Simple iterative accumulator combined with gated-carry, carry-completion detection, and skip-carry circuits produces three accumulators with decreased carry propagation times. Devices are used in machine control, measurement equipment, and computer applications to increase speed of binary addition. NAND gates are used in combining network

Mastin, W. C.

English

Methods for increasing speed of binary addition by decreasing carry propagation tim

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Fast carry accumulator design

August 1971	 Brief 71-10274 
NASA TECH BRIEF 
Marshall Space Flight Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield; Virginia 22151. Requests for individual copies or questions relating to the. Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Fast Carry Accumulator Design 
A simple iterative accumulator was combined 
with gated-carry, carry-completion detection, and 
carry-skip circuits to produce three accumulators 
with decreased carry propagation times. These 
devices can be used in machine control, measure-
ment equipment, and computer applications, to 
increase the speed of binary addition. NAND gates 
were used in the combining networks.
The average accumulation time was calculated 
using carry propagation lengths resulting from the 
accumulation of numbers with random bit distribu-
tions. The accumulation times, in order of highest 
to lowest, were: 6.50 us for the iterative accumulator; 
2.26 Ps for the carry-completion detection accumu-
lator; 2.14 Ps for the gated-carry accumulator; and 
1.24 Ps for the carry-skip accumulator. If the dis-
Accumulator Design t (isec) (t/t0) x 100 F F/FO Q 
Simple iterative accumulator 6.50 100 630 1 100 
Gated-carry accumulator 2.14 33 756 1.2 40 
Carry-completion detection (Avg) 14 818 1.3 18 
accumulator 0.880 
Carry-skip accumulator 1.24 19 1	 816	 1 1.3 1	 25
Comparison of Accumulator Designs 
All units offered a considerable decrease in accumu-
lation time without a significant difference in cost 
between them and the iterative accumulator. The 
comparison was formalized by the following criterion 
(see table):
Q = (t/t0)(F/F0) 100 
Q = dimensionless cost comparison indicator 
= accumulation time of circuit 
to = accumulation time of iterative accumulator 
F = cost figure of circuit 
F 0
 = cost figure of iterative accumulator 
Based on the above, the carry-completion detection 
accumulator appears to be the most desirable, and 
would be recommended for use wherever a variable 
accumulation rate is acceptable. However, it should 
be noted that the control circuitry for this unit is 
more complex than that for other accumulators.
tribution is not random, the accumulation time 
for the carry-completion detection circuit could be 
greater than that for the carry-skip circuit. 
The gated-carry circuit is applicable where fewer 
components are more desirable, and where the 
accumulation time obtained by this circuit is accept-
able. It offers the fastest accumulation time for 
fixed-time operation. Where a fixed accumulation 
rate is desirable, the carry-skip circuit is recom-
mended. 
Note: 
The following documentation may be obtained 
from:
National Technical Information Service 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.95)
IC' iiIiiiiietI ,scrleol 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights. 
https://ntrs.nasa.gov/search.jsp?R=19710000273 2020-03-17T02:20:53+00:00Z
Reference: 
NASA-TM-X-53983 (N70-29986), Fast Carry 
Accumulator Design 
Patent status: 
No patent action is contemplated b y NASA. 
Source: W. C. Mastin of
Astrionics Laboratory 
under contract to
Marshall Space Flight Center 
(M FS-20902) 
Brief 71-10274	 Category 01


Fast carry accumulator design

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