Building An ALU

1
Building An ALU

• Arithmetic and Logic Unit

2
Components

• AND Gate
• OR Gate
• NOT Gate
• Adder
• Multiplexor

3
One-bit ALU
4
One-bit ALU
Carry In
b Invert
a
M U X
b
Carry Out
5
32-bit ALU
a
b
Control
Carry In
32
32
a0
b0
a1
b1
a2
b2
a3
b3
a30
b30
a31
b31
32
Carry Out
Result
6
One-bit ALU -- MIPS
Control
Carry In
b Invert
a
Out
M U X
M U X
b
set
Carry Out
7
One-bit ALU -- MIPS High Bit
Control
Carry In
b Invert
a
Out
M U X
M U X
b
less
set
Overflow Detect
overflow
Carry Out
8
32-bit ALU -- MIPS
a
b
Control
Carry In
32
32
a0
b0
a1
b1
0
a2
b2
0
a3
b3
0
Zero
a30
b30
0
a31
Overflow
b31
0
32
Carry Out
Result
9
Register-Register Data Path
ALU Control
Reg 1 Read
5
Read Data 1
Zero
32
Reg 2 Read
ALU
5
Reg Write
5
Read Data 2
Overflow
32
Write Data
32
Clock
Write Enable