From: S_JUFFA@iravcl.ira.uka.de (|S| Norbert Juffa)
Newsgroups: comp.sys.intel
Subject: What you always wanted to know about math coprocessors for 80x86 2/4
Date: 19 Aug 1992 15:01:07 GMT
Organization: University of Karlsruhe (FRG) - Informatik Rechnerabt.
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         Cyrix EMC87    is basically a special version of the Cyrix 83D87.
                        In addition to the normal 387 operating mode, in
                        which coprocessor-CPU communication is handled thru
                        reserved IO-ports, it also offers a memory-mapped
                        mode of operation similar to the operation principle
                        of the Weitek Abacus. Please note that the EMC87 is
                        *not* compatible with Weitek's Abacus coprocessor.
                        They both use the same interface technique (memory
                        mapping) but while the EMC87 uses the standard 387
                        instruction set, the Weitek coprocessors use a
                        different instruction set of their own. Like the
                        Weitek Abacus, the EMC87 occupies a 64 kByte memory
                        block starting at physical address C0000000h. It can
                        therefore only be accessed in the protected or virtual
                        modes of the 386 CPU. DOS programs can access the
                        EMC87 with the help of DOS-extenders or memory
                        managers like EMM386 which run in protected/virtual
                        mode themself. Since the EMC87 provides also the
                        standard CPU interface via IO-ports, it can be used
                        just like any other 387 compatible coprocessor and
                        delivers the same performance as the Cyrix 83D87 in
                        this mode. However, using the memory mapped mode of
                        the EMC87 provides a significant speed advantage.
                        The traditional 387 CPU- coprocessor interface via
                        IO-ports has an overhead of about 16-20 clock cycles.
                        Since the Cyrix 83D87 executes some operations like
                        addition and multiplication in much less time, its
                        performance is limited by the CPU-coprocessor
                        interface. The memory-mapped mode has much less
                        overhead and allows all coprocessor instructions to
                        be executed at full speed and with no penalty. For
                        this reason, Cyrix introduced the EMC87 in 1990.
                        In a test, the EMC87 at 33 MHz ran the single
                        precision Whetstone benchmark at 7608 kWhetstones/sec,
                        while the Cyrix 83D87 at 33 MHz had a speed of
                        only 5049 kWhetstones/sec, an increase of 50.6% [63].
                        In another test, the EMC87 ran a fractal computation
                        at two times the speed of the Cyrix 83D87 and 2.6
                        times as fast as an Intel 387DX [64]. A third test
                        found the EMC87's overall performance to be 20%
                        higher than the performance of the Cyrix 83D87
                        [65]. The Cyrix FasMath EMC87 has also been sold
                        as Cyrix AutoMATH by Cyrix. The two chips are 100%
                        identical. Unlike the Cyrix 83D87, which fits into
                        the 68-pin 387 coprocessor socket, the EMC87 comes
                        in a 121-pin PGA and requires the 121-pin EMC
                        (Extended Math Coprocessor) socket. Note that not
                        all boards have such a socket, a notable exception
                        being IBM's PS/2s, for example. Originally, Cyrix
                        claimed support for the fast memory mapped mode of
                        the EMC87 from a lot of software vendors (including
                        Borland and Microsoft). However, there are only
                        very few applications that make use of it, among
                        them Evolution Computing's FastCAD 3D, MicroWay
                        Inc.'s NDP FORTRAN-386 compiler and Intusofts's
                        Spice [63]. I haven't seen the EMC being offered
                        for about nine month now. It may be that Cyrix
                        has discontinued this product due to lack of
                        sufficient software support. The EMC87 was available
                        in 25 and 33 MHz versions at the end of 1991.
         Cyrix 387+     seems to be the successor to the Cyrix 83D87. On
                        ordering a Cyrix coprocessor about a month ago,
                        I was automatically supplied with a 387+. In my
                        tests, I found the Cyrix 387+ to be about five
                        to 10 percent *slower* than the Cyrix 83D87. However,
                        some instructions like the square root (FSQRT) now
                        ony run at half the speed at which they ran in the
                        83D87 (see performance results below). I also found
                        the transcendental functions on the 387+ to be a bit
                        more accurate than those implemented in the 83D87.
                        Why Cyrix has brought out a new coprocessor slower
                        than the 83D87 I don't know. I have written to Cyrix
                        about this question but haven't received a reply yet.
                        Maybe the new coprocessor solves the one small
                        hardware compatibility problem the 83D87 had (see
                        above paragraph on the 83D87). It could also be that
                        Cyrix had to design around the three Intel patents
                        Intel claims the 83D87 has violated. I have no idea
                        wether the Cyrix 387+ is to replace the 83D87 or
                        if both chips will coexist in the market. Like the
                        83D87, the 387+ is available for speeds of up to
                        40 MHz.
         Cyrix 83S87    is the SX version of the Cyrix 83D87. Just like the
                        Cyrix 83D87 is the fastest 387 compatible coprocessor,
                        the Cyrix 83S87 is the fastest of the 387SX compatible
                        coprocessor [1]. Besides being the fastest 387SX
                        'clone', the Cyrix 83S87 also features the most
                        accurate transcendental functions. The 83S87 is
                        packaged in a 68-pin PLCC  and is available in 16,
                        20 and 25 MHz versions. Due to the advanced power
                        saving features of the Cyrix coprocessor, the typical
                        power consumption of the 20 MHz version is about
                        350 mW [67].
         ULSI 83C87     is a 387 'clone' that came out in early 1991, well
                        after the IIT 3C87 and Cyrix 83D87. Like all clones,
                        it is somewhat faster than the Intel 387DX. Especially
                        the basic arithmetic functions are fast, while the
                        transcendental functions show only a slight speed
                        improvement over the Intel 387DX (see benchmark
                        results below). In my tests, the ULSI had the most
                        inaccurate transcendental functions. However, the
                        maximum relative error is still within the limits
                        set by Intel, so this is probably not an important
                        issue in all but very few applications. The ULSI
                        shows some minor flaws in the tests for IEEE-754
                        compatiblity, but this, too, is unimportant under
                        typical operating conditions. ULSI claims that the
                        program IEEETEST, which was used to test for IEEE
