IBM 650 - Hardware

Hardware

The basic 650 system consisted of three components:

• Console Unit (IBM 650)
• Power Unit (IBM 655)
• Card Reader/Punch Unit (IBM 533 or IBM 537)

Optional components:

• Disk Unit (IBM 355) Systems with a disk unit were known as a IBM 650 RAMAC 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 (five characters per word) at addresses 0000 to 1999, which is approximately 8.5 KB in today's units. A Model 4, introduced in 1959, doubled the drum capacity to 4,000 words. 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 microseconds on the IBM 7040 to 5000 microseconds on the IBM 650) for looking up 46 entries as long as both were programmed in assembler. One higher-level language made the IBM 7040 dramatically slower at table-look-up.

The optional Auxiliary Unit (IBM 653), was introduced on May 3, 1955, providing up to three features:

• Sixty 10-digit words of magnetic core memory at addresses 9000 to 9059; a small fast memory (this device gave a memory access time of 96µs, a 26-fold raw improvement relative to the rotating drum), needed for a tape and disk I/O buffer
• Three four-digit index registers at addresses 8005 to 8007; drum addresses were indexed by adding 2000, 4000 or 6000 to them, core addresses were indexed by adding 0200, 0400 or 0600 to them. If the system had the 4000 word memory drum then indexing was by adding 4000 to the first address for index register A, adding 4000 to the second address for index register B, and by adding 4000 to each of the two addresses for index register C. (the indexing for 4000-word systems only applied to the first address). The 4000-word systems required transistorized read/write circuitry for the drum memory and were available before 1963.
• Floating point – arithmetic instructions with am eight-digit mantissa and two-digit characteristic (offset exponent) – MMMMMMMMCC, providing a range of ±0.00000001E-50 to ±0.99999999E+49

The IBM 533 reader punch unit could only read a maximum of 26 columns of alphanumerics from cards in mostly fixed columns. An expansion allowed more but certainly not over 50, as only ten words could be read from a card (five characters per word).

The IBM 650 (pictured here) at the Haus zur Geschichte der IBM Datenverarbeitung (House for the History of IBM Data Processing), Sindelfingen, is still running (as of May 2004) and will process an income tax program of the time, with input and output on punched cards.

The IBM 7070, announced 1960, was designed to provide a "transistorized IBM 650" upgrade path. The IBM 1620, introduced in 1959, addressed the lower end of the market. Both were decimal machines, but neither had a compatible instruction set.