IA-32
VMWare
Windows IA-32
Code Morphing
Crusoe VLIW
Introduction
1-23