last updated December 31, 2016
That was the bandwidth of the memory bus of ACS.
I worked (for perhaps a week) on a scheme to provide line printers scaled
to the speed of the machine. Acres of printers. It was bizarre but fun.
-- Norman Hardy, personal correspondence
At one point T.J. Watson [Jr.] visited and the machine was described to him.
At the end he asked, "But where is the unit record equipment?" There was no
answer to that. Later John Cocke described 20 parallel railroad tracks each
with a train traveling 60 mph. The cars of each train were full of punched
That was the bandwidth of the memory bus of ACS.
I worked (for perhaps a week) on a scheme to provide line printers scaled to the speed of the machine. Acres of printers. It was bizarre but fun.
-- Norman Hardy, personal correspondence
As the quote above indicates, the ACS-1 design was very much an out-of-the-ordinary design for IBM in the latter part of the 1960s. In his book, Data Processing Technology and Economics, Montgomery Phister, Jr., reports that as of 1968:
In the midst of this environment in 1968, the ACS project experienced a dramatic change of direction. The original architecture had been designed for the perceived performance and floating-point-precision needs of customers such as the national labs, much like the CDC 6600. However, Gene Amdahl repeatedly argued for a S/360-compatible design to leverage the software investment that IBM was making in the S/360 architecture and to provide a wider customer base.
Amdahl wanted foremost to see a successful product be developed by the ACS project. He and Max Paley had been told of a rumor that "the Research Supercomputer plan was to be developed, then found that the market size was not sufficient to permit IBM to meet the government's antitrust constraints on its product announcements, then IBM could cancel the program and have the technology which was developed available for future product programs without having the technology development costs attached to these products, thus allowing a lower price to be assigned to those products!" He felt that the ACS architecture team was not orienting their design and development so as to avoid such a cancellation, and he did not want to be associated with a failed program.
Because of his strong and repeated advocacy for a S/360-compatible design, in 1967 Amdahl was frozen out of the daily activities of the ACS project. During the next year, and with help from John Earle starting in early 1968, Amdahl developed an outline of a competing design that was S/360 compatible but arguably faster and cheaper than ACS-1. In May of 1968, Amdahl's arguments won the day, and IBM.management dictated a changeover to a S/360-compatible ACS design.
The sequence of events leading to the change in the instruction set architecture for ACS includes:
Actually, the machines which we are trying to supplant in the marketplace are not S/360 computers, but are really 6600's and 6800's. If we are going to try to use the program compatibility argument on these customers, then perhaps our machines should be 6600 compatible. Clearly, this is not an acceptable strategy.
Conti's memo states in part:
The comparison indicates the lack of any clear superiority of the ACS architecture and machine organization over a suitable System/360 machine organization. ... ACS architecture is superior to 360 architecture for a strictly high performance processor. The superiority, however, is in the 10-20% range. This superiority is due to a very few features of the ACS. First, the provision of the full address literal in storage to register instructions. Second, the ability to do fixed point and logical operations in the floating point area. Third, the branching structure which permits separation of the three functions associated with the branch: the testing of the condition, the generation of the branch, and the actual exit. Associated with the branching structure, of course, is the ACS definition of 24 condition bits registers and the skip instruction. ... Further, it would seem that the importance of the architecture is more than nullified when one considers how dependent performance is upon the algorithm chosen to solve a problem and the quality of the code produced toward that end. It is apparent that the ACS machine organization is more sensitive than the AEC/360 machine organization to the question of code optimization."
Conti also mentioned a "pseudo skip" feature designed at Poughkeepsie as a hardware alternative to the ACS-1 skip architecture. This is likely US 3,553,655, "Short forward conditional skip hardware." The inventors were Model 91 team members Anderson and Sparacio.
The decision to pursue S/360 compatibility caught most of the ACS team members off guard. Based on the mandate given at the founding of the laboratory, most project members felt that IBM management would never hinder their efforts at building the world's fastest supercomputer by requiring S/360 compatibility. However, several of the more senior members had noticed that they were giving more and more presentations during early 1968 and knew that a change in direction was being considered for ACS at company headquarters in New York.
