Assignment 28 (Assigned November 8)

ALUs generates condition codes to help control unit in a microprocessor to guide the flow of a program. For example, if a program wants to compare two numbers and then perform the next operation depending on if number A is greater than or equal to B or number B is greater than A. The ALU can be used to subtract B from A and store the sign of the result (we are not interested in the actual result). Similarly, in an another instance we may be interested in comparing A and B to determine if A is equal to B or not, we can subtract A from B and check if result is zero or not. There are four condition code typically used: CARRY (to indicate if a carry is generated from MSB), SIGN (MSB itself), ZERO (to indicate if all bits are zero from ALU), and OVFL (to indicate if the operation resulted in arithmetic overflow). CARRY, SIGN, and OVFL make sense only for ADD or SUB operations. Is there a relationship between CARRY, SIGN and OVFL if we are adding or subtracting 2's complement numbers.

Table 11.3 in the book gives 16 arithmetic and 16 logical functions performed by an ALU circuit. Four bits F3-F0 specify the function and M specifies arithmetic or logical operation required. C is carry input to ALU. What will be the output if A=0011, B=0101, F3-F0 = 0101, C=0 (C'=1) and M=0.

Suppose you have to waste one cycle in the following data path (you may have to do it if, for example, input is not ready). This is equivalent of a NOOP (No operation) instruction. Provide a set of control signals to waste one cycle.