Hardware Reference
In-Depth Information
Logical unit
Carry in
AB
INVA
A+B
A
Output
ENA
B
B
ENB
Sum
Enable
lines
F
0
Full
adder
F
1
Decoder
Carry out
Figure 3-18.
A 1-bit ALU.
A
7
B
7
A
6
B
6
A
5
B
5
A
4
B
4
A
3
B
3
A
2
B
2
A
1
B
1
A
0
B
0
F
0
F
1
1-bit
ALU
1-bit
ALU
1-bit
ALU
1-bit
ALU
1-bit
ALU
1-bit
ALU
1-bit
ALU
1-bit
ALU
INC
O
7
O
6
O
5
O
4
O
3
O
2
O
1
O
0
Carry
in
Carry
out
Figure 3-19.
Eight 1-bit ALU slices connected to make an 8-bit ALU. The en-
ables and invert signals are not shown for simplicity.
Years ago, a bit slice was an actual chip you could buy. Nowadays, a bit slice is
more likely to be a library a chip designer can replicate the desired number of
times in a computer-aided-design program that produces an output file that drives
the chip-production machines. But the idea is essentially the same.
