Microcode inside the Intel 8087 floating-point chip: register exchange
righto.com36 points by pwg 2 hours ago
36 points by pwg 2 hours ago
Author here if anyone has questions about the 8087's microcode...
I worked in Systems Validation at Intel when the 8087 was current. Intel had an engineer dedicated to validating customer bug reports and reproducing them. Day in, day out, that's pretty much all he did. Sooooo many corner cases, and so many opinions on what the 'right' thing to do was when you lost precision[1].
[1] I'd say that over half of the bug reports were people who were annoyed that doing fp instructions in one order got them the right answer but in another order got them the wrong answer.
80 bits always seemed a strange choice for floating point, but as soon as you said there’s a 16-bit exponent and a 64-bit fraction part, it made sense.
I assume microcode was a choice for both ease of development/testing/changes and saving die space. Would there come a point later on where performance could be gained by converting the microcode into a full set of discrete logic, or is that not worth the effort?
Usually, it's not worth the effort of converting microcode into discrete logic to get performance. Among other things, it's a mess to try to fix a bug.
A few exceptions: The different models of the IBM System/360 mainframe are almost all microcoded, except for the high-end machines, which were hard-wired for performance. The design of the Apollo Guidance Computer is microcode, but the implementation is discrete logic. The 8086 and derivatives are microcoded, except NEC created a faster hard-wired version, the V33.
Wouldn't it be simpler for Intel to have designed a chip, with those 8 identical instructions (xfer, shift, add, arith, far jmp, far call, local jmp, misc), but read/executed from normal RAM accessible by the user, perhaps with a tiny cache, instead all these ROM/microcode special compression/hidden architecture shenanigans?
That's basically the RISC approach, using simple one-clock instructions instead of complex microcoded instructions. In the case of the 8087, it made sense to use microcode because the 8087 is running in parallel with the regular 8086 processor. If the 8087 is constantly fetching micro-instructions from RAM, it will get in the way of the 8086. (Note that RISC chips rapidly added floating-point units, even though that goes against the strict RISC ideology.)
This is exactly the theory of the RISC and VLIW processors, which replaced, respectively, the vertical microprograms and the horizontal microprograms stored in ROMs, which were used in the processors of the seventies, with normal programs with simple instructions, which were normally executed from fast cache memories, thus achieving the same speed as microprograms.
However, when the 8087 was designed, RISC and VLIW processors were still in the future, because a fast cache memory allowing the execution of an instruction per clock cycle was still far too expensive in comparison with a microprogram ROM.