Arithmetic Logic Unit

Description:

• Combinational logic circuit that combines a variety of operations into a single unit.
• Has:
  • Addition/Subtraction (4-bit Full Adder)
  • Logical (OR, And gates)
  • Bit Shifter
  • Comparisons
• Inputs: A, B and the code for OPeration
• Outputs: Y (output) and result flag

ALU Operation Encoding in Control Logic:

• BSEL: B’s selector
• LOP: logical operation
• SOP: Bit Shifter op
• OSEL: Output selector

OP Name	OP	BSEL	CI	LOP	SOP	OSEL	Operation
ADD	000	00	0	X	X	00	Y = A + B + CI
SUB	001	01	1	X	X	00	Y = A + $\bar{B}$ +1
AND	011	00	X	0	X	01	Y = A and B
OR	100	00	X	1	X	01	Y = A OR B
SHL	101	X	X	X	0	10	Y = A >> 1
SHR	110	X	X	X	1	10	Y = A << 1
PASS A	111	10	0	X	X	00	Y = A