A 487 is a full 486DX with integrated FPU and CPU.
It just disables the other CPU installed.

With earlier CPU/FPU pairs, there was a protocol for the two processors to work together in a system. The two sockets communicated.
A 486 was designed with an integrated FPU, so there is no socket protocol, the communication is all internal.
Therefore the only way to use an FPU is to have a single processor with integrated FPU.

I have no idea what you mean by 'high performance ALU unit' though. There is only one 486 CPU. A 486SX is a 486 with the FPU disabled. A 487 is a 486 that fits into the 'FPU' socket of a 486SX board.
They all have the same ALU.

Scali wrote:

I have no idea what you mean by 'high performance ALU unit' though. There is only one 486 CPU. A 486SX is a 486 with the FPU disabled. A 487 is a 486 that fits into the 'FPU' socket of a 486SX board.
They all have the same ALU.

He probably meant something like using an SX2 or a 486-class processor from another maker with poor FPU performance, then use an i487 to enhance its floating point capabilities.

alexanrs wrote:

He probably meant something like using an SX2 or a 486-class processor from another maker with poor FPU performance, then use an i487 to enhance its floating point capabilities.

Ah right, well that won't be possible, because there's no way to communicate between them.
I think the reason for the 487 is that many 486SX systems were sold with a surface-mount CPU soldered to the board, rather than in a socket. So you could not remove the CPU and replace it with a regular 486DX. Hence the solution of putting a '487' in a second socket, which disables the onboard CPU.

thank you all for the fast replies;
maybe its easier to explain, despite my bad english skills, on the example of matze's machine from another thread:

by matze79 » 2015-4-24 @ 15:05 […]
Show full quote

by matze79 » 2015-4-24 @ 15:05

Cyrix 486 DRx2 25/50, 1Kb L1 - 50Mhz
Intel 16-33Mhz 387DX @ 25Mhz
OPTI 25Mhz Chipset, No Cache, Unknown Mainboard with 8xSIPP Slot

so matze is using a 486 with the pins of a 386 and therefore he can add an external 387dx and use the powerful ALU of the Cyrix and the fast FPU of the 387; now, as you seem to agree, the same 486 processor with newer pinout, so that it could fit the 486 motherboard, wont allow for this?
it seems its not the innerts of the 486 CPU that disallow the external FPU, its just the pinout?
if i understand you well, just placing that processor on a 486 mainboard will disallow the here used by matze external fpu which works perfectly on older mainboards... madness
so maybe its the 486 chipset that needs to learn how to use separate ALU and FPU?

mrau wrote:

it seems its not the innerts of the 486 CPU that disallow the external FPU, its just the pinout?

It's both I suppose.
There is no FPU interface, because the FPU is integrated in the CPU. So you couldn't create any pins to the outside.
Note that what you quoted uses a 387 FPU, not a 487.
This is because that 486 uses the 386 socket/bus, and 386 does have external FPU support. So technically it's a 386 system, just with a more powerful ALU than a regular 386, resulting in 486-like performance.

It is not about the chipset, it is also about the CPU itself. Some instructions require both the CPU and the FPU to perform a task at the same time (eg the CPU has to generate an address on the bus, and the FPU has to read/write the data).
The 486 simply isn't designed like that.

The i487 is really the first Overdrive chip, which is what it should have been called in the first place.

Anonymous Coward wrote:

The i487 is really the first Overdrive chip, which is what it should have been called in the first place.

But then you could argue that Intel should have named all other OverDrive chips something else...
Because only the i487 and one version of the 486 OverDrive (ODP) used a second socket on the motherboard, leaving the main CPU in place, but disabling it.
The other type of 486 OverDrive (ODPR) and all later OverDrive CPUs would replace the main CPU in the same socket.

Weren't there actually two different types of Overdrive chips with slightly different pinouts? I thought one of them was designed to disable the existing chip a la 487.
At least, this is the impression I get from reading my DTK pkm 0031y motherboard manual, a board that predates ZIF socket 2 and 3 (has a 168 pin socket for regular 486s, and a special 169 pin socket for i487 and overdrives).

I think the markings are ODP (OverDrive Processor) and ODPR (OverDrive Processor Replacement).

There are 487s that where placed under the 486 CPU.

But i think they will only work with 486S Fascache's from Cyrix.

http://xhoba.x86-guide.com/en/collection/Cyrix-487.html

The Performance is about the same of the Cyrix 487/387 for 387 FPU Socket.

Yes, from what I could find, the Cyrix 486S is a 486DLC, but modified to work in a 486 socket.
So like the 486DLC, it is actually a 386-like CPU, with no FPU integrated. So the 487S will use the 386-387 interface that was already present in the 486DLC.