The decision to switch ACS to the S/360 architecture seemed to ignore the architectural benefits sought and obtained by the design team; so, several original project members quit and went back to New York to the Research Division, including Phil Dauber and Herb Schorr. John Cocke decided to take a sabbatical at the Courant Institute of Mathematical Sciences at New York University. (While there he worked on a well-known compiler textbook with Jacob Schwartz; the textbook was made available in draft form but never formally published.)
Because of the architectural incompatibility, Amdahl's design, now termed the ACS/360, had to discard the innovative branching and instruction skipping schemes. The new design provided a strongly-ordered memory model as well as precise interrupts.
To achieve a profit for the ACS program, Amdahl asked IBM management to approve three ACS/360 models: the high-performance design, a 1/3 performance version, and a 1/9 performance version. He felt that these performance goals would be a good fit with the System 360 marketing plans. He remembers that IBM Corporate Marketing evaluated the targets and reported: "1) the supercomputer alone was a loss leader! 2) the supercomputer plus the 1/3 performance computer was break-even! And 3) the supercomputer plus both the 1/3 performance computer and the 1/9 performance computer was normal profit -- 30% pre-tax!" He remembers meeting with "the Divisional President and Vice President who worked on me for over an hour to get me to modify the multipliers of my 3 computers (which would increase their cost without increasing their market acceptance nearly as much)." Amdahl countered with a recommendation to shut down ACS. In May 1969, IBM management cancelled ACS.
Additional possible factors in the compatibility and cancellation decisions:
The Model 90 series was initiated by IBM as a program to advance the state of computer art and to serve a limited number of sophisticated data processing users. With its program objectives met and all deliveries now scheduled over the next 12 months, IBM has stopped accepting orders for these computers.
Finally my business sense overcame my pride and I belatedly figured out that we couldn't compete with Control Data for the same reason that General Motors can't compete with Ferrari in building two-hundred-mile-an-hour sports cars. Supercomputers had become so highly specialized that even if we came up with a design equal to theirs, it would never fit in with the rest of our product line, our style of selling, our volume and profit targets, and so on.He goes on to say that his "temper was partly to blame for IBM's erratic behavior in the supercomputer market".
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After the cancellation a large number of ACS engineers wanted to stay in California. Several chose to work on disk drive systems at the IBM San Jose facility, including Mooney. Robelen and Galtieri left IBM to form MASCOR (Multi Access Systems Corp.), and Beebe, Buelow, Clements, Tobias, Zasio, and others left IBM to join MASCOR. Amdahl resigned in September of 1970 and formed his own company shortly thereafter. Many of the former ACS engineers at MASCOR joined Amdahl after MASCOR closed, due to MASCOR being unable to obtain the necessary additional venture capital to stay afloat.
The S/360 Model 195 was announced in August 1969 after the ACS cancellation, and a vector processing task force was started in Poughkeepsie that same month.
Excerpt from Gene Amdahl interview in April-June 1997, IEEE Design and Test of Computers, p. 7 and p. 13:
Amdahl: ... While I was [in California], IBM decided to set up the Advanced Computer Systems Laboratory to develop a very high-end machine. I had been named an IBM Fellow ..., and an IBM Fellow was entitled to work on whatever he wanted to, provided he did not impact the product line.
I went to work on this Advanced Computer Systems project, and I concluded after three or four months that it would be a much more manageable project if the machine were made compatible with System/360 instead of being an entirely different architecture. This did not endear me to the manager of the project, so I got sort of sent to Coventry. Anyone on that project who was seen talking to me - or me talking to them - got called out to a meeting.
D&T: So in a sense you were excommunicated?
Amdahl: That's right. They also had a very bright logic designer who had designed the most significant parts of the system they were working on, but who was virtually unmanageable. So they decided to get rid of him, and they gave him to me to work with. I told him what I wanted him to design in the 360 area, and lo and behold, we were able to get slightly higher performance at about 75% of the cost. And we wouldn't need to write a new operating system.
