IBM 5100 Portable Computer Design (1977)

The IBM 5100 Portable Computer Design is a five-page engineering paper by A. J. Cubbon of IBM General Systems Division, Rochester, presented at the APL ITL Symposium, Endicott, New York, 11–13 January 1977 (Volume 2, pages 52–56, IBM document reference 77A000835). It is marked IBM Internal Use Only.

It matters because it is IBM's own account of how the IBM 5100 was built, written by the people who built it, and it states things about the machine that appear nowhere in the customer manuals or in the CE documentation.

What it says that nothing else does

The processor. IBM describes it as "a fast two-chip 8-bit processor" with 49 16-bit microinstructions, operating on four interrupt levels. Almost every secondary account calls the same processor a 16-bit design built from thirteen bipolar gate arrays. Both descriptions come from real evidence and they are hard to reconcile; the argument is set out on the IBM PALM processor page.
Interrupt priority. In IBM's order: 1) Keyboard; 2) Tape and Printer; 3) Communications and Serial I/O; 4) Normal Operations.
What was actually designed for this machine. "The only hardware specially designed for the 5100 is the power supply and the covers." The keyboard is the standard IBM solid-state keyboard already used on the System/32 and the 3740; the read/write storage is the same part used in System/32 and some System/370 models; the ROM is the same technology as the 3740's; the printer is the matrix printer already used on System/32 and the 3740.
Storage. A new 48K-bit-per-chip FET ROM, giving up to 248K bytes of storage in the desktop unit. All three storage types — read/write and the two ROMs — are MOS FET LSI.
Why APL is System/370 code. Two IBM 1130 APL implementations were evaluated and rejected (overlay pages cost too much time and too much user storage) before the team chose APLSV in late June 1974 — 15 digits of precision against the 1130's six. The emulator and the three interface routines were written in six months and occupy less than 10K of ROM; IBM records that no error had been found in it since final testing in early 1975.
Availability. Announced September 1975, available worldwide in April 1976.
The ROS quirk. The nonexecutable ROS is attached through the I/O adapter and read with a GET microinstruction rather than through a storage address register — which is why the language interpreters cannot simply be jumped into.
The patch mechanism. Because the processor can fetch instructions from read/write storage as well as from executable ROS, portions of ROM can be superseded by instructions in RAM.

What it does not contain

No schematics, no parts list, no component values, no voltages. It is a conference paper, not a service document — for service data use the IBM 5100 Maintenance Information Manual and the IBM 5100 Portable Computer Parts Catalog. The paper's own cited reference is SY31-0405-2 (1976), an earlier edition of the manual hosted here.

Download the paper here: IBM 5100 Portable Computer Design (1977).pdf

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