INTRODUCTION - Structured Computer Organization - page 3

Rather than thinking in terms of translation or interpretation, it is often simpler
to imagine the existence of a hypothetical computer or virtual machine whose ma-
chine language is L1. Let us call this virtual machine M1 (and let us call the ma-
chine corresponding to L0, M0). If such a machine could be constructed cheaply
enough, there would be no need for having language L0 or a machine that executed
programs in L0 at all. People could simply write their programs in L1 and have the
computer execute them directly. Even if the virtual machine whose language is L1
is too expensive or complicated to construct out of electronic circuits, people can
still write programs for it. These programs can be either interpreted or translated
by a program written in L0 that itself can be directly executed by the existing com-
puter. In other words, people can write programs for virtual machines, just as
though they really existed.
To make translation or interpretation practical, the languages L0 and L1 must
not be ''too'' different. This constraint often means that L1, although better than
L0, will still be far from ideal for most applications. This result is perhaps discour-
aging in light of the original purpose for creating L1—relieving the programmer of
the burden of having to express algorithms in a language more suited to machines
than people. However, the situation is not hopeless.
The obvious approach is to invent still another set of instructions that is more
people-oriented and less machine-oriented than L1. This third set also forms a lan-
guage, which we will call L2 (and with virtual machine M2). People can write
programs in L2 just as though a virtual machine with L2 as its machine language
really existed. Such programs can be either translated to L1 or executed by an
interpreter written in L1.
The invention of a whole series of languages, each one more convenient than
its predecessors, can go on indefinitely until a suitable one is finally achieved.
Each language uses its predecessor as a basis, so we may view a computer using
this technique as a series of layers or levels , one on top of another, as shown in
Fig. 1-1. The bottommost language or level is the simplest and the topmost lan-
guage or level is the most sophisticated.
There is an important relation between a language and a virtual machine. Each
machine has a machine language, consisting of all the instructions that the machine
can execute. In effect, a machine defines a language. Similarly, a language defines
a machine—namely, the machine that can execute all programs written in the lan-
guage. Of course, the machine defined by a certain language may be enormously
complicated and expensive to construct directly out of electronic circuits but we
can imagine it nevertheless. A machine with C or C++ or Java as its machine lan-
guage would be complex indeed but could be built using today's technology.
There is a good reason, however, for not building such a computer: it would not be
cost effective compared to other techniques. Merely being doable is not good
enough: a practical design must be cost effective as well.
In a certain sense, a computer with n levels can be regarded as n different virtu-
al machines, each one with a different machine language. We will use the terms