Information Technology Reference
In-Depth Information
Fig. 2.18. Truth table and schema
of a flip-flop circuit built from NOR
gates. Inputs S = 1 and R = 1 are not
allowed, the outputs marked by * are
indeterminate.
R
S
1
0
0
0
1
R
Q
Q
Q
0
1
1
0
0
0
1
0
1
0
0
*
1
Q
1
*
S
Fig. 2.19. Truth table and schema of
clocked RS flip-flop.
(a)
(b)
R
S
Q
Q(t+1)
R
0
0
0
0
Q
0
0
1
1
0
1
0
1
Clock
0
1
1
1
1
0
0
0
1
0
1
0
Q
1
1
0
*
S
1
1
1
*
the “main memory” of the computer and can be used to store results from reg-
isters as well as storing data needed by the registers for the different stages of
the calculation. Between the registers and main memory, modern computers
now incorporate several levels of memory that can be accessed more quickly
than the main memory. These fast-access levels constitute the “cache memory”
that is used to store the most frequently used data in order to avoid time delays
that would be incurred in retrieving data from the slower memory.
Finally, since main memory is expensive, computer engineers introduced
“secondary memory.” This uses cheaper and slower technologies but allows
data to be transferred to the main memory as and when required. Initially,
data for this secondary memory was recorded on punched cards or paper tape
and fed in manually to the machine by computer operators. The use of cards
for secondary memory was superseded by magnetic tapes rather than much
more expensive magnetic core memory. Magnetic tapes holding computer
data became so common that many movies showing computers in operation
showed images of spinning tape drives as an icon for a computer. Much clever
engineering has been devoted to making tape drives extremely reliable and
fast. However, there is one problem that no amount of engineering can solve:
Fig. 2.20. How long does it take to get
the data? This figure shows an analogy
suggested by Jim Gray to illustrate the
different data access times and the
importance of memory hierarchy in
computers. On the left, the access time
is given in CPU clock ticks. For a typical
1 gigahertz clock this is one nano-
second. To relate these times to human
timescales, on the right we translate a
clock tick to one minute. The drawing in
the middle illustrates how far we could
have traveled during the time to retrieve
data from the different elements of the
computer memory hierarchy.
Andromeda
2,000 Years
10
9
Ta pe /Optical
Robot
10
6
Disk
Pluto
2 Years
1.5 hr
Sacramento
100 Memory
10 On Board Cache
This Campus
This Room
My Head
10 min
2 On Chip Cache
1 Registers
1 min
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