D&T: What happened?
Amdahl: We ended up having a shoot-out. The two of us who did the 360-compatible version won. We established that in fact we could achieve more performance at lower cost.
D&T: So you basically got a new architecture through this 360-compatible machine?
Amdahl: Yes, but the company decided not to build it because it would have destroyed the pricing structures. In the first place, it would have forced them to make higher-end machines. But with IBM's pricing structure, the market disappeared by the time performance got to a certain level. Any machine above that in performance or price could only lose money.
... I've always aimed to create a product that was useful to a great many people. So I didn't want to continue working on high-end machines at IBM when their pricing policy made that virtually impossible. ...
Excerpt from Gene Amdahl interview in July 2007, IEEE SSCS E-News:
(Q) Was IBM still planning ways to use your design talent?
(A) ... [discussing events of 1965] ... A few months later I was asked to consider attaching my Fellow activities to a new lab IBM was starting called Advanced Computer Systems, ACS, which would be designing a super computer, hopefully to serve the Livermore and Los Alamos labs. The project would be developing a computer proposed by a group from IBM research. I knew quite a bit about it and liked much, but not all, of the plan. I agreed to do it, but being a Fellow, I did not report to their management. For a few weeks I tried to make some changes in areas I didn't like, but to no avail. I recognized that with the requirement to develop the computer design, the technology and the total software support, that there was no way they could possibly find a big enough market to meet IBM's antitrust requirements of profitability. I didn't want to be associated with a loss-leading project, like had happened to STRETCH in the 1960s, so I thought about the problem and came up with a different approach - design the computer to be System 360 compatible and at the highest speed we could achieve. This would eliminate all of the software development cost. To make it profitable we could design one or two smaller machines with the performance spacing of the existing 360 product line, thus sharing the technology development costs over a much larger market and maybe meeting the profitability requirements.
(Q) How did you fare in the design challenge and the consequences?
(A) I presented my alternative to the project managers only to have it rejected out of hand, for they were wedded to the architecture they had developed. I was pondering how to separate myself from the impending loss leader when their top logic designer got into some trouble. The managers considered him unmanageable, but couldn't fire him so they found the solution, transfer him to me! I was delighted for he was responsible for the design of the most performance determining part of their computer. I knew that if he did the design of that part of the 360 alternative, there could be no charges of faulty design. It took a bit over two weeks to describe enough of my performance approaches before he recognized that it was really feasible to compete with the other design. He then went into it wholeheartedly and actually was able to achieve a slightly higher performance and a somewhat smaller cost. Bob Evans came out to ACS with about five technical people and they held a shoot-out. We won and I was made the lab manager. The first thing I did was have the two smaller computers costed. I then submitted the three system plan to corporate pricing. The single highest speed computer was a loss leader. The second smaller computer added made a break-even program. Adding the third even smaller computer came out with normal profit! IBM management decided not to do it, for it would advance the computing capability too fast for the company to control the growth of the computer marketplace, thus reducing their profit potential. I then recommended that the ACS lab be closed, and it was.
The ACS/360 design was based on this overall structure:
In summer 1968, Ed Sussenguth investigated making the ACS/360 into a multithreaded design by adding a second instruction counter and a second set of registers to the simulator. Instructions were tagged with an additional "red/blue" bit to designate the instruction stream and register set; and, as was expected, the utilization of the functional units increased since more independent instructions were available.
IBM patents and disclosures on multithreading include:
Sidebar: ES/9000 high-end processors
The ACS/360 structure appears to have influenced the design of the IBM ES/9000 high-end ("520-based") processors some twenty-odd years later. The ES/9000 high-end processors were structured as:
For more information on the high-end ES/9000 processors, see J.S. Liptay, "Design of the IBM Enterprise System/9000 high-end processor," IBM Journal or Research and Development, vol. 36, no. 4, 1992, pp. 713-731.
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