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INTRODUCTION
iAPX 88 PERFORMANCE IS
COST
EFFECTIVE
Figure
1-25
shows the 8088's memory speed
requirements compared to other 8-bit micro-
processors. The memory access times listed
refer to the time available from when the
address first comes out
of
the
CPU
during a
memory read machine cycle until the data
must be available coming back from the
memory into the
CPU.
The
8088
running at 5MHz allows 460ns for
memory devices to receive the address and
return the data. The fastest Z80 and the fas-
test
6809
allow only 140ns and 320ns
respectively for the same activity to take
place. This means
that
the
8088
can offer its
full performance while using slower and pre-
sumably cheaper memories than any other
high-performance
8-bit
micropro-
cessor.
Note that according to the benchmark
reports in the Appendix, the 5MHz
8088
use
slower memories while offering
an
average of
30%
more performance than either the
2MHz
6809
or
the 6MHz Z80B.
How does the
8088
offer higher performance
yet use slower memory devices? The main
reason
is
that parallel instruction fetch and
execute using the instruction pipeline allows
CPU
8088
5MHz
MEMORY
ACCESS
460
NS
TIME
the bus interface to be much more relaxed
while execution takes place at the full speed.
The
8088
can
run
at
full speed using readily
available 450ns
EPROM
devices whereas its
counterparts, the 68B09 and Z80B require
wait states in their machine cycles to do the
same.
PROCESSOR EXTENSIONS FOR
FLOATING
POINT
ARITHMETIC
AND
HIGH
SPEED
I/O
Up to
now,
we have justified that the
8088
CPU
itself offers a lot
of
performance, and many
systems
will
be built around the
8088
as the
only central processing unit. Note that there
are other ways to expand on the
8088
architec-
ture to add additional processing power to the
basic
CPU. These additional processing mod-
ules are called processor extensions. There are
two processor extension chips that can be
added to the
iAPX
88
system (Fig.
1-26).
Numerics Processor Extension
The iAPX 88/20
is
an
optional numerics
processor extension
(NPX) added alongside
the
8088
CPU. This configuration has the
effect of adding the additional set of numerics
instructions to the
8088
instruction set. The
NPX
picks its own instructions out
of
the
68B09
Z80A Z80B
2MHz
4MHz
6MHz
320
NS
250
NS
140
NS
LONGER
ACCESS
TIME
MEANS
SLOWER
(AND
CHEAPER)
MEMORIES
CAN
BE
USED
WITH
iAPX
88
Figure 1-25. iAPX
88
Longer Memory Access Time
1-17