8088 vs NEC V20
Robert L. Doerr
a DROP-IN replacement for the 8088 but that it does work in almost
all instances. I believed it talked about a slightly different clock that
the V20 requires. Does anyone know of a list of the differences on
the clock? I could also use a list of differences in the execution cycles
between the two. I have an old 8088 based computer that I would
like to put a V20 in its place but when I do it works for a while then
locks up. If I can get a complete list of the differences then I can
try to determine if it is a hardware problem (with the clock) or one
that is caused by the fact the some operations can complete in
less clock cycles and it is throwing off some timing loops in the
firmware.
Let me know if you have any data on this or know of an on-line
resource that can help.
Regards,
Robert
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anon...@bogus_address.con
On 1999-12-10 rdo...@bizserve.com said:
>I once read a post that mentioned that the NEC V20 chip is not
>really a DROP-IN replacement for the 8088 but that it does work in
>almost all instances. I believed it talked about a slightly
>different clock that the V20 requires. Does anyone know of a list
>of the differences on the clock? I could also use a list of
>differences in the execution cycles between the two. I have an old
>8088 based computer that I would like to put a V20 in its place but
>when I do it works for a while then locks up. If I can get a
>complete list of the differences then I can try to determine if it
>is a hardware problem (with the clock) or one that is caused by the
>fact the some operations can complete in less clock cycles and it
>is throwing off some timing loops in the firmware.
>Let me know if you have any data on this or know of an on-line
>resource that can help.
Later V20 chips had a number printed on the chip immediately following
the 'V20.' This number indicates the maximum speed (in MHz) at which
the chip can operate. For instance: 'V20 8' indicates a maximum speed
of 8 MHz.
If you drop a 'V20 8' into a 10 MHz XT-class machine, you're cruisin'
for a bruisin'.
If the chip has no number following the 'V20,' chances are it's an
older 4.77 MHz version, and definitely won't work (for long) at faster
speeds.
Robert L. Doerr
The computer has a clock speed of 5Mhz which I eventually want to try to
run at a higher speed. I tried a couple of different V20 up to the 10Mhz
version with the same results. I should have noted before that it will
always lock at the same point. At that point in the program it is doing
I/O and it could be that the program is trying to read the I/O ports before
the data is ready and it just hangs. I had heard that the clock for the
V20's was slightly different so I am just trying to determine whether the
problem is in the hardware or strickly a software issue. Any detailed
data I can get that covers the differences between these processors
will help.
bill_h
>
> I once read a post that mentioned that the NEC V20 chip is not really
> a DROP-IN replacement for the 8088 but that it does work in almost
> all instances. I believed it talked about a slightly different clock that
> the V20 requires. Does anyone know of a list of the differences on
> the clock? I could also use a list of differences in the execution cycles
> between the two. I have an old 8088 based computer that I would
> like to put a V20 in its place but when I do it works for a while then
> locks up. If I can get a complete list of the differences then I can
> try to determine if it is a hardware problem (with the clock) or one
> that is caused by the fact the some operations can complete in
> less clock cycles and it is throwing off some timing loops in the
> firmware.
>
> Let me know if you have any data on this or know of an on-line
> resource that can help.
Somebody nudge Allison awake. She worked in some capacity for NEC
on these things and knows all about their quirks........
Bill
Tucson
Allison
>> I once read a post that mentioned that the NEC V20 chip is not really
>> a DROP-IN replacement for the 8088 but that it does work in almost
It is IN ALL CASES. Note though it's instruction set is extended
some, it can execute 8080 code directly (in 8080 mode) and
some internal operations take fewer clock cycles.
>> the clock? I could also use a list of differences in the execution cycles
>> between the two. I have an old 8088 based computer that I would
Find a databook for the V20. The list is far too long to type on an
instruction by instruction basis. Generally any instruction that
needs to form a long address (20bit) takes fewer cycles.
Allison
Real address is: Allisonp @ world DOT std DOT com
++++BULK Email severely not wanted+++
Allison
>I/O and it could be that the program is trying to read the I/O ports before
>the data is ready and it just hangs. I had heard that the clock for the
>V20's was slightly different so I am just trying to determine whether the
>problem is in the hardware or strickly a software issue. Any detailed
>data I can get that covers the differences between these processors
>will help.
No difference worth talking about. Your problem is your pushing logic
around the CPU (memory and IO) that can not run at 10mhz. I bet you
get the same results from a 10mhz intel8088.
FYI, I'm running a DTK 10mhz turbo 8088 box with a V20 no problem.
Keep in mind it was meant to run at 10mhz!
The V20 difference is it took fewer clock cycles to do certain things
like form long addresses. So it's overall execution speed for some
cases could be 10% faster. The speed difference was more internal
parallelism not any external clock timing difference.
