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Can you use an 80287 in place of an 8087-1?

jscipione

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I’m building a micro-8088 kit and I have a couple of quick questions.

Can you use an 80287 in place of an 8087-1 math coprocessor? I happen to have a 10mhz rated 80287 chip (and a 10mhz rated 8088-1 chip.) I can’t find any info on if the chip is compatible with an 8088 though.

Can you use a 486 motherboard as an ISA backplane if you take the main processor out?

What is a cheap eeprom programmer to program the BIOS ROM, PS/2 gal, and other chip? Will a Pick It 3 work? TL866II Plus go for around $60 which more than I’d like to pay.
 
Not without immense modification

The 287 does not have a built in mmu

Funny part is even though the 287 is IEEE compliant it actually is typically slower due to the missing on die mmu

There were however things like the 287turboplus that would allow a 16bit 287 to plug in place of an 8bit XT bus 8087

It should be possible to custom design your board with effort to interface a 287 alongside an 8086.

After the 80187 fiasco…
I always believed it was unfortunate that Intel didn’t include universal FPU compatibility both on the board and chip, doing so would have added under 500 transistors to enable 8/16 support on all FPUs and to 286 motherboards.
Ah well, can’t rewrite history
 
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That’s a no.

Are any of my other ideas possible? Use a 486 mb as a backplane? Cheap eeprom programmer?
 
Use a 486 mb as a backplane?

Likely not without inflicting massive damage on the board. You could try tracking down the datasheets for whatever chipset happens to be on your particular motherboard and see if you can glean from them if the chipset tristates all of its bus drivers if the CPU isn't present, but lacking that you'd probably be in a position to have to do "evil things" to the board (cutting traces) to isolate the bus connectors from anything that might drive them.
 
There were however things like the 287turboplus that would allow a 16bit 287 to plug in place of an 8bit XT bus 8087
The 287turbo(plus) was an upgrade for 286 systems was it not? I thought these were just dinguses with separate clock crystals to run the 287 in async mode.

Still, the option for a 287 on an 808x would be very nice. There are 808x faster than 10MHz, but no matching FPU. Would also be nice to get the 387 instructions.
 
The 287turbo(plus) was an upgrade for 286 systems was it not? I thought these were just dinguses with separate clock crystals to run the 287 in async mode.

Yes. There is absolutely no way to adopt an 80287 to work directly in place of an 8087, the bus interface is completely and fundamentally different. Widgets to overclock a 287 were fairly common because natively the original 80287 divided the system clock fed to the 286 by 3 instead of 2.
 
Yes. There is absolutely no way to adopt an 80287 to work directly in place of an 8087, the bus interface is completely and fundamentally different. Widgets to overclock a 287 were fairly common because natively the original 80287 divided the system clock fed to the 286 by 3 instead of 2.
I see you say “directly” which is true but…

The PCSG 'Breakthrough 286', was 12MHz, with onboard cache and a socket for '287 co-processor, and yes, it has a cable to jack-in at the 8088 socket.

So yeah there were certainly 286 upgrade cards for XTs that would also allow a 287 to pop in as well

I remember “coprocessor “ cards that popped in an XT, some even had something as strange as a 68000 to speed up spreadsheets.
 
So yeah there were certainly 286 upgrade cards for XTs that would also allow a 287 to pop in as well

Sure, but like Chuck already said, that’s not what we were talking about. The 287 sockets on those 286 upgrade cards are speaking directly with the 286 using their own protocols, the claim being evaluated was if it were possible to put an 80287 into an 8087 socket without upgrading the CPU. And that’s a hard no… with one tiny exception I will clarify below:

I remember “coprocessor “ cards that popped in an XT, some even had something as strange as a 68000 to speed up spreadsheets.

Sure, but how is that relevant? You could theoretically interface any digital computer ever made to an ISA bus connector with suitable glue and write software that throws compute jobs at it, that doesn’t mean your device is a direct replacement for an 8087.

To really veer off into the weeds, in principle at least *with a 286 or higher CPU* you could replace the Intel math coprocessor with “something else” and transparently redirect/translate the x87 code so your arbitrary other hardware could execute the desired operation. This is possible because all later iterations of the x87 FPU are “slaves” to the main CPU, which handles all instruction fetching, and do the work of ”trapping” the FPU instructions and routing them to the coprocessor. It just so happens that these later CPUs, if the FPU is not present, also have the ability to process any trapped instructions through a specified error handler which can, for instance, consist of emulator software. Here’s an old thread about such software:


but as that thread also makes clear the 8088/8086 CPUs CANNOT do the same trick. With those CPUs x87 instructions just look like No-Ops, they have no capacity to trap them. With those CPUs the 8087 actually sits parallel on the bus and independently intercepts the instructions meant for itself; its relationship to the primary CPU is completely different from the 80287’s and the latter lacks the necessary hardware to run in that sort of role.

In short, the only way to *transparently* sub something else for an 8087 would be to build a dingus that sticks in the 8087 socket, replicates the 8087’s bus interface (which, again, is basically a subset to the 8086’s), and grabs the opcodes off the wire to shove into whatever actual compute hardware you care to use. (which I guess could maybe be an 80287 if you’re a true glutton for punishment and enjoy doing pointless things…) I’m sure it’d be very doable with a modern FPGA, but not so trivial for 1980’s vintage discrete glue.
 
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