Carry Save adder:
The carry-save adder block is the same circuit as the full adder
The name “carry-save” arises from the fact that we save the carry-out word instead using it immediately to calculate a final sum.
Mainly used in multiplier to add partial products (Wallace tree)
Carry Skip adder (n-bits):
Total adder is divided by (n/M) equal length bypass stage, each of which contain M bits
An approximate total propagation time can be derived esimated as
tp = tsetup + M.tcarry + (n/M -1).tbypass + (M-1).tcarry + tsum
tp is linear in the number of bits n, since the worst case delay is generated at the first bit position and ripples through the last block, skip around (n/M-2) bypass stage and is consumed at the last bit position without generating an output carry.
Carry Select adder
CSA breaks the addition problem into smaller group
Total adder is divided by (n/M) equal length bypass stage, each of which contain M bits
Instead of waiting on the arrival of the output carry, both 0 and 1 possibilities are analyzed
When carry out settles, either the result of 0 or 1 path is selected by the multiplexer, which can be performed with a minimal delay
Delay can be estimated as
tadd = tsetup + M.tcarry + (n/M).tmux + tsum
tsetup: fixed overhead to create the generate and propagate signal
tcarry: the propagation delay through a single bit
tbypass:the delay through the bypass mux of a single stage
tsum: the time to generate the sum of the final stage