Information Technology Reference
In-Depth Information
consisting of a command (
cmd
), an address (
addr
), and an input data word (
in wrd
).
cmd
is either
READ
,
WRITE
,or
NO
-
OP
.A
NO
-
OP
command does nothing whereas a
READ
com-
mand changes the value of
out wrd
to the value of the data word at address
addr
.A
WRITE
command replaces the data word at address
addr
with the value of
in wrd
.
The random-access memory holds data as well as
programs
, collections of instructions
for the CPU. The CPU executes the
fetch-and-execute cycle
in which it repeatedly reads an
instruction from the random-access memory and executes it. Its instructions typically include
arithmetic, logic, comparison, and jump instructions. Comparisons are used to decide whether
the CPU reads the next program instruction in sequence or jumps to an instruction out of
sequence.
The general-purpose computer is much more complex than suggested by the above brief
sketch of the RAM. It uses a rich variety of methods to achieve high speed at low cost with the
available technology. For example, as the number of components that can fit on a semiconduc-
tor chip increases, designers have begun to use “super-scalar” CPUs, CPUs that issue multiple
instructions in each time step. Also, memory hierarchies are becoming more prevalent as de-
signers assemble collections of slower but larger memories with lower costs per bit to simulate
expensive fast memories.
1.4.4 Other Models
There are many other models of computers with memory, some of which have an infinite
supply of data words, such as the
Tu r i n g ma c h i n e
, a machine consisting of a
control unit
(an
FSM) and a
tape unit
that has a potentially infinite linear array of cells each containing letters
from an alphabet that can be read and written by a
tape head
directed by the control unit. It
is assumed that in each time step the head may move only from one cell to an adjacent one on
the linear array. (See Fig.
1.7
.) The Turing machine is a standard model of computation since
no other machine model has been discovered that performs tasks it cannot perform.
The
pushdown automaton
is a restricted form of Turing machine in which the tape is
used as a pushdown stack. Data is entered, deleted, and accessed only at the top of a stack. A
0
1
2
m
−
1
b
Ta p e Un i t
Control
Unit
Figure 1.7
The Turing machine has a control unit that is a finite-state machine and a tape unit
that controls reading and writing by a tape head and the movement of the tape head one cell at a
time to the left or right of the current position.
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