                        compatibility, contains many personal interpretations
                        of the IEEE standard by the program's author and
                        states that there is no ANSI-certified IEEE-754
                        complicency test. While this is most probably true,
                        it is also a fact that the IEEE test vectors used in
                        IEEETEST are sort of an industry standard and that
                        Intel's 387, 486, and RapidCAD chips pass it
                        without a single failure. Since the ULSI Math*Co
                        83C87 fails some of the tests, it is certainly less
                        than 100% compatible with Intel's chips, although
                        this will hardly make any difference in typical
                        operating conditions. The ULSI 83C87 is also not fully
                        compatible with the Intel 387DX in that is does
                        not implement the precision control feature of
                        Intel's coprocessor [58]. While all the internal
                        operations of 80x87 coprocessors are usually done
                        with the maximum precision available (double extended
                        presision with 64 mantissa bits), the 80x87 also
                        offer the possiblity to force lower precision to
                        be used for the basic arithmetic functions add,
                        subtract, multiply, divide, and square root. This
                        feature was included for compatiblity with existing
                        floating-point implementations at the time the 8087
                        was devised. All coprocessors except the ones from
                        ULSI support this feature. Since precision control
                        is rarely used, this incompatibility with the Intel
                        387DX does not pose major problems. IEEE-754 mentions
                        precision control, but requires it only for those
                        systems that don't have the possibility to store
                        single and double precision results. Therefore, the
                        standard does not call for precision control in the
                        387 coprocessor, so the ULSI 83C87's failure to
                        provide rounding control does not constitute a
                        conflict with the IEEE-754 standard for floating
                        point arithmetic. Like the other 387 'clones', the
                        83C87 does not support asynchronous operation of the
                        CPU and the coprocessor. This means that the 83C87
                        always runs at the full speed of the CPU. The ULSI
                        83C87 is available in 20, 25, 33, and 40 MHz versions.
                        The ULSI is produced in high perfromance, low power
                        CMOS. Power consumption at 20 MHz is max. 800 mW
                        (400 mW typical), at 25 MHz it is max. 1000 mW
                        (500 mW typical), at 33 MHz it is max. 1250 mW
                        (625 mW), and at 40 MHz the ULSI Math*Co 83C87
                        consumes max. 1500 mW (750 mW typical) [58]. The
                        83C87 is packaged in a 68-pin ceramic PGA. ULSI
                        coprocessors come with a lifetime warranty. ULSI
                        Systems, Inc. will replace the coprocessor up to
                        three times free of charge should it ever fail.
         ULSI 83S87     is the SX version of the ULSI 83C87 for operation
                        with an Intel 387SX or an AMD Am387SX. It is
                        functionally equivalent to the 83C87. To aid low
                        power laptop designs, the ULSI 83S87 features an
                        advanced power saving design with a sleep mode and
                        a standby mode with only minimal power requirements.
                        Power consumption under normal operating conditions
                        (dynamic mode) is max. 400 mW at 16 MHz (300 mW
                        typical), max. 450 mW at 20 MHz (350 mW typical),
                        and max. 500 mW at 25 MHz (400 mW typical) [58].
                        The ULSI 83S87 is packaged in a 68-pin PLCC.
         Intel RapidCAD is not a coprocessor, strictly seen, although it
                        is marketed as one. Rather, it is a CPU replacement.
                        It is basically an Intel 486DX without the cache and
                        with a 386 pinout. RapidCAD is delivered as a set of
                        two chips. RapidCAD-1 goes into the 386 socket and
                        contains the CPU and FPU, RapidCAD-2 goes into the
                        coprocessor socket and contains a PAL that generates
                        the Ferr signal that is normally generated by a
                        coprocessor and used by the motherboard circuitry to
                        provide 287 compatible coprocessor exception handling
                        in 386/387 systems. The RapidCAD instruction set is
                        compatible with the 386, so it doesn't know the 486
                        specific instructions like BSWAP. Since the RapidCAD
                        CPU core is very similar to 486 CPU core, most of the
                        register to register instructions execute in the same
                        number of clock cycles as on the 486. The use of the
                        386 bus interface causes instructions that access memory
                        to execute at about the same speed as on the 386. The
                        integer performance on the RapidCAD is definitely
                        limited by the low memory bandwidth provided by the
                        386 bus interface (2 clock cylces per bus cycle)
                        and the lack of an internal cache. CPU instructions
                        often execute faster than they can be fetched from
                        memory, even with a big and fast external cache.
                        Therefore, the integer performance of the RapidCAD
                        exceeds that of a 386 by at most 25%. This value
                        was derived by running some programs that use
                        mostly register-to-register operations and few
                        memory accesses. This finding is supported by the
                        SPEC ratings that Intel reports for the 386-33
                        and the RapidCAD-33. While the 386-33 has a
                        SPECint of 6.4, the RapidCAD has a SPECint of 7.3
                        [28], a 14% increase. Note that these tests used
                        the old (1989) SPEC benchmarks suite. While CPU
                        instructions often execute in one clock cycle on
                        the RapidCAD, FPU instructions always take more
                        than seven clock cycles. They are therefore rarely
                        slowed down by the low memory bandwidth provided
                        by the 386 bus interface. My tests show a 70%-100%
                        performance increase for floating-point intensive
                        benchmarks (see below) over a 386 based system
                        using the Intel 387DX math coprocessor. This is
                        consistent with the SPECfp rating reported by Intel.
                        The 386/387 at 33 MHz is rated at 3.3 SPECfp, while
                        the RapidCAD is rated at 6.1 SPECfp at the same
                        frequency, a 85% increase. This means that a system
                        that uses the RapidCAD is faster than any 386/387
                        combination, regardless of the type of 387 used
                        (Intel 387DX or faster clone). The diagnostic disk
                        for the RapidCAD also gives some application
                        performance data for the RapidCAD compared to the
                        Intel 387DX:

