vp@csd4.csd.uwm.edu (Victoria Piaskowski) (09/05/89)
Here is a summary of my questions regarding the replacement of an 8086/8088
CPU with a V20 NEC chip.
The actual speed of the microprocessor is determined by the crystal (that
generates the clock signal), NOT the microprocessor itself. The different
ratings are the MAXIMUM speeds that the parts should be run, so you can put a
100 MHz chip in a 4.77 MHz PC, and you still get 4.77 MHz performance. If the
price is not too different (a couple of bucks?), I would get the 10 MHz one,
even though I can only run it at 8 MHz. It would make the chip more reliable.
If you are not changing the clock rate, keep the same 8087. There should be no
problem at all. Note that the 8087 is likely to have its own clock rate,
independent of the CPU clock.
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I am not sure about the coprocessor (my gut feeling says it will be ok).
You should change your 8088-2 to the 8 MHz V20 chip. Your CPU will
still be 8 MHz but there will be a 20 to 30% speedup.
Harold Shapiro (hs0i+@andrew.cmu.edu)
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Replacing the 8088 with a V20 does not affect the clock speed--that
is controlled by a crystal on the motherboard, so if you replace
your 8088-2 with a v20 qualified for 8MHz or higher all should
be just fine. I have a 10mhz XT clone which came with an 8088-1
and I added an 8087-1, and later replaced the 808801 with a 10MHz
rated v20[230z, and have had absolute NO problems with compatibility.
Floating point math on the 8087 continues to work just fine. (I have
a Mandelbrot program which exercises the *#&$*@W& out of it, too!)
As to how much faster it will be, well, that depends on what you
are doing! I find, roughly, about 5-10% overall improvement on mine.
Soome benchmark programs will show a 5% increase, others a hundred percent
or more. The notorious Norton SI shows a CPU rating of 4.1 with the
V20 or 2.1 with the 8088, but that is known to be a poor test.
Most other benchmark programs I have run consider it to be around
2.6 times an XT with the v20 and in the 2.1-2.5 range with the 8088.
Not a huge improvement, but whaddaya want for 12.95???
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The 8 MHz V20 will work just fine.
Since your machine runs at 8MHz, buying the 10MHz chip would be useless.
The 8MHz chip will work perfectly with the 8MHz co-processor. I know
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I would sugguest the 8 MHz one, since the 10 MHz V20 will actually be
running at 8 MHz unless you change the oscillator on the system board (and
that will mess up a lot more than just the 8087 -- take my word for it).
Other than that, either the 8 or 10 MHz versions should work fine with your
8087-2. One other problem is that some add-in cards (I have been told, I have
never seen one myself) do not work correctly with a V20 because some
of the control signals are not the same duty cycle as they are on an Intel
8088 or 8086.
Charles
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I have an 8088-2 (8MHz) machine (Leading Edge Model D) that
also has an 8087-2 coprocessor. I would like to replace the
8088-2 with a V20 but which one?
Use the 8-MHz V20. You can use a 10-MHz V20, but if your CPU clock
doesn't go that high, you're just wasting money.
Can I put the 10MHz V20 in my machine and still have it work
with my 8087-2 coprocessor?
Yes. As long as your CPU clock is running at 8 MHz, I don't think it
makes any difference whether your CPU chip is rated for 8 or 10 MHz.
Remember, your CPU clock rate is determined by the crystal and other
circuitry on the motherboard -- *not* by the rating of the CPU chip.
The only way the CPU chip's speed rating comes into play is that, if
you use a slow CPU at a fast clock rate, it probably won't work (or it
may fail sporadically, which of course is just as bad as hard failure).
If I use the 8MHz V20 how much faster will my system be
compared to the 8088-2?
It depends on what kinds of stuff you do. I've heard estimates ranging
all the way from 5% to over 30%. The internal differences between an
8088 and a V20 are rather complex, so it isn't just a straightforward
matter of an N% speedup.
The book I have says that the 8087-1 coprocessor is for an 8086
running at 10 MHz or less but they do not have one listed for
an 8088 running at 10MHz or less. I do not want to give up my
8087-2 coprocessor but I want to maximize my CPU speed. Which
V20 should I use?
To the best of my understanding, a given 8087 chip works equally well
with either an 8088 or an 8086. In any case, assuming your existing
8087-2 works OK with your existing 8088-2, the 8087-2 should still work
if you replace the 8088-2 with an 8-MHz V20.
What you *cannot* do is use an 8087 with a 286 (or 386) CPU -- or, con-
versely, use an 80287 with an 8088 (or 8086) CPU.
Also, as I recall, an 8088/8086/V20 system automatically runs an 8087
coprocessor at a slower clock rate than the CPU -- 2/3 of the CPU rate,
I think. This is from memory; I don't have a coprocessor chip in my
own system at home. Hence, the speed rating for a system's 8087 chip
can be a bit lower than for the CPU chip.
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Use the V20 that matches the system clock in your machine. The
system clock does not change when you change CPU chips. In your
case you said you have an 8mhz Leading Edge, so use an 8mhz V20. The
speed at which the V20 runs depends upon the system clock, not the
rated speed of the chip. If you use a chip that is rated slower than
your system clock you run the risk of the system not working right.
If you use a chip that is rated faster than the system clock then you
have wasted money by buying a chip that is faster than you need -- if
you plug a 10mhz V20 into a system with an 8mhz clock you will be
running a 10mhz capable V20 at 8 mhz.
When you replace your 8088 with a V20 if the V20 works properly then
the 8087 should also work properly - the speed ratings of the two
chips do not have to match. A 10mhz V20 will work fine with an 8mhz
8087, but in your case both will be running at 8mhz since your
system clock is 8mhz.
Depends upon what your application program does. Real world speed
ups are typically not astounding - on the order of 5-10%.
Use the 8086 ratings for the 8088 as well.
See above. Use the V20 which matches your system clock, in your
case an 8mhz V20. Unless of course you plan to replace your 8mhz
crystal on the motherboard with a faster crystal (say 10mhz) in
which case buy a 10mhz V20. Note that replacing your crystal can
be tricky - the rest of the computer may not work at 10mhz. >
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About 30% overall. On CPU (not memory access) upto 100%-200% faster.
Even more if you compile for 80186 code, say with Turbo C.
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Vickyleonard@bucket.UUCP (Leonard Erickson) (09/06/89)
One bit of mis-information needs to be corrected. An 8087 is run at the *same* clock rate as the CPU (8088, 8086, V20, V30). This is because it processes the instructions synchronously (in step with the processor). The other math chips run asyschronously, and are usually run at 2/3 the clock rate of the CPU. -- Leonard Erickson ...!tektronix!reed!percival!bucket!leonard CIS: [70465,203] "I'm all in favor of keeping dangerous weapons out of the hands of fools. Let's start with typewriters." -- Solomon Short
akcs.amparsonjr@vpnet.UUCP (Anthony M. Parson, Jr.) (09/15/89)
Yes, I've read the same thing that only the 8088/8087 combination runs at
the same speed of the motherboard. On Norton's SI, my v20/8087 at 8 mhz is
considered as fast as an 8 mhz AT (I forget it it means an AT with an 80287,
or without), and I got something similar on some other benchmark as well.
Tony
(missing handle)