IBM 650

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The IBM 650 (photo) was one of IBM’s early computers, and the world’s first mass-produced (photo) computer. It was announced in 1953, and over 2000 systems were produced between the first shipment in 1954 and its final manufacture in 1962. Support for the 650 and its component units was withdrawn in 1969.

The 650 is a two-address, bi-quinary coded decimal machine (both data and addresses were decimal), with memory on a rotating drum. The 650 was specifically designed for users of existing IBM unit record equipment (electro-mechanical punched card-processing machines) upgrading from so-called Calculating Punches, like the IBM 604 model, to computers proper.



The basic 650 system consisted of three components:

Optional components:

  • Disk Unit (IBM 355) Systems with a disk unit were known as a IBM RAMAC 650 Data Processing System
  • Card Reader Unit (IBM 543)
  • Card Punch Unit (IBM 544)
  • Control Unit (IBM 652) Magnetic Tape Controller
  • Auxiliary Unit (IBM 653) Core storage, index registers, floating point arithmetic
  • Auxiliary Alphabetic Unit (IBM 654)
  • Magnetic Tape Unit (IBM 727)
  • Inquiry Station (IBM 838)
  • Tape To Card Punch IBM 46 Model 3
  • Tape To Card Punch IBM 47 Model 3
  • Alphabetical Accounting Machine IBM 407

The rotating drum memory (photo) provided 2,000 signed 10-digit words of memory (5 character per word) at addresses 0000 to 1999. A word could not be accessed until its location on the drum surface passed under the read/write heads during rotation (rotating at 12,500 rpm, the non-optimized average access time was 2.5 ms). Because of this timing restriction, the second address in each instruction word was the address of the next instruction. Programs could be optimized by placing instructions around the drum based on the expected execution time of the previous instruction. One specialized instruction, 'Table lookup', could high-equal compare a reference 10 digit word with 46 consecutive following words on the drum in one 5ms revolution and then switch to the next track in time for the next 46 words (there were fifty words per track/revolution). This feat was about one third the speed of a one-thousand times faster binary machine in 1963 (1500 microsecs on the IBM7040 to 5000 microsecs on the IBM650 for looking up 46 entries as long as both were programmed in assembler. One higher level language made the IBM7040 dramatically slower at table-look-up. An upgraded 4,000 word drum became available in 1959.

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