|R3 ← R1 + R2||Addition|
|R3 ← R1 - R2 (R1 + R2' + 1)||Subtraction|
|R2 ← R2'||Complement (really a logic operation)|
|R2 ← -R2 (R2' + 1)||Negation|
|R1 ← R1 + 1||Increment|
|R1 ← R1 - 1||Decrement|
Increment and decrement can be done with combinational incrementers and decrementers, counter registers, or by adding a 1. ( Where does the 1 come from? )
Multiply and divide are not often implemented as microoperations due to the amount of time they require. They are usually implemented as a multi-clock-cycle routine of shifts and adds.
Figure 4-7: 4-bit adder/subtractor.
Figure 4-9: Arithmetic circuit. Analyze all select input combinations.