                        Application    Time w/ 387DX  Time w/ RapidCAD  Speedup

                        AUTOCAD 11            32 sec            52 sec      63%
                        AutoShade/Renderman  108 sec           180 sec      67%
                        Mathematica(Windows) 103 sec           139 sec      35%
                        SPSS/PC+ 4.01         14 sec            17 sec      21%

                        RapidCAD is available in 25 MHz and 33 MHz versions.
                        It is distributed through other channels than the
                        other Intel math coprocessors. Therefore, I have been
                        unable to obtain a data sheet for it. The RapidCad-1
                        chip gets quite hot when operating and it can be
                        assumed that its power consumption is similar to
                        the 486-33. Therefore, I recommend extra cooling
                        for this chip (see the paragraph below on the 486 for
                        details). The RapidCAD-1 is packaged in a 132-pin
                        PGA, just like the 80386, and the RapidCAD-2 is
                        packaged in a 68-pin PGA like a 80387 coprocessor.
         Intel 486DX    is not a coprocessor. This chip, brought out in
                        1989 functionally combines the CPU (a heavily pipelined
                        implementation of the 386 architecture) with an
                        enhanced 387 (the floating-point unit, FPU) and
                        8 kB of unified code/data cache on one chip. Of
                        course, this description is simplified, for a
                        detailed hardware description, see [52]. The
                        486DX offers about two to three times the integer
                        performance of a 386 at the same frequency.
                        Floating point performance is about three to four
                        times as high as on the Intel 387DX at the same
                        clock rate [29]. Since the FPU is on the same
                        chip as the CPU, the considerable communication
                        overhead between CPU and coprocessor in a 386/387
                        system is omitted, letting FPU instructions run
                        at the full speed permitted by the implementation.
                        The FPU also takes advantage of the on-chip cache
                        and the highly pipelined execution unit. Besides
                        the higher speed, the 486 FPU features more accurate
                        transcendental functions than the Intel 387DX
                        coprocessor according to tests run by me (see below).
                        To achieve better interrupt latency, FPU instructions
                        with a long execution time have been made abortable
                        in the case an interrupt occurs during their
                        execution. The concurrent execution of CPU and
                        coprocessor instructions typical for 80x86/80x87
                        systems is still in existence on the 486, but
                        some FPU instructions like FSIN have nearly no
                        concurrency with CPU instructions, indicating
                        that they make heavy use of both, CPU and FPU
                        resources [53, 1]. The 486DX comes in a 168 pin
                        ceramic PGA (pin grid array). It is available in
                        25 MHz and 33 Mhz versions. Since the end of 1991,
                        there is also a 50 MHz version available done in
                        a CHMOS V process (the 25 MHz and 33 MHz are
                        produced using the CHMOS IV process). Maximum
                        power consumption is 3500 mW for the 25 MHz 486
                        (2600 mW typical), 4500 mW for the 33 MHz version
                        (3500 mW typical), and 5000 mW (4000 mW typical)
                        for the 50 MHz chip. Due to the considerable amount
                        of heat produced by these chips, and taking into
                        consideration the slow air flow provided by the
                        fan in garden variety PC tower cases, I recommend
                        an extra fan directly above the CPU for safer
                        operation. If you measure the surface temperature
                        of an i486 in a normal tower case without extra
                        cooling after some time of operation, you may well
                        come up with something like 80 - 90 degrees Celsius
                        (that is 176 - 194 degrees Fahrenheit for those not
                        familiar with metric units) [54,55]. You don't need
                        the well known and expensive IceCap(tm) to effectively
                        cool your CPU. A simple fan mounted directly above
                        the CPU can bring the temperature down to about 50
                        to 60 degrees Celsius (122 - 140 degrees Fahrenheit)
                        depending on the room temperature and the temperature
                        within the PC case (which depends on the total power
                        dissipation of all the components and the cooling
                        provided by the fan in the power unit). According
                        to a simple rule known as Arrehnius' Law, lowering
                        the temperature by 10 degrees Celsius slows down
                        chemical reactions by a factor of two, thus lowering
                        the temperature of your CPU by 30 degrees should
                        prolong the live of the device by a factor of eight
                        due to the slower aging process. If you are reluctant
                        to add a fan to your system because of the additional
                        noise, settle for a low-noise fan like those
                        available from the German manufacturer Pabst (this
                        is not meant to be an advertisement. I am just the
                        happy owner of such a fan. Besides that, I have no
                        connections to the firm).
         Intel 486DX2   is the name for Intel latest generation of 486 CPUs.
                        Using the DX2 suffix instead of simply DX is meant
                        to be an indicator that these are clock-doubled
                        versions. A normal 486DX operates at the frequency
                        provided by the incoming clock signal. A 486DX2
                        generates a new clock signal from the incoming clock
                        by means of a PLL (phase locked loop). In the DX2,
                        this clock signal has twice the frequency of the
                        incoming clock, hence the name clock-doubler. All
                        internal parts of the 486DX2 (cache, CPU core, FPU)
                        run at this higher frequency. Only the bus interface
                        runs at the normal speed. That way, a 486DX-50 can
                        run on a motherboard designed for 25 MHz operation.
                        Since motherboards for 50 MHz operations are much
                        harder to design than those for 25 Mhz, this makes
                        a 486DX2-50 system easier to built and cheaper than
                        a 486DX-50 system. For all operations that don't
                        access off-chip resources (e.g. register operations)
                        a 486DX2-50 provides exactly the same performance as
                        a 486DX-50 and twice the performance of a 486DX-25.
                        However, since the main memory in a 486DX2-50 systems
                        still operates at 25 MHz, all instructions involving
                        memory accesses are potentially slower than in a
                        486DX-50 system, whose memory also runs at 50 Mhz.
                        The internal cache of the 486 helps this problem a
                        bit, but overall performance of a 486DX2-50 is still
                        lower than that of a 486DX-50, although Intel's
                        documentation [32] shows this drop to be quite small.
                        It depends a lot on the code one runs, though. The
                        nice thing about the 486DX2 is that it allows easy
                        upgrading of 25 and 33 Mhz 486 systems, since the
                        486DX2 is completely pin-compatible with the 486DX.
                        Just take out the 486DX and plug in the new 486DX2.
                        Note that power consumption of the 486DX2-50 equals
                        that of the 486DX-50 (4000 mW typical), and that the
                        486DX2-66 exceeds this by about 30%. These chips get
                        really hot in a standard PC case with no extra cooling.