William Stulz
for the 8088 and 8088-2 intel processors. They came in 5,8, and 10mhz
versions. There is no harm done using a faster rated chip than the original
but don't use a slower one. They require no modification to the system
hardware. The difference is the way they process internally. The V20 is able
to read, decode, and execute an instruction sent to it simultaneously while
the intel chip does these one step at a time. Some machines used these out
of the factory with no hardware or software incompatabilities. If your
programs depend on a co-processor make sure it is rated at the same speed as
the main processor. I haven't seen a spec sheet on these in a lot of years
but they are mentioned in a lot of the older pc upgrade and repair guides -
try your public library.
bst...@bellatlantic.net
"Robert L. Doerr" <rdo...@bizserve.com> wrote in message
news:38514F68...@bizserve.com...
> I once read a post that mentioned that the NEC V20 chip is not really
> a DROP-IN replacement for the 8088 but that it does work in almost
> all instances. I believed it talked about a slightly different clock that
> the V20 requires. Does anyone know of a list of the differences on
> the clock? I could also use a list of differences in the execution cycles
> between the two. I have an old 8088 based computer that I would
> like to put a V20 in its place but when I do it works for a while then
> locks up. If I can get a complete list of the differences then I can
> try to determine if it is a hardware problem (with the clock) or one
> that is caused by the fact the some operations can complete in
> less clock cycles and it is throwing off some timing loops in the
> firmware.
>
> Let me know if you have any data on this or know of an on-line
> resource that can help.
>
Betty W. Monroe
change in Zenith Z-100 PC's (Z-150's) and it worked just fine. MS-DOS and
8088 software ran unchanged (actually a bit faster, the V-20 has hardware EA
(effective address) calculation, which the actual intel 8088 did in
microcode), and it could then run actual 8080 and Z-80 object code, and run a
CP/M environment in hardware at full speed.
Barry Watzman
Don Maslin
: I have done a straight "pull the 8088 and plug the V-20 into the same socket"
: change in Zenith Z-100 PC's (Z-150's) and it worked just fine. MS-DOS and
: 8088 software ran unchanged (actually a bit faster, the V-20 has hardware EA
: (effective address) calculation, which the actual intel 8088 did in
: microcode), and it could then run actual 8080 and Z-80 object code, and run a
: CP/M environment in hardware at full speed.
Erm! I think you mean 8080 only. The V20 does not understand Z-80
peculiar instructions IIRC.
- don
: Barry Watzman
Paul Schlyter
Betty W. Monroe <BWM...@ibm.net> wrote:
> I have done a straight "pull the 8088 and plug the V-20 into the same socket"
> change in Zenith Z-100 PC's (Z-150's) and it worked just fine. MS-DOS and
> 8088 software ran unchanged (actually a bit faster, the V-20 has hardware EA
> (effective address) calculation, which the actual intel 8088 did in
> microcode), and it could then run actual 8080 and Z-80 object code,
specific code. Therefore, CP/M on a V20 won't be able to run for
instance Borland's Turbo Pascal for CP/M, because it used Z80
specific code.
> and run a CP/M environment in hardware at full speed.
8080 mode ran CP/M perhaps 20-30 times faster than an 8088 emulating
an 8080. But already a 33 MHz 80386 would emulate an 8080 at about
the same speed as a 4.77 HMz V20 in 8080 mode, running at "full
speed". And a modern Pentium III will easily emulate an 8080 at a
speed perhaps 50 times faster than a V20 at "full speed".
(will there ever be a Pentium-III compatible version of the V20,
running at 500+ MHz? Of course, that V20 should also understand
Z80 specific operation... :-)
Yep, it's kind'a an upside-down world: modern emulators of old
8-bit hardware running many times faster than the original hardware
itself...
--
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Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
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WWW: http://hotel04.ausys.se/pausch http://welcome.to/pausch
Charles E. Bortle, Jr.
In case anyone is interested, I have two *.zip files
I can email attach. (I will go ahead and send them to Robert).
One is called VIntro and contains a program that lists the V20
instruction set. It was written by Juan Jimenez in (or about) 1985.
The other is called Lemule10 and is a CP/M emulator which I wrote that runs
on a PC with a V20 replacing the 8088. It should also run on
an machine with a V30 replacing the 8086.
--
Charles cbr...@ix.netcom.com
"For God So Loved The World, That He Gave His
Only Begotten Son, That Whosoever Believeth
In Him Should Not Perish, But Have Everlasting
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Robert L. Doerr <rdo...@bizserve.com> wrote in message
news:38514F68...@bizserve.com...
Eric J. Korpela
Paul Schlyter <pau...@merope.saaf.se> wrote:
> [The V20 ] could then run actual 8080 and Z-80 object code,
>
>The V20 understands 8080 code, but it does not understand Z80
>specific code.
IIRC the V20/30 does include an undefined opcode interrupt. Does anyone
know if this interrupt is active in 8080 mode? I've always meant to test it,
but never have. If so, it would make implementation of Z80 opcodes
possible.......
Eric
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kor...@ssl.berkeley.edu | stopped.
