Virtual machines

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1 Virtual machines In a machine there are 6 levels of instructions, to move from one to the lower one a translator is needed. Level 2 is that of the instruction set architecture (ISA), which represents the bunch of instructions the machine can execute. It is divided into: • User ISA: visible to an application program; • System ISA: can be seen only by the operative system, used to perform its specific task. Another important component is the application binary interface (ABI) which allows the program to access to hardware resources and other available systems. It is composed by: • User ISA • System calls, which allows a program to indirectly interact with Os-managed resources Not possible to run on instructions that were not meant for that machine, but the hardware can be virtualised: Java runs in every architecture for which an interpreter exists, as it uses a specific set of instructions which is translated by the Java Runtime Environment into machine instructions. The same technique can be used to run on a machine software which is meant for another one, by using emulators. They understand the behaviour of the CPU and of the RAM and reproduce it into the physical system on which it is running. Both the ABI and the ISA are reproduced. A virtual machine is a logical abstraction able to provide a virtual execution environment or a full system one. It maps virtual resources in physical ones and it uses the physical machine instructions to execute virtual ones, the host is the software that runs in the virtual machine while the guest is the underlying platform that supports it. There are two types of virtual machines. 1. Process virtual machines The ABI provides the interface between the physical machine and the virtual one, onto which it is possible to execute processes…