                        See the above paragraph for more detailed information
                        on this problem.
         Intel 487SX    is the coprocessor intended for use in 486SX systems.
                        The 486SX is basically a 486DX without the floating-
                        point unit (FPU) [48, 50]. Originally Intel sold
                        486DXs with a defective FPU as 486SXs but it has
                        now completly removed the FPU part from the 486SX
                        mask for mass production. The introduction of the
                        486SX in 1991 has been viewed mainly as a marketing
                        'trick' by Intel to take market share from the 386
                        based systems once AMD became successful with their
                        Am386 (AMD has taken as much as 40% of the 386 market
                        due to some superior features such as higher clock
                        frequency, lower power consumption, and a fully
                        static design). A 486SX at 20 MHz delivers a bit
                        less integer performance than a 40 MHz Am386. To add
                        floating-point capabilities to a 486SX based system,
                        it would be easiest to swap the 486SX with a 486DX
                        which includes the FPU. However, Intel has prevented
                        this easy solution by giving the 486SX a slightly
                        different pin out [48, 51]. Since only three pins
                        are assigned differently, clever board manufacturers
                        have come out with boards that accept anything from
                        a 486SX-20 to a 486DX2-50 in their CPU socket and
                        provide a clean upgrade path this way. A set of
                        three jumpers ensures correct signal assignment to
                        the pins for either configuration. To upgrade systems
                        without this feature, one has to buy the 487SX and
                        put it into the "Performance Upgrade Socket" present
                        in most 486SX systems. Once the 487SX was available,
                        it was quickly found out that it is just a normal
                        486DX with a slightly different pin out [49]. Inserting
                        the 487SX effectively shuts down the 486SX in the
                        486SX/487SX system, so the 486SX could be removed
                        once the 487SX is installed. Since the shut down is
                        logical, not electrical, the 486SX still uses power
                        if used with the 487SX, although it is unoperational.
                        Technically speaking, the solution Intel chose was
                        the only practical way to provide a 486SX system with
                        the high level of floating-point performance the
                        486DX offers. The CPU and FPU have to be on the same
                        chip, otherwise the FPU can not make use of the cache
                        on the CPU chip and there would be considerable
                        overhead in CPU-FPU communication (similar to a
                        386/387 system), nullifying most of the arithmetic
                        speedups over the 387. That the 486SX, 487SX, and
                        486DX are not pin-compatible seems to be purely for
                        marketing reasons. To upgrade a 486SX based system,
                        Intel also offers the OverDrive chip, which is just
                        the same as a 487SX with internal clock doubling. It
                        goes also goes into the "Performance Upgrade Socket"
                        found in 486SX systems. The OverDrive roughly doubles
                        the performance of a 486SX/487SX based system. For a
                        explanation of clock doubling, see the description
                        of the 486DX2 above. As the 486SX, the 487SX is
                        available in 20 MHz and 25 MHz versions. At 20 MHz,
                        the 487SX has a power consumption of max. 4000 mW.
                        It is available in a 169 pin ceramic PGA (pin grid
                        array).
         Weitek 3167    was introduced in 1989 to provide the fastest
                        floating point performance possible on a 386 based
                        system at that time. The Weitek Abacus 3167 is not
                        a real coprocessor, strictly speaking, but rather
                        a memory mapped peripheral device. The Weitek 3167
                        was optimized for speed wherever possible. Besides
                        using the faster memory mapped interface to the CPU
                        (the 80x87 uses IO-ports), it does not support many
                        of the features of the 80x87 coprocessors, allowing
                        all of the chip's ressources to be concentrated on
                        the fast execution of the basic arithmetic operations.
                        For a more detailed description of the Weitek 3167 see
                        the first chapter of this document. In benchmark
                        comparisons, the Weitek 3167 provided up to 2.5 times
                        the performance of an Intel 387DX coprocessor. For
                        example, on a 33 MHz 3167 the Whetstone benchmark
                        performed at 7574 kWhetstones/sec compared with the
                        the 3743 kWhetstones/s for the Intel 387DX. Note
                        however that these are single precision results and
                        that the Weitek 3167's performance would drop to
                        about half the stated rate for double precision,
                        while the value for the Intel 387DX would not change
                        much. Anyhow, before the advent of the Intel RapidCAD,
                        the Weitek 3167 usually beat all 387 compatible
                        coprocessors even for double precision operations
                        [63,65,69]. For typical applications the advantage
                        of the Weitek 3167 over the 387 clones is much smaller.
                        In a benchmark test using AutoDesk's 3D-Studio the
                        Weitek 3167 performed at 123% of the Intel 487DX's
                        perfromance comapred with 106% for the Cyrix FasMath
                        83D87 and 118% for the Intel RapidCAD. The Weitek
                        Abacus 3167 is packaged in a 121-pin PGA that fits
                        into an EMC socket provided by most 386 based systems.
                        It does *not* fit into the normal coprocessor socket
                        designed to hold a 387 compatible coprocessor in a
                        68-pin PGA. To get the best of both worlds, one might
                        want to use a Weitek 3167 and a 387 compatible
                        coprocessor in the same system. These coprocessors
                        can coexist in the same system just fine. Only problem
                        is that most 386 based systems contain only one
                        coprocessor socket, usually of the EMC (extended math
                        coprocessor) type. Thus, you can install either a
                        387 coprocessor or a Weitek 3167, but not both. There
                        are little daughter boards available though that fit
                        into the EMC socket and provide two sockets, an EMC
                        and a standard coprocessor socket. At 25 MHz, the
                        Weitek 3167 has a power consumption of max. 1750 mW.
                        At 33 MHz, the max. power consumption is 2250 mW.
         Weitek 4167    is a memory mapped coprocessor that has the same
                        architecture as the 3167 and is designed to provide
                        486 based systems with the highest floating point
                        performance available. It executes coprocessor
                        instructions at three to four times the speed of
                        the Weitek 3167. Although it is up to 80% faster
                        than the Intel 468 in some benchmarks [1,69], the
                        performance advantage for real application is more
                        like 10%. The introduction of the 486DX2 processors
                        has more or less obliterated the need for a Weitek
                        4167, since the DX2 CPUs provide the same performance
                        and all the additional features the 80x87 has over
                        the Weitek Abacus. The Weitek 4167 is packaged in
                        a 142-pin PGA package that is only slightly smaller
                        than the 486's package. At 25 MHz, it has a max.
                        power consumption of 2500 mW [32].

