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Adder Build (Synthesis)

Adding a pair of bit lists, lsb first.

Ripple carry adder:

fun add c (nil, nil) = [c]
|   add c (a::at, b::bt) =
    let val s = gen_xor(a, b)
        val c1 = gen_and(a, b)
        val c2 = gen_and(s, c)
        in (gen_xor(s, c))::(add (gen_or(c2, c1)) (at, bt))

Faster adder: use wide gates: use functions like gen_addl

Carry argument is replaced with a list of generate and propagate pairs from the earlier stages.

The ripple carry adder is generated by the ML fragment above. The kogge stone is frequently used as a practical, synthesisable adder that is fast and not critical over its layout.

47: (C) 2008-11, DJ Greaves, University of Cambridge, Computer Laboratory.