CDC 1604

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The CDC 1604 was a 48-bit computer designed and manufactured by Seymour Cray and his team at the Control Data Corporation (CDC). The 1604 is known as one of the first commercially successful transistorized computers. (The IBM 7090 was delivered in November 1959.) Legend has it that the 1604 designation was chosen by adding CDC's first street address (501 Park Avenue) to Cray's former project, the ERA-Univac 1103.[1] A cut-down 24-bit version, designated the CDC 924, was also produced.

The first 1604 was delivered to the US Navy in 1960 for applications supporting major Fleet Operations Control Centers in Hawaii, London, and Norfolk, Virginia. By 1964, over 50 systems were built. The CDC 3000 succeeded the 1604.

A 12-bit minicomputer, called the CDC 160, was often used as an I/O processor in 1604 systems. A stand-alone version of the 160 called the CDC-160A was arguably the first minicomputer.[2]


CDC 1640 registers
47 . . . 14 . . . 00 (bit position)
Operand registers (48 bits)
A Accumulator
Q Mask register
Program counter (15 bits)
  P Program counter
Index registers (15 bits)
  1 Index 1
  2 Index 2
  3 Index 3
  4 Index 4
  5 Index 5
  6 Index 6

Memory in the CDC 1604 consisted of 32K 48-bit words of magnetic core memory with a cycle time of 6.4 microseconds. It was organized as two banks of 16K words each, with odd addresses in one bank and even addresses in the other. The two banks were phased 3.2 microseconds apart, so average effective memory access time was 4.8 microseconds. The computer executed about 100,000 operations per second.

Each 48-bit word contained two 24-bit instructions. The instruction format was 6-3-15: six bits for the operation code, three bits for a "designator" (index register for memory access instructions, condition for jump (branch) instructions) and fifteen bits for a memory address (or shift count, for shift instructions).

The CPU contained a 48-bit accumulator (A), a 48-bit mask register (Q), a 15-bit program counter (P), and six 15-bit index registers (1-6). Internal integer representation used one's complement arithmetic. Internal floating point format was 1-15-32: one bit of sign, fifteen bits of offset (biased) binary exponent, and thirty-two bits of binary mantissa.

The most-significant three bits of the accumulator were converted from digital to analog and connected to a tube audio amplifier contained in the console. This facility could be used to program audio alerts for the computer operator, or to generate music. Those familiar with the inner workings of the software could often hear what parts of a task were being performed by the CDC 1604; as a debugging aid, for example, a never-ending repetitive musical phrase indicated the program was stuck in a loop.

Uses and applications

In 1960 one of the first text-mining applications, Masquerade, was written for the Marathon Oil Company in Findlay, Ohio. Masquerade was a text-mining program that used syntactic structures underlying text data to mask out words and phrases for searching purposes.

During 1969, Fleet Operations Control Center, Pacific (FOCCPAC at Kunia) on Oahu in Hawaii launched an Automated Control Environment (ACE) using a cluster of five CDC 160As to supervise a multi-tasking network of four CDC 1604's.

The Minuteman I was the first U.S. solid-rocket ICBM system to be fielded. There were two entirely separate ground station designs which were developed independently. The smaller, more elegant, single silo design incorporated two redundant CDC 1604 computer systems, each equipped with dual cabinets containing four 200 bpi magnetic tape drives. The computers were used to pre-compute guidance and aiming control information. Results based on current weather and targeting information were downloaded into the missile prior to launch. Model displays of both of these ICBM ground station designs, including block models of the CDC 1604 computers, may be viewed at the Octave Chanute Aerospace Museum in Rantoul, Illinois.

The third version of the PLATO computer-based educational system was implemented on a CDC 1604-C.

JOVIAL was used as the CDC 1604's main programming language, while octal was used to program shared services supported by the CDC 160A. NAVCOSSACT based at the Washington Navy Yard provided systems and training support.

According to Irving John Good, the CDC 1604 was used to compose the "drawing" Sailboat by Sam Schmitt and Stockton Gaines.

Similar machines

The 1604 design was used by the Soviet nuclear weapons laboratory. Their BESM-6 computer, which entered production in 1968, was designed to be somewhat software compatible with the CDC 1604, but it ran 10 times faster and had additional registers.


  1. Curiously, a very detailed 1975 oral history with CDC's computer engineers does not confirm this legend: when the "1604" question was asked, the insiders laughed and responded: "It was quite popular at the time that this was the origin" and "We've never been able to substantiate it. However, there's still lots of people who believe it." Page 21 of the oral history provides the official CDC explanation for 1604: the original goal was to support 16K of memory and 4 tape units.
  2. Ed Thelen. "CDC 160A". Retrieved April 15, 2011.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>

External links