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Unformatted text preview: 4.1.2 104 ARM Organization and Implementation Example 4.2 Complete the ARM2 4-bit carry logic circuit outlined in Figures 4.1 1 and 4. 12 on page 89. The 4-bit carry look-ahead scheme uses the individual bit carry generate and propagate signals produced by the logic shown in Figure 4.12. Denoting these by G[3:0] and P[3:0], the carry-out from the top bit of a 4-bit group is given by:
Cout = G + P.(G + P.(G + P.(G + P.Cin))) Therefore the group generate and propagate signals, G4 and P4, as used in Figure 4. 1 1 are given by: G4 P4 = G + P.(G + P.(G + P.G)) =P.P.P.P These two signals are independent of the carry-in signal and therefore can be set up ready for its arrival, allowing the carry to propagate across the 4-bit group in just one AND-OR-INVERT gate delay.
Exercise 4.2.1 Write a logic expression for one bit of the ALU output generated by the circuit shown in Figure 4.12 in terms of the inputs and the function select lines, and hence show how all the ALU functions listed in Table 4. 1 on page 90 are generated. Estimate the gate count for the ripple-carry adder and for the 4-bit carry look-ahead adder, basing both designs on the circuit in Figure 4.12 and varying only the carry scheme. How much does the extra speed of the carry look-ahead scheme cost in terms of gate count? How does it affect the regularity, and hence the design cost, of the adder? Exercise 4.2.2 The ARM Instruction Set Summary of chapter contents
In Chapter 3 we looked at user-level ARM assembly language programming and got the general flavour of the ARM instruction set. In this chapter we will look in finer detail at the instruction set to see the full range of instructions that are available in the standard ARM instruction set. Some ARM cores will also execute a compressed form of the instruction set where a subset of the full ARM instruction set is encoded into 16-bit instructions. These instructions are Thumb' instructions, and are discussed in...
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- Spring '09