Vviewpoints
DOI: 10.1145/1859204.1859216
Historical reflections
iBM’s single-processor
supercomputer efforts

Insights on the pioneering IBM Stretch and ACS projects.

IMaGine a CpU designed to is- sue and execute up to seven instructions per clock cycle, withaclockrate10timesfaster than the reigning supercomputer. Imagine a design team of top experts in computer architecture, compilers, and computer engineering— including two future ACM A.M. Turing Award recipients, five future IBM Fellows, and five future National Academy of Engineering members. Imagine that this team explored advanced computer architecture ideas ranging from clustered microarchitecture to simultaneous multithreading.

Is this a description of the latest microprocessor or mainframe? No, this is the ACS- 1 supercomputer design from more than 40 years ago.

In the 1950s and 1960s IBM undertook three major supercomputer projects: Stretch (1956–1961), the Sys- tem/360 Model 90 series, and ACS (both 1961–1969). Each project produced significant advances in instruction-level parallelism, and each competed with supercomputers from other manufacturers: Univac LARC, Control Data Corporation (CDC) 6600, and CDC 6800/7600, respectively.

Of the three projects, the Model 90
series ( 91, 95, and 195) was the most
successful and remains the most well
known today, in particular for its out-
of-order processing of floating-point
operations. But over the past few years,
new information about the other two
efforts has surfaced, and many previ-
ously unseen documents are now phys-
ically collected and available online
at the Computer History Museum in
Mountain View, CA.

Stretch

Many histories recount that Stretch
began in 1954 with efforts by Steve
Dunwell. 2, 11 Less known is that Gene
Amdahl, designer of the IBM 704 scien-
tific computer, was assigned to design
Stretch. 10 Stretch was targeted at the
needs of the Livermore and Los Alamos
nuclear weapons laboratories, such as
calculations for hydrodynamics and
neutron diffusion. Unlike the vacuum-
tube-based 704, which had identical
machine and memory cycle times of
12 microseconds, the transistorized
Stretch was expected to have a 100-nano-
second machine cycle time and a two-
microsecond memory cycle time. In
response to this logic/memory speed
imbalance, Amdahl worked with John
Backus to design an instruction look-
ahead scheme they called asynchronous
non-sequential (ANS) control. 10

image CoUrteSy oF ibm