1

I am taking comp architecture course, and we are given a task to write 32 bit look-ahead adder in VHDL. Instructor provided the instruction and diagram.instructor's diagram While reading, Wikipedia article, I stumbled on the following diagram. wiki-diagram
Since C4 is already available, what is the purpose of PG & GG outputs, ??? I am trying to find, if the instructors diagram is complete. Thanks !

newprint
  • 217
  • 4
  • 8

1 Answers1

0

The block diagram of a 16-bit CLA is divided into four 4-bit groups and comprises a combinatorial circuit, namely the look-ahead carry generator. The term Gk denotes the group- generated carry, whereas Pk denotes the group-propagated carry. All four groups generate their individual internal carries and then the sum bits. The section-carry generate, GG, and the section-carry propagate, PP, are also computed.

travisbartley
  • 4,823
  • 3
  • 23
  • 41
  • First diagram is 32 bit CLA, made of two 16 bit CLAs. I understand purpose of `Pi` and `Gi`(where i is some number). But what can I do with `GG` and `PP` ? – newprint Sep 25 '12 at 01:44
  • 1
    I think these might be useful for producing recursive adding. For instance, imagine a 64-bit adder with 16 4-bit groups and 4 4-bit CLAs. To compute the final sum, you could use a top-level 4-bit CLA which takes input from the the other 4 4-bit CLAs. So you can stack them hierarchically. If you only need a 32-bit adder the section-carry generate and section-carry propagate are not needed. – travisbartley Sep 25 '12 at 01:55