Portable Computer

IBM 5100

Introduced in September 1975, the 5100 Portable Computer was IBM’s first production personal computer (six years before the best-seller IBM PC). The 5100 was intended to put computer capabilities at the fingertips of engineers, analysts, statisticians and other problem-solvers, but not for business purposes.

If the size and weight of the 5100 seems (25 kg) huge by today’s standards, then the IBM 5100 was very slim compared to a late-1960’s IBM computer with the equivalent capability. Such a machine would have been nearly as large as two desks and would have weighed about half a ton.

The 5100 was much more powerful that its predecessors, like Altair 8800 (and much more expensive however—it was sold for between $8975 and $19975). It featured built in CRT display, keyboard, BASIC interpreter and mass storage (tape drive). It has also a much more advanced design: A microcoded 16-bit CPU executing an interpreter which in turn interprets a subset of the IBM 360 (or IBM/3) mainframe instruction set!

Available options:

  • carrying case
  • IBM 5103 printer, dot matrix, tractor feed, 132 column, 80 char/s bidirectional
  • IBM 5106 external tape drive
  • communication adapter
  • serial I/O adapter

IBM 5100 Computer

The IBM 5100 Computer

Depending on options installed, the 5100 can run the APL (see the IBM 5100 APL reference manual) and/or BASIC programming languages and can have 16K, 32K, 48K or 65K RAM (see the table below of twelve different models).

MemoryProgramming language
16KA1 – $8,975B1 – $9,975C1 – $10,975
32KA2 – $11,975B2 – $12,975C2 – $13,975
48KA3 – $14,975B3 – $15,975C3 – $16,975
64KA4 – $17,975B4 – $18,975C4 – $19,975

The 5100 has an internal five inch CRT, displaying 16 lines of 64 characters. Because the characters are so small, IBM provided a three-position switch to allow the user to select the display of all 64 characters of each line, or only the left or right 32 characters (interspersed with spaces).

Mass storage was provided by a 1/4-inch cartridge tape drive using DC300 cartridges to store 204 KB on 300 feet tape.

On a 5100 with both languages (APL and BASIC), the user’s choice of language is selected by a toggle switch on the front panel.

Instead of being written in the native microcode instruction set of the processor, the 5100’s language interpreters (APL and BASIC) are written for more sophisticated virtual machines, and the microcode emulates those machines. This was done in order to economize on the amount of ROM (read-only memory) needed to implement the language interpreters, and possibly to speed the software development. The APL microcode emulates a subset of the System/360 instruction set, while the BASIC microcode emulates the System/3.

IBM offered three Problem-Solver Libraries, contained in magnetic tape cartridges, with the IBM 5100 to provide more than 100 interactive routines applicable to mathematical problems, statistical techniques and financial analyses.

The IBM 5100 hasn’t a discrete CPU (Central Processing Unit) as in modern computers, the circuit board seen below is the “processor” (it has 15 large chips).

CPU of IBM 5100 Computer

The CPU of IBM 5100 Computer

The 5100 Portable Computer was withdrawn from marketing in March 1982.


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