         Chips & Technologies has shipped samples of their 38700 and
         38700SX coprocessors, which are compatible with the Intel 387DX
         and Intel 387SX coprocessors, respectively. Both have already
         been tested in [1]. However, C&T's German distributor (Rein
         Elektronik, Nettetal) states that these coprocessors will
         become generally available not before 4Q 1992. The samples
         tested in [1] showed about the same performance as the Cyrix
         83D87.



         Pricing

         Due to a recent price slashing by Cyrix and subsequently by Intel
         for 387 coprocessors, prices have dropped significantly for all
         287 and 387 compatible coprocessors with hardly any price difference
         between manufacturers. 387DX compatible coprocessors typically sell
         for ~US$ 100 for all speeds except for 40 MHz versions which are
         typically ~US$ 130. 387SX compatible coprocessors sell for ~US$ 90
         regardless of speed with the exception of the 33 MHz version, which
         are ~US$ 100. The Intel 287XL sells for ~US$ 100, while the IIT 2C87
         and Cyrix 82S87 sell for about US$ 70. 8087s may be more expensive,
         the price of an 8087-10 being US$ 150. I bought the Intel RapidCAD
         for US$ 320 and haven't seen it offered for a better price. I see
         the Weitek Abacus 3167-33 being offered for US$ 780 and the 4167-33
         being offered for US$ 1100. This price information reflects the
         price situation as of 08-14-92. Prices can be expected to drop
         slightly in the near future.

         If you have a demand for high floating-point performance, you
         should consider to buy a 486 based system rather than buying
         a 386 based system with an additional coprocessor. A 386 mother
         board for 33 MHz operation sell for ~ US$ 300, together with the
         coprocessor, costs total ~ US$ 400. A 486-33 ISA-board sells for
         US$ 650. While the 486-33 system is 60% more expensive than the
         386/387 system, it also provides 100% more integer and floating-
         point performance (twice the performance). If you want to push
         your 386 based system to maximum floating-point performance and
         can't switch to a 486 based system for some reason, I recommend
         the Intel RapidCAD. It is both faster [1] and cheaper than installing
         a Weitek Abacus 3167 with your 386, which used to be the highest
         performing combination before the RapidCAD came out. Similarily,
         the introduction of the 486DX2 clock-doubler chips have obliterated
         the need for a Weitek 4167 to get maximum floating-point performance
         out of a 486 based system. A 486DX2-66 performs at or above the
         performance level of a 33 Mhz Weitek 4167, even if the latter
         uses single precision rather than double precision. The 486DX-66
         is rated by Intel at 24700 double precision kWhetstones/sec and
         3.1 double precision Linpack MFLOPS. Of course, these benchmarks
         used the highest performance compilers available. But even with
         a Turbo Pascal 6.0 program, I managed to squeeze 1.6 double precision
         MFLOPS out of the 486DX2-66 for the LLL benchmark (for a description
         of the benchmarks mentioned, see the paragraph on benchmarks below).
         Although I haven't yet seen 486DX2-66 processors seen offered to
         the end users for upgrade purposes, I'll recommend the 486DX2-66
         to those that need highest floating-point performance and are
         planning on buying a new PC. The price difference between a
         33 MHz 486DX motherboard and a 486DX2-66 motherboard is around
         US$ 600, well below the price for the Weitek Abacus 4167.



         Operation

         In a 80x86/80x87 system CPU instructions and coprocessor
         instructions are executed concurrently. This means that
         the CPU can execute CPU instructions while the coprocessor
         executes a coprocessor instruction at the same time. The
         concurrency is restricted somewhat by the fact that the
         CPU has to aid the coprocessor in certain operations. As
         the CPU and the coprocessor are fed from the same instruction
         stream and both instruction streams may operate on the same
         data, there has to be a synchronizing mechanism between the
         CPU and the coprocessor.

         8086/8087 or 8088/8087 system, both of the chips look at the
         opcodes coming in from the bus. To do this, both chips have
         the same BIU (bus interface unit) and the 8086 BIU sends the
         status signals of its prefetch queue to the 8087 BIU. This
         assures that both processors always decode the same instructions
         in parallel. Since all coprocessor instruction start with the
         bit pattern 11011, it is easy for the 8087 to ignore all other
         instructions. Likewise the CPU ignores all coprocessor instructions
         except if they access memory. In this case, the CPU computes
         the address of the LSB (least significant byte) of the memory
         operand and does a dummy read. The 8087 then takes the data                                                                                    and does a dummy read.
         from the data bus. If more than one meory access is needed to
         load an memory operand, the 8087 requests the bus from the CPU,
         generates the consecutive addresses of the operand's bytes
         and fetches them from the data bus. After completing the operation,
         the 8087 hands bus control back to the CPU. Since 8087 and CPU
         are hooked up to the same synchronous bus, they have to run at
         the same speed. This means that with the 8087, only synchronous
         operation of CPU and coprocessor is possible. Another 8087
         coprocessor instruction can only be started if the previous one
         has been completed in the NEU (numerical execution unit) of the
         8087. To prevent the 8086 from decoding a new coprocessor
         instruction while the 8087 is still excuting the previous
         coprocessor instruction, the following mechanism is used: The
         compilers and assemblers automatically generate a WAIT instruction
         before each coprocessor instruction. The WAIT instruction tests
         the /TEST pin until its input becomes "LOW". In 8086/8087 systems,
         the 8086 /TEST pin is connected to the 8087 BUSY pin. As long
         as the NEU executes a coprocessor instruction, it forces its
         BUSY pin "HIGH". Thus the WAIT instruction in front of every
         coprocessor instruction stops the CPU until a still executing
         previous coprocessor instruction has finished. The same
         synchronization is used before the CPU accesses data that
         was written by the coprocessor. A WAIT instruction after the
         coprocessor instruction that writes to memory causes the CPU to
         stop until the coprocessor has transferred the data to memory,
         after which the CPU can safely access the data.

