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Хорошо. А FasMath соответственно даёт выигрыш производительности? В табличке он обозначен как Cyrix EMC87?

http://www.chipdb.org/img-cyrix-automath-33-449.htm

А Weitek 3167 они полюбому хотели спецсофт или отчасти работали и так?

dps написал:

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А Weitek 3167 они полюбому хотели спецсофт или отчасти работали и так?

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Если подойти к вопросу критично, это даже не совсем математический сопроцессор в привычном понимании:

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The architecture of the Weitek chips differs significantly from the 80x87. Strictly speaking, the Weitek Abacus 3167 and 4167 are not coprocessors in
that they do not transparently extend the CPU architecture; rather, they could be described as highly-specialized, memory-mapped IO devices. But as
the term "coprocessor" has been traditionally used for these chips, they will be referred to as such here.

The Weitek coprocessors have a RISC-like architecture which has been tuned for maximum performance. Only a small instruction set has been implemented in
the chip, but each instruction executes at a very high speed (usually only a few clock cycles each). Instructions available include load/store, add,
subtract, subtract reverse, multiply, multiply and negate, multiply and accumulate, multiply and take absolute value, divide reverse, negate,
absolute value, compare/test, convert fix/float, and square root. In contrast to the 80x87 family, the Weitek Abacus does not support a double extended
format, has no built-in transcendental functions, and does not support denormals. The resources required to implement such features have instead
been devoted to implement the basic arithmetic operations as fast as possible.

While the 80x87 coprocessors perform all internal calculations in double extended precision and therefore have about the same performance for single
and double-precision calculations, the Weitek features explicit single and double-precision operations. For applications that require only single-
precision operations, the Weitek can therefore provide very high performance, as single-precision operations are about twice as fast as their double-
precision counterparts. Also, since the Weitek Abacus has more registers than the 80x87 coprocessors (31 versus 8), values can be kept in registers more
often and have to be loaded from memory less frequently. This also leads to performance gains.

The Weitek's register file consists of 31 32-bit registers, each one capable of holding an IEEE single-precision number. Pairs of consecutive single-
precision registers can also be used as 64-bit IEEE double-precision registers; thus there are 15 double-precision registers. The Weitek register
file has the standard organization like the register files in the 80386, not the special stack-like organization of the 80x87 coprocessors.

To the main CPU, the Weitek Abacus appears as a 64 KB block of memory starting at physical address 0C0000000h. Each address in this range
corresponds to a coprocessor instruction. Accessing a specified memory location within this block with a MOV instruction causes the corresponding
Weitek instruction to be executed. (The instructions have been cleverly assigned to memory locations in such a way that loads to consecutive
coprocessor registers can make use of the 386/486 MOVS string instruction.) This memory-mapped interface is much faster than the IO-oriented protocol
that is used to couple the CPU to an 80287 or 80387 coprocessor. The Weitek's memory block can actually be assigned to any logical address using the MMU
(memory management unit) in the 386/486's protected and virtual modes. This also means that the Weitek Abacus *cannot* be used in the real mode of those processors, since their physical starting address (0C0000000h) is not within the 1 MByte address range and the MMU is inoperable in real mode.
However, DOS programs can make use of the Weitek by using a DOS extender or a memory manager (such as QEMM or EMM386) that runs in protected/virtual
mode itself and can therefore map the Weitek's memory block to any desired location in the 1 MByte address range.

Typically the FS segment register is then set up to point to the Weitek's memory block. On the 80486, this technique has severe drawbacks, as using the
FS: prefix takes an additional clock cycle, thereby nearly halving the performance of the 4167. Most DOS-based compilers exhibit this problem,
so the only way around it is to code in assembly language [75]. The Weitek Abacus 3167 and 4167 are also supported by the UNIX operating system

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to Max1024 - советую начать с матчасти, хотя-бы прочтите внимательнее эту тему, ибо то что вы написали выше читать дико.
Во первых к A80486SX2-50 (кстати бывают и SX2-66) сопр IIT X2 487DLX25/50 никак не приспособить, поскольку 486SX
не работают совместно с сопроцессором. Они им заменяются. Либо на лоха - A80487SX (до 25MHz) что тупо развод на деньги,
а проще на A80486DX2 - большинство SX матерей их поддерживают, в отличии от просто 486DX. Как вариант - 486 OVERDRIVE

Как ни странно, но с внешним сопроцессором могут работать 486DX (встроенный в ядро при этом отключается)
Имя этому чуду Weitek 4167 и работают они на особых мамках типа таких. Но это уже отдельная тема.

А IIT X2 487DLX25/50 предназначен для супер-386-х вроде Техасца TI486SXL2-50 и Cyrix Cx486DRx2 25/50GP - оба с 8kb кеша L1
Хотя и это не максимальные варианты, существуют супер 386-е связки Cyrix Cx486DRx2 33/66GP + ULSI US83D87-C DX2 66MHz
Но такое вы вряд-ли когда нибудь найдёте, даже на 50MHz версии весьма редки и стоят довольно неприличных денег.

Badman написал:

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Мне казалось в свое время, что самый быстрый из 486SX это GREEN CPU U5 486-40, Виндовс на нем летала. По ощущениям, он существенно обгонял равночастотные процессоры всех других производителей.

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Вообще-то так и есть, UMC GREEN CPU U5SX 486-40 (U5S-SUPER40) уделает AMD A80486SX2-66