In a typical microprocessor ALU, the results of every arithmetic operation are usually (but not always) transferred to the accumulator register, as in Figures 6-10 and 6-14. In most microprocessor ALUs, the result of each arithmetic operation is also used to control the states of several special flip-flops called flags. These flags are used by the microprocessor when it is making decisions during the execution of certain types of instructions. The three most common flags are:

S (sign flag). This FF is always in the same state as the sign of the last result from the ALU.

Z (zero flag). This flag is set to 1 whenever the result from an ALU operation is exactly 0. Otherwise, it is cleared to 0.

C (carry flag). This FF is always in the same state as the carry from the MSB of the ALU.

Using the adder/subtractor of Figure 6-14 as the ALU, design the logic circuit that will implement these flags. The sum outputs and C₄ output are to be used to control what state each flag will go to upon the occurrence of the TRANSFER pulse. For example, if the sum is exactly 0 (i.e., 0000), the Z flag should be set by the PGT of TRANSFER; otherwise, it should be cleared.

Figure 6-14

Figure 6-10

Transcribed Image Text:

Transfer pulse 8 B₂B₂ B₂ B₂ D 12 7 B₂ CLK A3 Im A3 6 D B register 11 5 B₂ CLK A₂ A₂ D 10 B₁ 3 74LS283 A₁ A₁ CLK BoBo 2 D Bo CLK A A₂ ADD SUB 44 45 Col