Philips Semiconductors FAST Products

Product specification

74F181

Arithmetic logic unit

March 3, 1989

4

When the Mode Control input (M) is High, all internal carries are

inhibited and the device performs logic operations on the individual

bits as listed. When the Mode control input is Low, the carries are

enabled and the device performs arithmetic operations on the two

4-bit words. The device incorporates full internal carry look-ahead

output, or for carry look-ahead between packages using the signals

P (Carry Propagate) and G (Carry Generate). P and G are not

affected by carry in. When speed requirements are not stringent, it

can be used in a simple ripple carry mode by connecting the Carry

high-speed operation, the device is used in conjunction with the

74F182 carry look-ahead circuit. One carry look-ahead package is

required for each group of four 74F181 devices. Carry look-ahead

can be provided at various levels and offers high speed capability

over extremely long word lengths.

The A=B output from the device goes High when all four F outputs

are High and can be used to indicate logic equivalence over 4-bits

when the unit is in the subtract mode. The A=B output is

open-collector and can be wired-AND with other A=B outputs to give

a comparison for more than 4 bits. The A=B signal can also be used

lists the arithmetic operations that are performed without a carry in.

An incoming carry adds a one to each operation. Thus select code

LHHL generates A minus B minus 1 (two’s complement notation)

without a carry in and generates A minus B when a carry is applied.

Because subtraction is actually performed by complementary

addition (one’s complement), a carry out means borrow; thus, a

carry is generated when there is no underflow and no carry is

generated when there is underflow. As indicated, this device can be

used with either active-Low inputs producing active-Low outputs or

with active-High inputs producing active-High outputs. For either

case, the table lists the operations that are performed to the

operands labeled inside the logic symbol.

MODE-SELECT FUNCTION TABLE

MODE SELECT INPUTS

ACTIVE HIGH INPUTS & OUTPUTS

ACTIVE LOW INPUTS & OUTPUTS

S3

S2

S1

S0

Logic (M=H)

Arithmetic** (M=L) (Cn=H)

Logic (M=H)

Arithmetic** (M=L) (Cn=L)

L

L

L

L

A

A

A

A minus 1

L

L

L

H

A+B

A+B

AB

AB minus 1

L

L

H

L

AB

A+B

A+B

AB minus 1

L

L

H

H

Logical 0

minus 1

Logical 1

minus 1

L

H

L

L

AB

A plus AB

A+B

A plus (A+B)

L

H

L

H

B

(A+B) plus AB

B

AB plus (A+B)

L

H

H

L

A

⊕B

A minus B minus 1

A

⊕B

A minus B minus 1

L

H

H

H

AB

AB minus 1

A+B

A+B

H

L

L

L

A+B

A plus AB

AB

A plus (A+B)

H

L

L

H

A

⊕B

A plus B

A

⊕B

A plus B

H

L

H

L

B

(A+B) plus AB

B

AB plus (A+B)

H

L

H

H

AB

AB minus 1

A+B

A+B

H

H

L

L

Logical 1

A plus A*

Logical 0

A plus A*

H

H

L

H

A+B

(A+B) plus A

AB

AB plus A

H

H

H

L

A+B

(A+B) plus A

AB

AB plus A

H

H

H

H

A

A minus 1

A

A

H = High voltage level

L

= Low voltage level

*

= Each bit is shifted to the next more significant position.

** = Arithmetic operations expressed in two’s complement notation.