         With the help of an additional chip, the 8087 can also be inter-
         faced to the 80186 [36]. The 80186 was the CPU in some PCs (e.g.
         from Philips, Siemens) in the 1982/1983 time frame, but with
         the introduction of the IBM AT which used the 80286, it lost all
         significance for the PC market. The 80C186 (CMOS version of the
         80186) nowadays sells as an embedded controller and can be combined
         with a 80C187 coprocessor which is based on the internals of the
         Intel 387 [37].

         The 80287 CPU-interface is totally different from the solution
         used in the 8087. Since the 80286 implements memory protection
         via an MMU based on segmentation, it would have been much to
         expensive to duplicate the whole protection logic on the coprocessor
         for an interface solution similar to the 8087. In a 80286/80287
         system, the CPU fetches and stores all opcodes and operands for
         the coprocessor. Information is passed through ports F8h - FFh.
         As these ports are accessible under program control, care must
         be taken to not accidentally perform write operation to them, as
         this could corrupt the information in the math coprocessor.
         The execution unit of the 80287 is practically identical to that
         of the 8087, that is, nearly all coprocessor instructions execute
         in the same number of clock cycles on both coprocessors. Due to
         the additional overhead of the CPU/coprocessor interface (at
         least ~40 clock cycles), a 8 MHz 80286/80287 combination can be
         slower than a 8086/8087 system running at the same speed for
         floating point intensive programs. Additionally, most of the
         older 286 boards were configured to run the coprocessor at 2/3
         the speed of the CPU, making use of the ability of the 80287
         to run asynchronous with the CPU. The 80287 has a CKM pin that
         causes the incoming system clock to be divided by three for
         the coprocessor if it is tied to ground. The 80286 always
         divides the system clock by two internally. Thus the ratio 2/3.
         However, when the CKM (ClocK Mode) pin is tied high on the 80287,
         it does not divide the CLK input. This feature has been exploited
         by the maker of coprocessor speed sockets. These sockets tie
         CKM high and supply their own CLK signal with a built-in oscillator,
         thereby allowing the 80287 or compatible to run at a much higher
         speed than the CPU. With an IIT or Cyrix 287 one can have a
         20 MHz coprocessor running with a 8 MHz 80286. Note however that
         the floating-point performance in such a configuration does not
         scale linearly with the coprocessor clock, since all the data
         has to be passed through the much slower CPU. If the coprocessor
         executes mostly simple intructions such as addition and multiplication
         doubling the coprocessor clock in a 10 MHz system to 20 MHz does
         not show any performance increase at all [24]. The 80C287 by AMD
         is a 100% clone of the original Intel 80287, but is produced in
         CMOS not in NMOS as the original Intel chip. This makes for lower
         power consumption.

         The 80287XL, the Cyrix 82S87, and the IIT 2C87 contain the internals
         of a 387 coprocessor, but are pin-compatible to the original 287.
         However, these chips divide the system clock by two internally,
         as opposed to three in the original Intel 80287. Since the 80286
         also divides the system clock by two, they usually run synchronously
         with the CPU. They can also run asynchronously, though.

         The 8087/8087 combination can be characterized as a cooperation of
         partners with equal rights, while the 80286/287 is more a master-
         slave relationship. This makes synchronization much more easy, since
         the complete instruction and data flow of the coprocessor goes thru
         the CPU. Before executing most coprocessor instructions, the 80286
         tests its /BUSY pin which is hooked up to the 287 coprocessor and
         signals if the 80287 is still executing a previous coprocessor
         instruction or has encountered an exception. The 80286 then waits
         until the 80287 is not busy before loading the coprocessor instruction
         into the coprocessor. Therefore, a WAIT instruction before every
         coprocessor instruction is not required. These WAITs are permissible,
         but not necessary in 80287 programs. The second form of WAIT
         synchronisation after the coprocessor has written a memory operand is
         still necessary on 286/287 systems.

         The coprocessor interface in 80386/80387 systems is very similar to
         the one found in 286/287 systems. However, to prevent corruption
         of the coprocessor's contents by programming errors, the IO-ports
         800000F8 - 800000FF are used which are not user accessible. The
         interface has been optimized and uses 32-bit transfers. The overhead
         of the interface has been reduced to about 16-20 clock cycles. For
         some operations on the 387 'clones', that take less than 16 clock
         cycles to complete this effectively limits the execution rate of
         coprocessor instructions. The only sensible solution to provide
         even higher floating point performance was to integrate the CPU
         and coprocessor functionality onto the same chip. This is what
         Intel did with the 80486. The FPU in the 486 also benefits from
         the instruction pipelining and from the integrated cache.



         Performance

         Several computer magazines have published performance comparisons
         at the application level for the 387 coprocessors and Weitek's
         ABACUS 3167 and 4167 chips [1,25,68,70]. Applications tested included
         AutoCAD R11, RenderStar, Quattro Pro, Lotus 1-2-3, and AutoDesk's
         3D-Studio. For most tests, performance improvements for the 387
         clones over Intel's 387DX were small to marginal, the clones running
         the applications no more than 5% to 15% faster than the Intel 387DX.
         In the test of 3D-Studio, one of the few programs that supports
         the Weitek Abacus, the Weitek 3167 improved performance by 23%
         over an Intel 387DX and the 4167 improved performance by 10% over
         the 486 [1].


         The Intel Math Coprocessor Utilities Disk that accompanies the
         Intel 387DX coprocessor has a demonstration program that shows
         the speedup of certain application programs when run with the
         Intel coprocessor vs. a system with no coprocessor.

