Functional Description
The DM74LS181 is a 4-bit high speed parallel Arithmetic
Logic Unit (ALU). Controlled by the four Function Select
inputs (S0–S3) and the Mode Control input (M), it can per-
form all the 16 possible logic operations or 16 different
arithmetic operations on active HIGH or active LOW oper-
ands. The Function Table lists these operations
DM74LS181 devices. Carry lookahead 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 four 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 four bits. The A = B signal can also be used with
the Cn+4 signal to indicate A > B and A < B.
When the Mode Control input (M) is HIGH, all internal car-
ries 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 per-
forms arithmetic operations on the two 4-bit words. The
device incorporates full internal carry lookahead and pro-
vides for either ripple carry between devices using the Cn+4
The Function Table 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 (2s complement notation) without a carry
in and generates A minus B when a carry is applied.
Because subtraction is actually performed by complemen-
tary addition (1s 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 indi-
cated, 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 oper-
ands labeled inside the logic symbol.
output, or for carry lookahead between packages using the
signals P (Carry Propagate) and G (Carry Generate). In the
ADD mode, P indicates that F is 15 or more, while G indi-
cates that F is 16 or more. In the SUBTRACT mode, P indi-
cates that F is zero or less, while G indicates that F is less
than zero. 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 output
(Cn+4) signal to the Carry input (Cn) of the next unit. For
high speed operation the device is used in conjunction with
the 9342 or 93S42 carry lookahead circuit. One carry loo-
kahead package is required for each group of four
Function Table
Mode Select
Inputs
Active LOW Operands
& Fn Outputs
Active HIGH Operands
& Fn Outputs
Logic
Arithmetic
(Note 2)
Logic
Arithmetic
(Note 2)
S3
S2
S1
S0
(M = H)
(M = L) (Cn = L)
(M = H)
(M = L) (Cn = H)
L
L
L
L
L
L
L
H
L
A
A minus 1
A
A
AB
AB minus 1
AB minus 1
minus 1
A + B
A B
A + B
L
L
H
H
L
A + B
Logic 1
A + B
B
A + B
L
L
H
L
Logic 0
AB
minus 1
L
H
H
H
H
L
A plus (A + B)
AB plus (A + B)
A minus B minus 1
A + B
A plus AB
L
L
H
L
B
(A + B) plus AB
A minus B minus 1
AB minus 1
A plus AB
L
H
H
L
A
B
A
B
L
H
L
A + B
AB
H
H
H
H
H
H
H
H
A B
A plus (A + B)
A plus B
A + B
L
L
H
L
A
B
A
B
A plus B
L
H
H
L
B
AB plus (A + B)
A + B
B
(A + B) plus AB
AB minus 1
A plus A (Note 1)
(A + B) plus A
(A + B) plus A
A minus 1
L
H
L
A + B
Logic 0
AB
AB
H
H
H
H
A plus A (Note 1)
AB plus A
Logic 1
A + B
A + B
A
L
H
L
H
H
AB
AB minus A
A
H
A
Note 1: Each bit is shifted to the next most significant position.
Note 2: Arithmetic operations expressed in 2s complement notation.
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