ALU

Question 1

describe the ALU

Answer 1

• implements a set of logic commands
• allows for the selection of which commands should be output
• fixed set of functions:
  
  1. each function can take at most two multi-bit arguments
     2. outputs the result of the boolean function and a set of flags

Question 2

ALU design issue

Answer 2

hardware/software trade-off
- hardware = faster but more expensive
- software = slower less expensive

Question 3

what are the ALU control bit functions

Answer 3

1. za = zero input a
2. na = negate input a
3. zb = zero input b
4. nb = negate input b
5. f = select beterrn & and +
6. no = negate output

Question 4

describe our version of the ALU

Answer 4

• simple: no multiplication/ division, no floating point, no exponentiation
• logic circuits for ADD & AND
• logic circuits to select between them
• logic circutis that can negate or zero an input (x 2)
• logic circuit to negate the output
• logic circuit to handle flags