On 28/06/2013 4:23 PM, Rui Sá wrote:
Yes indeed, it's a 237-pin socket, which is not compatible with the
i386, which has 132 pins.

However, what's the Cyrix CX-83D87-33 coprocessor doing on that board,
as I don't think it's compatible with any 486 CPU: the i486DX included
its own FPU (faster than the 80387) so there won't even be a socket
for a coprocessor on its motherboard.

The cheaper i486SX lacked the FPU and some boards included a socket to
add a math coprocessor later but AFAIAA it was only compatible with
the i487SX, which is the same size as the i486SX itself.

Oh yes, this is bringing back some major old-school memories. The 486SX
was a 486DX with its onboard coprocessor disabled. Intel then sold a
487SX "coprocessor", which was in reality a fully-functional 486DX which
then disabled the original 486SX, and took over all processing functions
from it, including the CPU & FPU. You basically had a dual-processor
board where one processor got disabled.

However, this was not the only way to get back FPU functionality back on
a 486SX. Some people engineered around Intel's 487SX socket design and
instead put a 387 socket on the motherboard. It's possible to attach a
387 FPU copro on the 486SX, since the CPU and FPU were asynchronously
attached. The coprocessor could be clocked separately from (& usually
lower than) the CPU, since the days of the 386. I remember that I had
once attached a 287 coprocessor to a 386DX CPU, so the ability to do
something similar was still available on the 486 generation. A 387 copro
was not nearly as fast as the internal FPU on a 486DX or 487SX, but it
served its purpose.


This board not only has a socket for a coprocessor (which means it
came with an i486SX), but it's smaller than the i487SX socket and has
installed an 80387-compatible FPU and probably some special logic to
make it work (asynchronously?) with the i486SX, making it clearly
slower than a full i486DX for FPU operations. Were the savings worth
the effort, the extra logic and the slower FPU speed?...

Well, things were very expensive back then, so a lot of cheapskates may
have taken any opportunities save some bucks. The cost differential from
buying a full 486DX could've been more than buying a 486SX with a 387
copro; and certainly much better than buying a 486SX + 487SX combo which
was the ultimate waste of money, since it completely disables a
perfectly fine CPU.

Yousuf Khan