Hardware Reference
In-Depth Information
6
MACHINE LEVEL
The theme of this topic is that a modern computer is built as a series of levels,
each one adding functionality to the one below it. So far, we have seen the digital
logic level, microarchitecture level, and instruction-set architecture level. Now it is
time to move up another level, into the realm of the operating system.
An
operating system
is a program that, from the programmer's point of view,
adds a variety of new instructions and features, above and beyond what the ISA
level provides. Normally, the operating system is implemented largely in software,
but there is no theoretical reason why it could not be put into hardware, just as
microprograms normally are (when they are present). For short, we will call the
level that it implements the
OSM
(
Operating System Machine
) level. It is shown
in Fig. 6-1.
Although the OSM level and the ISA level are both abstract (in the sense that
they are not the true hardware level), there is an important difference between
them. The OSM-level instruction set is the complete set of instructions available to
application programmers. It contains nearly all of the ISA level instructions, as
well as the set of new instructions that the operating system adds. These new in-
structions are called system calls. A
system call
invokes a predefined operating
system service, effectively, one of its instructions. A typical system call is reading
some data from a file. We will typeset system calls in
lowercase Helvetica
.
The OSM level is always interpreted. When a user program executes an OSM
instruction, such as reading some data from a file, the operating system carries out
this instruction step by step, just as a microprogram would carry out an
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