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.

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mitthrowaway2 an hour ago | parent | next [-]

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.

	▲	kens an hour ago \| parent [-]
		I count approximately 2014 transistors (including pull-ups) for the 69-bit adder. Each block of four bits takes approximately 117 transistors.

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Aardwolf 3 hours ago | parent | prev | next [-]

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...

	▲	kens an hour ago \| parent [-]
		Even by the time of the Pentium, they had moved to much more complicated adders like Kogge-Stone. I wrote about it here: https://www.righto.com/2025/01/pentium-carry-lookahead-rever...

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sebgan 4 hours ago | parent | prev | next [-]

No immediate questions, but happy to have some great weekend reading. A quick pass through finds one of the best and clearest explainers I've seen. Thanks for this and all the materials you produce.

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m1333 2 hours ago | parent | prev [-]

> 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...

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JdeBP an hour ago | parent [-]

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.

	▲	kens an hour ago \| parent [-]
		In more detail, the microcode engine normally executes one micro-instruction per cycle. For addition, the engine is blocked for one extra cycle to give the result time to percolate through the adder. 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.