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A project log for PEAC Pisano with End-Around Carry algorithm

Add X to Y and Y to X, says the song. And carry on.

Yann Guidon / YGDESYann Guidon / YGDES 04/13/2021 at 01:110 Comments

The design of the corresponding digital circuit will not make your head spin a lot.

In fact it's almost trivial. There are not enough circuits to hide tricks, though:

Apart from that, this circuits is probably as simple as you can get. The critical datapath is one MUX and one ADD so this thing runs FAST.

The "finish" circuit is simple as well :

  1. Select which variable contains the last computation
  2. Split the data into its two halves
  3. Merge them with a XOR

Et voilà.

You get 32 bits if your input word was 32 bits wide.

Ideally, the simplest enhancement that could be done is to "recirculate" the Carry Out from one adder to the Carry Input of the next adder, but apart from x86 (and eventually some POWER cores ?) playing with the carry signals is "a whole different matter".

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