<a href="http://sag-www.ssl.berkeley.edu/~korpela">Click for home page.</a>
Robert L. Doerr
I found a copy of the old message I had read a while ago regarding the
differences between the NEC V20 and the 8088. Any one care to
comment on these? The problem that I have could be related to the
POP CS instruction or that some of the instructions that take less
cycles to perform (If they are inside a software timing loop). I'd
like to find out if the differences outlined below are true....
----------------------------------------------------------------------------------------------------------------
The NEC V20 is NOT a direct plug-in replacement for the 8088,
but it's close enough that in the systems that it works in it's a cheap
upgrade with a lot of potential. It's actually a design-in replacement that usually
works as a drop-in.
The V20 takes a LOT less power, which is good. It is actually an
enhanced 186 for operations. That means some instructions
operate 'differently' than an 8088. Some games like LodeRunner
that wouldn't run on the 286 also wouldn't run on PCs upgraded
with a V20. The patch involved a POP CS instruction, if I remember
all the way back to 1983!!.
The V20 is faster than the 8088 in how it does a lot of opcodes, so
that's good. Think of it as a real mode only 286 without
the abiltiy to go beyond 1Meg in physical addressing space.
The V20 also had the capability of going back into 8080 mode,
and would actually run a version of CP/M that ran on PC's with
the V20. The 8080 mode was actually enhanced to run not
only like an 8080 with 64K of memory, but also as an 8080
with separate code and data space, each 64K. If you were
a CP/M freak, this was INCREDIBLE!
Finally, the V20 has a hardware difference that can cause
problems if you try to drop it in place of an 8088. It is NOT
a direct drop-in, as uninformed people think. The V20 used
a 50-50 clock, like the 80186 part from Intel. The 8088 used
a 1/3-2/3 clock. When you drop in a V20, you have to make
sure that the shorter 1/3 clock period of the 8088 motherboard
still meets spec for the equivilent speed the 50-50 clock the
chip thinks it's working with. In most cases, the V20 chips
far exceed their rated spec, and since the spec is temperature
dependant, in most PCs just dropping in a 6mhz V20 works
in place of the standard 8088 at 4.77Mhz, but where there is
a heat issue (like a laptop) or where the original 8088 was
faster than stock, the V20 can get 'flakey'. However, this is
also tempered by the fact that the V20 runs MUCH cooler than the
8088. Just try touching an operational 8088 and then a V20
once the replacement is made.
Would I drop in a V20 in old machines? You bet! Especially
for all the stuff I write myself. Just getting access to the multiple
shift instructions and the other stuff with the .186 directive in the
assembler is worth it!
Mike Yetsko
N1DVJ
Paul Binns wrote in message <36418...@nemo.idirect.com>...
>I'm looking at the specs for the Tandy 1500HD, which
>lists the CPU as being a v20 NEC.
>
>I've seen references to this CPU many times over the
>years, but have never understood where it stands in
>the technology. Is it basically an 8088, 8-bit CPU?
>
>My books don't even mention it, so that's why I'm
>asking. Would appreciate any and all replies.
>
> -=Paul=-
Andreas von Gorup
Robert L. Doerr wrote
>I once read a post that mentioned that the NEC V20 chip is not really
>a DROP-IN replacement for the 8088 but that it does work in almost
>all instances. I believed it talked about a slightly different clock that
There's a not identical implemented opcode AAD too:
-------------------- snip ----------------
Operands to AAM and AAD Instructions
Those of you who have examined 86 family opcodes with an eagle
eye will have noticed a somewhat spurious "0A" opcode generated
after every AAM or AAD instruction. The opcode is there to
provide the constant divisor or multiplicand for the instruction.
Believe it or not, there wasn't enough room in the microcode of
the original 8086 to hold this constant! Although Intel has
never announced the generality of AAM and AAD, it is there: you
can substitute any other constant for 0A (decimal 10), and that
constant will be used. A86 supports this by letting you give a
constant byte-sized operand to AAM or AAD. Particularly useful
are the instructions AAM 16, which unpacks AL into nibbles AH and
AL; and AAD 16, which reverses the process, packing nibbles AH
and AL into AL.
WARNING: A couple of my users point out to me that the AAD
instruction with a general operand won't work on the NEC V20 and
V30 chips. The operand is assumed to be 10 no matter what it
really is. Since a large number of PC "speed up" kits involve
switching to NEC chips, this will be seen on many PC's. You
should not use AAD with an operand if you want your program to
run on everybody's machine. Too bad. AAM works fine, though
----------------------- snip ----------------
from the documentation of Isaacson's A86 assembler
( http://www.eji.com ) The version I just downloaded is brandnew and
the last timestamp in the zipfile is 23-Nov-1999.
Andreas.
Paul Schlyter
Andreas von Gorup <go...@altavista.net> wrote:
> Robert L. Doerr wrote
> >I once read a post that mentioned that the NEC V20 chip is not really
> >a DROP-IN replacement for the 8088 but that it does work in almost
> >all instances. I believed it talked about a slightly different clock that
>
>
> There's a not identical implemented opcode AAD too: