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Author here for your 8087 questions. I find adders and ALUs interesting because they are key to the performance of a system and every system implements them differently.
I've been curious about transistor counts for floating point units for a while, but it's hard to find information about them.
It is interesting that over the years people have produced synthesizable RTL HDL for the 8086/8088 and later, with varying degrees of fidelity, but no-one seems to have produced similar for the 8087.
/* It's a bummer that there is addition but no vipition. */
I knew a guy who bred snakes but could never really get much out of his adders. Turns out what he needed to do was saw up some tree trunks to make rough platforms for them, and they bred like crazy. Adders can multiply really efficiently with log tables.
Author here for your 8087 questions. I find adders and ALUs interesting because they are key to the performance of a system and every system implements them differently.
> take two clock cycles to complete an addition. How does the clocking work exactly? The circuit is fed A and B and up down up down clock and then the output appears? How does the consumer (circuit) know when to read the result? Is there a "result is ready" flag? How long does the result stay stable? One full clock cycle? So many questions...
It is interesting that over the years people have produced synthesizable RTL HDL for the 8086/8088 and later, with varying degrees of fidelity, but no-one seems to have produced similar for the 8087.
Author here for your 8087 questions. I find adders and ALUs interesting because they are key to the performance of a system and every system implements them differently.
Any idea how much adder designs changed on modern CPUs compared to back then? I mean there's only so much you can optimize in those, I think...
It is interesting that over the years people have produced synthesizable RTL HDL for the 8086/8088 and later, with varying degrees of fidelity, but no-one seems to have produced similar for the 8087.
The ROM used different sized transistors to store two bits per transistor. That's pure analog territory, which most HDLs don't touch.
Author here for your 8087 questions. I find adders and ALUs interesting because they are key to the performance of a system and every system implements them differently.
Do you know about how many transistors are needed to implement the adder (or the FPU as a whole)? And how it scales with the width of the numbers (16 bit, 32 bit, etc)? I've been curious about transistor counts for floating point units for a while, but it's hard to find information about them.
Earlier quoted context omitted.
> take two clock cycles to complete an addition. How does the clocking work exactly? The circuit is fed A and B and up down up down clock and then the output appears? How does the consumer (circuit) know when to read the result? Is there a "result is ready" flag? How long does the result stay stable? One full clock cycle? So many questions...
The adder is not clocked. You can see from the diagrams that there are no clock inputs. The clock cycles comment is more an expression of the length of time that it takes before all of the carry rippling and whatnot settles down.
There is some complicated timing within a clock cycle with slightly delayed clocks and whatnot, for instance, to precharge the carry lines at the beginning of the operation. The 8087 is mostly synchronous with the clock, but they "cheat" in many places.
It is interesting that over the years people have produced synthesizable RTL HDL for the 8086/8088 and later, with varying degrees of fidelity, but no-one seems to have produced similar for the 8087.
AIUI, the 8087 was essentially at the extreme cutting edge of what was possible to produce with the technology of the time, and even Intel at the time was largely treating it as a likely-to-fail project.
My educated guess is that primarily simply no-one has needed this, and secondarily it's hard. They're running softwares that can do all of their floating point in software anyway and they just don't need an 8087 on an FPGA. And floating point on an FPGA uses a lot of area, if one is taking the easy route of just emulating the external behaviour rather than the much harder task of emulating the clever microarchitecture that reduces it all to just 1 adder.