CS 352 Lecture 12 – ALU
Dear students,
Now that we have hardware components that can perform boolean logic and arithmetic, let’s package them all together into a one-stop shop called the arithmetic-logic unit (ALU). This component will perform exactly one of many possible operations. Which operation it performs will be determined by 6 control pins, and it will also accept two multibit parameters named x and y. The six pins will have this influence:
if zx x = 0 if nx x = ~x if zy y = 0 if ny y = ~y if f result = x + y else result = x & y if no result = ~result
We’ll spend the first part of class today determining how exactly to set these pins so that they achieve 18 possible functions of x and y. Our function space looks something like this:
~? ({x, ~x, 0, 111...111} {&, |} {y, ~y, 0, 111...111})
To figure out how to set these pins, we’ll establish a few identities:
x & 111...111 = x x + 111...111 = x - 1 ~x = -x - 1 x + ~y = x - y - 1
Once we determine the pin configurations, we’ll sketch out a complete ALU using components we have defined earlier. Then we’ll have a memory-less computer!
Here’s your TODO list to complete before next time:
- Read chapter 2 in your textbook.
- On a quarter sheet, write the HDL for a chip named Add4. Assume the half and full adder chips already exist.
See you next class!
