precision as well as a much larger register set than System/360.7 Amdahl was
subsequently excluded and thereafter
worked on his own competing design.
Average access time to memory
in ACS- 1 was reduced by using cache
memory.b A new instruction prefetch
scheme was designed by Ed Sussenguth. Pipeline disruption from branches was minimized by using multiple
condition codes along with prepare-to-branch and predication schemes created by Cocke and Sussenguth. Compiler optimizations were viewed as
critical to achieving high performance,
and Allen and Cocke made significant
contributions to program analysis and
As for Stretch, detailed simulation
was critical. Developed by Don Rozenberg and Lynn Conway, the simulator
was used for documentation as well as
validation and improvement of the design. While working through the simulation logic for multiple instruction
decode and issue, Conway invented a
general method to issue multiple out-of-order instructions per machine cycle
that used an instruction queue to hold
pending instructions. Named “
Dynamic Instruction Scheduling,” 9 the scheme
met the timing constraints and was
Also like Stretch, advanced circuit
technology was key to the performance
goals. New circuits were developed with
switching times in the nanosecond
range, and new circuit packaging and
cooling approaches were investigated.
In 1968, based on Amdahl’s cost/
performance arguments for his own
design and the increasing cost projections for unique ACS- 1 software,
management decided to make ACS
System/360 compatible and appointed
Amdahl as director. The next year the
project was canceled when Amdahl
pushed for three different ACS-360
models. Instead, the cache-enhanced
Model 195 was announced, and approximately 20 were built.
Although the project fizzled, Silicon
Valley benefited from the collection of
talent assembled for the ACS project.
b Cache memory was a new concept at the time;
the IBM S/360 Model 85 in 1969 was IBM’s first
commercial computer system to use cache.
Amdahl stayed in California and began his own company, with two dozen
former ACS engineers joining him to
build the Amdahl 470. Other ACS project members moved to IBM’s San Jose
disk drive facility or joined other companies in the Valley.
Cocke went back to the East coast,
staying with IBM, and he later expressed
regret that very little of the ACS- 1 design
was ever published. 5 Indeed, had the
ACS- 1 been built, its seven-issue, out-of-order design would have been the pre-eminent example of instruction-level
parallelism. Instead, the combination
of multiple instruction issue and out-of-order issue would not be implemented
until 20 years later.
In discussing supercomputers in
his autobiography, Watson blamed the
“erratic” IBM efforts partly on his own
“temper,” and said he had come to
view IBM’s competition with CDC like
General Motors’ competition with Ferrari for building a high-performance
sports car. 12
In the 1970s the supercomputer industry pursued vector processing, with
CDC building the unsuccessful STAR-
100 in 1974 and Cray Research building
the very successful Cray- 1 in 1976. IBM
announced a vector add-on for its mainframes in 1985, but the extension did
not sell well. IBM funded Supercomputer Systems Incorporated between 1988
and 1993, but SSI went bankrupt before
delivering its multiprocessor computer.
More recently IBM built a number of
successful large clusters, including Blue
Gene and Roadrunner. In the June 2010
“Top500” Supercomputer list, four of
the top 10 supercomputers in the world
were built by IBM.
Although only one of these three proj-
ects was a commercial success, each sig-
nificantly contributed to the computer
industry. Fred Brooks has described
the impact Stretch had on IBM, and
especially on the System/360 architec-
ture. 3 The Model 90 series influenced
high-performance CPU design across
the industry for decades. The impact of
ACS was more indirect, through people
such as Fran Allen, Gene Amdahl, John
Cocke, Lynn Conway, and others. For ex-
ample, Amdahl was able to recruit ACS
engineers Bob Beall, Fred Buelow, and
John Zasio to further develop high-speed
ECL circuits and packaging for the Am-
dahl 470. Allen and Cocke disseminated
their work on program optimization,
and Cocke carried the idea of designing
a computer in tandem with its compiler
into later projects, one of which was the
IBM 801 RISC. Conway went on to co-
author a seminal book on VLSI design
and attributes much of her insight into
design processes to her ACS experience.
The Computer History Museum collection contains more than 900 Stretch
and ACS-related documents donated
by Harwood Kolsky, and many of these
are online at http://www.computerhis-tory.org/collections/ibmstretch/. These
documents shed light on contributions
to Stretch and ACS by Amdahl, Cocke,
and others, as well as recording the design trade-offs made during the Stretch
project and the studies made of Stretch
performance problems. The Museum
has several oral history interviews on-line at http://www.computerhistory.org/
collections/oralhistories/ and has made
recent talks available on its YouTube
channel at http://www.youtube.com/
user/ComputerHistory/. The Museum
also features a new exhibit, part of which
focuses exclusively on supercomputers,
including the CDC 6600 and Cray- 1.
1. allen, F. the history of language processor technology
in ibm. IBM Journal of Research and Development 25,
5 (Sept. 1981).
2. bashe, C. et al. IBM’s Early Computers. mit press,
Cambridge, ma, 1986.
3. brooks, F., jr. Stretch-ing is great exercise—it gets
you in shape to win. IEEE Annals of the History of
Computing 32, 1 (jan. 2010).
4. buchholz, W. Planning a Computer System: Project
Stretch. mcgraw-hill, inc., hightstown, nj, 1962.
5. Cocke, j. the search for performance in scientific
processors. Commun. ACM 31, 3 (mar. 1988).
6. pugh, e., johnson, l. and palmer, j. IBM’s 360 and
Early 370 Systems. mit press, Cambridge, ma, 1991.
7. Schorr, h. design principles for a high-performance
system. in Proceedings of the Symposium on
Computers and Automata (new york, april 1971).
8. Shriver, b. and Capek, p. just curious: an interview
with john Cocke. IEEE Computer 32, 11 (nov. 1999).
9. Smotherman, m. ibm advanced Computing Systems
10. Smotherman, m. ibm Stretch (7030)—aggressive
Uniprocessor parallelism; http://www.cs.clemson.
11. Spicer, d. it’s not easy being green (or “red”): the ibm
stretch project. Dr. Dobb’s Journal, july 21, 2001.
12. Watson, jr., t. and petre, p. Father Son & Co. bantam,
ny, 1990, 384.
Mark Smotherman ( email@example.com) is an
associate professor in the School of Computing at
Clemson University, SC.
Dag Spicer ( firstname.lastname@example.org) is Senior
Curator at the Computer history museum in mountain