Hardware Reference
In-Depth Information
Most modern computers consist of two or more levels. Machines with as
many as six levels exist, as shown in Fig. 1-2. Level 0, at the bottom, is the ma-
chine's true hardware. Its circuits carry out the machine-language programs of
level 1. For the sake of completeness, we should mention the existence of yet an-
other level below our level 0. This level, not shown in Fig. 1-2 because it falls
within the realm of electrical engineering (and is thus outside the scope of this
book), is called the
device level
. At this level, the designer sees individual transis-
tors, which are the lowest-level primitives for computer designers. If one asks how
transistors work inside, that gets us into solid-state physics.
Level 5
Problem-oriented language level
Translation (compiler)
Level 4
Assembly language level
Translation (assembler)
Operating system machine level
Level 3
Partial interpretation (operating system)
Level 2
Instruction set architecture level
Interpretation (microprogram) or direct execution
Microarchitecture level
Level 1
Hardware
Level 0
Digital logic level
Figure 1-2.
A six-level computer. The support method for each level is indicated
below it (along with the name of the supporting program).
At the lowest level that we will study, the
digital logic level
, the interesting ob-
jects are called
gates
. Although built from analog components, such as transistors,
gates can be accurately modeled as digital devices. Each gate has one or more dig-
ital inputs (signals representing 0 or 1) and computes as output some simple func-
tion of these inputs, such as AND or OR. Each gate is built up of at most a handful
of transistors. A small number of gates can be combined to form a 1-bit memory,
which can storea0ora1. The1-bit memories can be combined in groups of (for
example) 16, 32, or 64 to form registers. Each
register
can hold a single binary