         Application      Time w/o 387   Time w/ 387  Speedup

         Art&Letters          87.0 sec      34.8 sec     150%
         Quattro Pro           8.0 sec       4.0 sec     100%
         Wingz                17.9 sec       9.1 sec      97%
         Mathematica         420.2 sec     337.0 sec      25%


         The following table is an excerpt from [70]:

         Application      Time w/o 387   Time w/ 387  Speedup

         Corel Draw          471.0 sec     416.0 sec      13%
         Freedom Of Press    163.0 sec      77.0 sec     112%
         Lotus 1-2-3         257.0 sec      43.0 sec     597%


         The following table is an excerpt from [25]:

         Application      Time w/o 387   Time w/ 387  Speedup

         Design CAD, Test1    98.1 sec      50.0 sec      96%
         Design CAD, Test2    75.3 sec      35.0 sec     115%
         Excel, Test 1         9.2 sec       6.8 sec      35%
         Excel, Test 1        12.6 sec       9.3 sec      35%



         The performance statistics below were put together with the
         help of four widely known numeric benchmarks and two benchmarks
         developed by me. Three Pascal programs, one FORTRAN program,
         and two assembly language program were used. The assembly language
         programs were linked with Turbo-Pascal 6.0 for library support,
         especially to include the coprocessor emulator of the TP 6.0
         run-time library. The Pascal programs were compiled with Turbo
         Pascal 6.0 from Borland International, a non-optimizing compiler
         that produces 16-bit code. The FORTRAN program was compiled using
         MS FORTRAN 5.0, an optimizing compiler that generates 16-bit
         code. All programs except PEAKFLOP and SAVAGE, which use double
         extended precision, use double precision variables. Note that
         using a highly optimizing compiler producing 32-bit code you
         will see much higher performance for some benchmarks. For example,
         Intel rates the 33 MHz 386/387DX at 3290 KWhetstones/sec and 0.4
         double precision LINPACK MFLOPS [28,29]. The 33 MHz Intel 486 is
         rated by Intel at 12300 KWhetstones/sec and 1.6 double precision
         LINPACK MFLOPS [30]. The compilers used in these benchmarks run by
         the chip vendor are the ones that give the highest performance
         available. These compilers are in the US$ 1000+ price range.
         Some of them may be experimental or prereleased versions not
         available to the general public. The relative performance of
         one coprocessor to another could vary depending on the code
         generated by compilers. Non-optimizing compilers tend to generate
         a high percentage of operations which access variables in memory,
         while optimizing compiler produce code that contains many
         operations involving registers. Thus it is well possible that
         coprocessor A beats coprocessor B running benchmark Z if compiled
         with compiler C, but B beats A when the same benchmark is compiled
         using compiler D. All benchmark in this overview were run from
         floppy under a 'bare-bones' MS-DOS 5.0 without the CONFIG.SYS
         and AUTOEXEC.BAT files. This way, it was made sure no TSR or
         other program unnecessarily stole computing resources from the
         benchmarks.

         Coprocessor performance also depends on the motherboard, or more
         specifically the chip set used on the motherboard. In [34] and [35]
         identically configured motherboards using different 386 chip sets
         were tested. Among other tests a coprocessor benchmark was run
         which is based on a fractal computation and its execution time
         recorded. The following tables showing coprocessor performance
         to vary with the chip set have been copied from these articles
         in abridged form.

                       Cyrix                                   Cyrix
         chip set      387+                 chip set           83D87

         Opti, 40 MHz  24.57 sec   97.0%    PC-Chips, 33 MHz  26.97 sec   93.0%
         Elite,40 MHz  24.46 sec   97.4%    UMC,      33 MHz  27.69 sec   90.5%
         ACT,  40 MHz  23.84 sec  100.0%    Headland, 33 MHz  25.08 sec  100.0%
         Forex,40 MHz  23.84 sec  100.0%    Eteq,     33 MHZ  27.38 sec   91.6%

         This shows that performance of the same coprocessor can vary by
         up to ~10% depending on the chip set used on your board, at least
         for 386 motherboards (similar numbers for 286, 386sx, and 486 are
         unfortunately not available). The benchmarks for this article were
         run on a board with the Forex chip set, which is one of the fastest
         386 chip sets there is, not only with respect to floating-point
         performance [35].


         Description of benchmarks

         PEAKFLOP is the kernel of a fractal computation. It consists
         mainly of a tight loop written in assembly code and fine tuned
         to give maximum performance. All variables are held in the
         CPU's and coprocessor's registers, so the only memory access
         is for opcode fetches. The main loop contains three multiplications
         and five additions/subtractions. This ratio is fairly typical
         for other floating point intensive programs as well. The whole
         program fits nicely into even a very small CPU cache. Due to
         the nature of this program, its MFLOPS rate is hardly to be
         exceeded by any program that calculates anything useful. Thus
         the name PEAKFLOP. You will find the source code for PEAKFLOP
         in appendix B.

         TRNSFORM multiplies an array of 8191 vectors with a 3D-transformation
         matrix (a 4x4 matrix). Each vector consists of four double precision
         values. Multiplying vectors with a matrix is a typical operation in
         the manipulation (e.g. rotation) of 3D objects which are made up from
         many vectors decribing the object. This benchmark stresses addition
         and multiplication as well as memory access. For each vector, 16
         multiplications and 12 additions are used. About 256 kByte of data
         is accessed during the benchmark. TRNSFORM is implemented as an
         optimized assembler program linked with the Turbo Pascal 6.0 library.
         For the IIT 3C87, a special version was written that makes use of
         the special F4X4 instruction available on that coprocessor. F4X4
         does a full multiplication of a 4x4 matrix by a 4x1 vector in a
         single instruction. The full source code for the TRNSFORM program is
         in appendix B.

         LLL is short for Lawrence Livermore Loops [21], a set of kernels
         taken from real floating point extensive programs. Some of these
         loops are vectorizable, but since we don't deal with vector
         processors here, this doesn't matter. For this test, LLL was
         adapted from the FORTRAN original [20] to Turbo Pascal 6.0. By
         variable overlaying (similar to FORTRAN's EQUIVALENCE statement)
         memory allocation for data was reduced to 64 kB, so all data fits
         into a single 64 kB segment. The older version of LLL is used here
         which contains 14 loops. There also exists a newer, more elaborate
         version consisting of 24 kernels. The kernels in LLL exercise only
         multiplication and addition. The MFLOPS rate reported is the
         average of the MFLOPS rate of all 14 kernels as reported by the
         LLL program. LLL and Whetstone results (see below) are reported
         as returned by my COMPTEST test program in which they have been
         included as a measure of coprocessor/FPU performance. COMPTEST
         has been compiled under Turbo Pascal 6.0 with all 'optimizations'
         on and using my own run-time library, which gives higher perfor-
         mance than the one included with TP 6.0. My library is available
         as TPL60N15.ZIP from garbo.uwasa.fi and ftp-sites that mirror
         this site.

