Carry-lookahead adder

Chan P. K. et al Delay optimization of carry-skip adders and block carry-lookahead adders. In Proc. 10th Computer Arithmetic Symp., Grenoble, S. 154-164, June 1991. 

You have read in your source HDL followed by compile. An add function in the source HDL code mapped to an adder from the Synopsys Design Ware library. The current implementation is a ripple adder (rpl), but you require a carry-lookahead adder (cla) to be inferred. Does one have to start with reading in the source HDL and specifying constraints to chiange the implementation to a cla adder. 

The carry-lookahead adder is designed to speed up the addition process by overlapping the carry and sum operations. The result is diat tiie 2n logic level ripple-carry arithmetic process is reduced (in practice) to a log n logic level operation. ... 

A carry-lookahead adder introduces the propagate term P and the generate term G such that the carry out equation can be rewritten as ... 

Figure 6.10 Cell structure of a 4-bit carry-lookahead adder with overflow.

Figure 6.20 Top-level schematic of the carry-lookahead adder.

The statistics for the whole carry-lookahead adder are shown in Table 6.4. This is a special example in that there are two sjmthesized versions of... 

The best circuit to compare the carry-lookahead adder with is the ripple-carry architecture, RIPPLE I, shown in Table 6.2, as both have 4-bit operation and output an overflow signal. 

Table 6.5 Delays and gates along the critical path of the carry-lookahead adder. Bold figures are discussed in the text. R/F means the delay on a rising/falling input signal edge...

The Booth recoding algorithm of this multiplier required a behavioural implementation as opposed to the structured approach taken with the carry-lookahead adder. A behavioural style allows the multiplier to be implemented in different ways, such as a function or a process. The former ap-... 

Map Operators to library models. The description of a model may contain operators such as + and that can be synthesized in a variety of implementation styles. For example, an addition can be implemented using either a ripple-carry adder or a carry-lookahead addo. The designer can selectively replace operators by calls to specific models in a particular library. Any tq)erator that is not mapped to a library model will be implemented by default as combinational logic, with the exception of multiply ( ) and divide C / )> which must always be mapped. 

The 16-bit 2 s complement adder design is contained in Figure 8.12. This is an extension of the ripple-carry adders designed in Chapter 6. A ripple-carry adder has been used rather than a cany-lookahead adder and so speed has been traded for area. The adder provides addition, subtraction and comparison operations. 

---