IBM 3790

IBM 3790 was a computer system developed by the IBM Data Processing Division (DPD) announced in 1975, one of the first distributed computing platforms. It preceded the IBM 8100, announced in 1979.

It was designed to be installed in branch offices, stores, subsidiaries, etc., and to be connected to the central host mainframe, using IBM Systems Network Architecture (SNA).

Components

• The 3791 control unit was offered in two models, Model 1 and Model 2. Both models offered an integrated floppy disk unit and a Synchronous Data Link Control (SDLC) communications interface. Either control unit could optionally include an integrated 120 lines per minute (lpm) line printer, and could attach up to sixteen IBM 3277 display stations, up to three 3292 auxiliary control units, and up to four 3793 keyboard-printers. A 1200 baud internal or external modem was optional. The model 1 supported 8.3MB of disk storage and the model 2 up to 26.9MB.
• The 3792 auxiliary control unit had options for attachment of up to two dial-in IBM 2741 communications terminals, up to four 3793 display stations, and a line printer.
• The 3793 printer-keyboard.
• The 3411 model 1, Magnetic tape unit and controller (added in 1977) and up to three 3410 tape units attached to the 3411 unit.

Host software

• Function Support Program.
• Subsystem Support Services.
• User Application Support Program.

Reception

The 3790 failed to achieve the success IBM intended, due to several issues. It had a complex programming language, The 3790 Macro Assembler, and the customers found it difficult to deploy applications on it. The Macro Assembler ran only on an IBM mainframe and then the compiled and linked object was moved to the 3790 for testing.

The 3790 was designed as a departmental processor, but the requirement for an IBM mainframe development environment inhibited adoption in its target market of mid-size companies. The result was lackluster interest in the product. In addition the 3790 was priced higher than minicomputers of comparable processing power.

One of the products IBM released to help developers was Program Validation Services (PVS). With PVS, one could test a program in the mainframe environment using scripts. The scripts were cumbersome to create, and prone to errors. Since mainframe time was expensive and often difficult to obtain very few programmers used PVS for anything other than initial testing.

The manual for the Macro Assembler was bulky (about 4 inches thick) and difficult to use as a reference. Another programming issue was code design and size; the hardware architecture loaded code into memory on 2k segments, for optimal execution time it was critical to ensure that processing intensive loops did not cross the segment boundary and incur delays swapping segments in and out of memory.

IBM recognized the problems with the Macro Assembler and created an automated program generator named DMS. DMS later became Cross System Product (CSP) on the 8100. DMS was essentially a screen painter; it could do simple edits such as field range checking or numeric tests but more complex logic still had to be coded using the Macro Assembler.