         Linpack [5] is a well known floating-point benchmark that also
         heavily exercises the memory system. Linpack operates on large
         matrices and takes up about 570 kB in the version used for this
         test. This is about the largest program size a pure DOS system
         can accomodate. Linpack was originally designed to estimate
         performance of BLAS, a library of FORTRAN subroutines that
         handles various vector and matrix operations. It uses two routines
         from BLAS which are thought to be typical of the matrix operations
         used by BLAS. Both routines only use addition/subtraction and
         multiplication. The FORTRAN source code for Linpack can be
         obtained from the automated mail server netlib@ornl.gov. Linpack
         was compiled using MS Fortran 5.0 in the HUGE memory model (which
         can handle data structures larger than 64 kB) and with compiler
         switches set for maximum optimization. Linpack repeatedly does
         the same test. The number reported is the maximum MFLOPS rate
         returned by Linpack. Linpack MFLOPS ratings for a great number
         of machines are contained in [6]. This PostScript document is
         also available from netlib@ornl.gov.

         Whetstone [2,3,4] is a synthetic benchmark based upon statistics
         collected about the use of certain control and data structures
         in programs written in high level languages. Based on these
         statistics, Whetstone tries to mirror a 'typical' HLL program.
         Whetstone performance is expressed by how many theoretical
         'whetstone' instructions are executed per second. It was
         originally implemented in ALGOL. Unlike PEAKFLOP, LLL, and
         Linpack, Whetstone not only uses addition and multiplication
         but exercises all basic arithmetic operations as well as some
         transcendental functions. Whetstone performance depends on the
         speed of the coprocessor as well as on the speed of the CPU,
         while PEAKFLOP, LLL, and Linpack place a heavier burden on the
         coprocessor/FPU. There exists an old and a new version of
         Whetstone. Note that results from the two versions can differ
         by as much as 20% for the same test configuration. For this
         test, the new version in Pascal from [3] was used. It was
         compiled with Turbo Pascal 6.0 and my own library (see above)
         with all 'optimizations' on.

         SAVAGE tests the performance of transcendental function
         evaluation. It is basically a small loop in which the sin,
         cos, arctan, ln, exp, and sqrt functions are combined in a
         single expression. While sin, cos, arctan, and sqrt can be
         evaluated directly with a single 387 coprocessor instruction
         each, ln and exp need additional preprocessing for argument
         reduction and result conversion. According to [14], the Savage
         benchmark was devised by Bill Savage, and is distributed by:
         The Wohl Engine Company, Ltd., 8200 Shore Front Parkway,
         Rockaway Beach, NY 11693, USA. Usually, Savage is programmed
         to make 250,000 passes though the loop. Here only 10,000 loops
         are executed for a total of 60,000 transcendental function
         evaluations. The result is expressed in function evaluations
         per second. SAVAGE source code was taken from [7] and compiled
         with Turbo Pascal 6.0 and my own run-time library (see above).


         Benchmark results for 387 coprocessors, coprocessor emulators and
         the Intel RapidCAD and Intel 486 CPUs.


         40 MHz         PEAKFLOP TRNSFORM LLL     Linpack Whetstone Savage
                        MFLOPS   MFLOPS   MFLOPS  MFLOPS  kWhet/sec Func/sec

         386, EM87      0.0084   0.0080   0.0060  0.0060         31      502 ##
         386, Franke387 0.0369   0.0295   0.0233  0.0215        164     4002 $$
         386, TP 6 Emu  0.0316   0.0273   0.0200  0.0190        160     3794 %%
         Intel 387DX    0.9204   0.7212   0.3932  0.3211       2428    52677
         ULSI 83C87     1.2093   0.7936   0.3890  0.3120       2528    56926
         IIT 3C87       1.0196   0.7145   0.3834  0.3179       2663    58766
         IIT 3C87,4x4   1.0196   1.7244   0.3834  0.3179       2663    58766 ??
         Cyrix 387+     1.1305   0.8162   0.3945  0.3208       2946    80322
         Intel RapidCAD 2.2128   1.8931   0.7377  0.5432       4810    86957
         Intel 486      2.4762   2.1335   1.1110  0.8204       6195    98522


         33.3 MHz       PEAKFLOP TRNSFORM LLL     Linpack Whetstone Savage
                        MFLOPS   MFLOPS   MFLOPS  MFLOPS  kWhet/sec Func/sec

         386, EM87      0.0070   0.0040   0.0050  0.0050         26      418 ##
         Franke387      0.0307   0.0246   0.0194  0.0179        137     3335 $$
         386, TP 6 Emu  0.0263   0.0227   0.0167  0.0158        133     3160 %%
         Intel 387DX    0.7647   0.6004   0.3283  0.2676       2046    43860
         ULSI 83C87     1.0097   0.6609   0.3239  0.2598       2089    47431
         IIT 3C87       0.8455   0.5957   0.3198  0.2646       2203    49020
         IIT 3C87,4X4   0.8455   1.4334   0.3198  0.2646       2203    49020 ??
         Cyrix 387+     0.9286   0.6806   0.3293  0.2669       2435    66890
         Cyrix 83D87    1.013       N/A   0.333   0.273        2550      N/A
         Intel RapidCAD 1.8572   1.5798   0.6072  0.4533       3953    72464
         Intel 486      2.0800   1.7779   0.9387  0.6682       5143    82192

         For comparison:

                        PEAKFLOP TRNSFORM LLL     Linpack Whetstone Savage
                        MFLOPS   MFLOPS   MFLOPS  MFLOPS  kWhet/sec Func/sec

         i486DX2-66     4.1601   3.4227   1.6531  1.3010      10655   163934
         i486DX2-50     3.0589   2.6665   1.2537  0.9744       7962   123203
         i387, 20 MHz   0.2253   0.3271   0.1434  0.1171        952    21739 ++
         i387DX, 20 MHz 0.3567   0.4444   0.1484  0.1161       1034    24155 &&
         i80287, 5 MHz  0.0281   0.0310   0.0242  0.0222        150     3261 !!
         i8087,9.54 MHz 0.0636   0.0705   0.0321  0.0219        234     5782 **
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