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"Adder" is also a name for a snake.
Wonder if this was one of the inspirations for naming "Python" language too.
One of the first implementations of the interpreter must have tackled the "add" operation, so maybe the interpreter was just an adder in a prototype version.
I know that Monty Python is officially cited as the inspiration behind the name.
by albert_e - great post … thanks for all the work
personally I would like to see a compare and contrast between the Intel 8087 (built around an full width adder), 287 and the Weitek 1167 (built around a full width mac and barrel shifter)
as you note, all these parts were pushing the transistor limits of their day
PS. and the Inmos T800 had a log shifter … so a compromise between those extremes
by librasteve - /* It's a bummer that there is addition but no vipition. */by nine_k
- 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.
- 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.by JdeBP
- 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.by jcranmer
- A lot of applications where an embedded x86 core makes sense don't have a huge need for FP maths.by userbinator
- The ROM used different sized transistors to store two bits per transistor. That's pure analog territory, which most HDLs don't touch.
- Do you have any insights on how power was delivered to these circuits? Maybe it's done in the metal layers that were dissolved? Also, is it correct that there is no on die capacitance surrounding these circuits?
Thanks for the great article.
- The 8087 has one metal layer, which makes power distribution more challenging. You want to keep power distribution in the metal, so for the most part the pattern is two interdigitated trees for power and ground. There are a few places where the lines need to cross, which is accomplished with a short polysilicon connection underneath. The two clock lines are also kept in metal whenever possible.
The die photo at the start of the article shows some of the power distribution (the thick white lines around the edge and through the die). I have a close-up shot of the adder's metal layer in the article, showing the thick power and ground metal lines that run next to the adder.
As far as capacitors, there are some capacitors for specific things, but no decoupling capacitors. I think the capacitors are mostly to tweak the timing, if a signal needs to be delayed slightly.
by kens - 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.by kens
- > 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...
by m1333 - 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.
- 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...by Aardwolf
- 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.by sebgan